January 31, 2020

It’s no secret that energy storage contractors have grown increasingly concerned about the safety risks associated with lithium batteries. While storage pairs nicely with solar technology and the industry seems poised for growth, a single incident could quickly change the narrative, as it already has in Arizona and South Korea.

SepiSolar CEO Josh Weiner invited Chief Electrical Engineer Richard Dobbins to the latest Quick Talk for ideas on designing safe and effective battery systems. See why Richard believes firefighters have an important role to play in permitting and inspections as installers continue to gain experience connecting storage to the grid.

Video Transcript

Josh Weiner: Hello and welcome. My name is Joshua Weiner of SepiSolar. I’m here with Richard Dobbins of SepiSolar. And we’re here today to talk a bit about lithium battery safety.

The way I actually see an engineering company like ours is—some people come to us asking for plan sets, some people ask us for technical advice and consul, or utility interconnection processing and PE stamps and whatnot.

I feel like all the services and deliverables that SepiSolar offers are all just a lot of different ways of calling ourselves risk managers. The name of the game for an engineering company like us is to really manage the risk on all the projects we touch.

And these risks, they can be safety related, they can be performance related, they could be upfront risks during the first few months of construction all the way downstream to the next twenty-five years of in-service operation because there’s risks all over the place on these projects.

And so I’m really happy to be here with you, Richard, because you are a licensed electrical professional engineer, right?

Richard Dobbins: Yeah, absolutely.

Josh: Could you just tell us a little bit about yourself? How long have you been at SepiSolar? What do you do for us?

Richard: I’ve been at the company about five years now. I’ve been a registered electrical professional engineer for about two of those.

Josh: Congratulations.

Richard: Thank you. I’ve been around since just around the inception of that marketable energy storage. My first few months on the job was diving right into battery storage projects and getting a feel for—back then, it was lead acid—type of systems. And so I’ve been kind of seeing the slow progression to where we are now with energy storage and utility scale. Something that really grew from where it was five years ago.

Josh: It’s an interesting point. When people talk about storage in the context of solar, it’s almost a given: we’re talking about lithium storage. It’s so ubiquitous now.

But it’s funny to remember that it’s actually only probably about five years old as a real commercial industry. I mean, for grid scale, for utility interactive applications behind or in front of the meter to do things like manage power, energy, on the grid.

It’s actually a very recent phenomenon. It’s been in laptops and cell phones for decades. But as a utility interconnected resource, it’s actually brand new. Well, great.

As an electrical professional engineer, safety—I assume—is a big part of your life. Is that fair to say?

Richard: I would say, yeah.

Josh: So, when you’re looking at lithium battery systems, what are some of the things you look at or look for or think about when you’re designing the systems for safety?

Richard: Safety is a huge concern with these projects.

I tend to look at safety from a two-pronged approach: passive and active. Passive safety, things like the enclosure that the batteries are contained in, the clearances that the batteries are from other equipment or from buildings and residences, things like that. There are certain distances that are required by code to keep those units away from potential risks, right?

And then you have active safety, things like the battery management system, things like fire suppression systems. In the case that a fire does break out in the unit, you can have a chemical extinguisher that puts it out or that stops the fire from propagating.

So really, between looking at those two sorts of approaches for safety, you can design an effective safe system that you can say with certainty that it will prevent catastrophic damage or loss of life.

Josh: I like your distinct delineation between active and passive. It almost sounds like the difference between quality assurance and quality control.

Quality assurance is something in the process or in the product you design. But then quality control is like a physical person, grabbing something, inspecting it, checking it, going down a checklist, and putting it into service.

And software is really good at that, right? Or BMSs and being able to—or people, human beings going in and checking something actively.

And then there’s the design aspect of it and engineering, what you try to do for it.

Well then how about common pitfalls that you see? Not only do we design engineer lithium systems, we get a lot of clients who ask us to review their lithium systems, either pre-construction or post-construction.

I’m just curious. What have you seen other people do with regard to lithium battery safety? And do you think it’s good? Do you think we got more work to do? Or how’s the industry doing from what you see out there?

Richard: I think as time goes on, the more exposure that people have to these kinds of projects, the more comfortable they’re going to be with the designs that they come up with or the first thoughts they have, right? The first pass of the design. Things like clearances and separation distances and even in the enclosure, like the passive elements that I mentioned before.

Those tend to get overlooked at the beginning, right? Most people who are used to PV systems are used to clearances for ventilation or for working clearances, you know, three feet and all that. With lithium batteries, it has to be a lot greater because of the risk of fire.

Josh: And that’s kind of what happened with APS’s battery fire, I think, too. Actually that was a case where there was a meltdown of a specific pack in a specific rack, and it melted into an aluminum pile (or molten metal pile) but then the saving grace, if you will, or the lesson learned, was because of the spacing in between the different racks.

Yes, there was a fire. We can talk about how it’s inherent to the technology; there may be nothing we can do about it. But, with some good spacing and ventilation requirements, we can at least prevent it from propagating, going from a little problem to a much bigger problem.

Richard: And that’s a big thing I have. Space is always an issue, right? With designers and with contractors. They want to fit as much as they can in the smallest space possible. They don’t want to waste space. So they want to densely populate the area, right? They want to fit as many batteries as they can. But you have to take into account that safety clearance.

Josh: That’s a serious kind of juxtaposition or conflict of interest. One of the promises and one of the promotions of lithium batteries is that they are so energy dense and don’t require a lot of footprint.

But that ends up being one of the risks, when you have that much energy too tightly packed, you end up with these problems like luckily with the APS battery fire was able to avoid in this particular case.

And then there are standards like NFPA 55,  UL9540, with and without the A. Do you think these are adequate?  Is the industry evolving to a point where all the codes are done and standards are done and everybody’s safe? Just follow the guidelines? Follow the rules?

Richard: I think that’s heading in the right direction. We obviously still have a lot to learn, as evidenced by the issues, the failures, that are breaking out, the ones we read about in the news, the ones we hear about through the grapevine.

There is a lot of work to be done with the safety regulations and just the industry catching up to those regulations. But we are definitely headed in the right direction with requiring those to be followed.

Josh: Do you think there’s a reason why folks underestimate the risks of lithium batteries? I mean, I almost feel like it’s a given. As I mentioned earlier, when we talk about battery storage, we just know we’re talking about lithium battery storage. Why do we just go straight to that? And are we underestimating the risks? And if so, how?

Richard: I think it just comes down to the age of the technology and the general awareness of the population. Like you mentioned at the beginning: It’s everywhere. But it’s only been maybe the last five years that people started plugging into their houses and using it to control their utility energy.

Josh: Do you think they’re so comfortable with it in their cell phones and laptops and now cars that—it’s just: “Yeah. OK, it’s already in everything. Just throw it on the side of the house. Back up my loads when the grid goes down.” It doesn’t even factor into the equation.

Richard: I think so. And to your point about energy density: the amount of lithium in your phone or in your laptop is really small compared to the amount of lithium that you’re going to be installing to back up your house or to create demand charge mitigation. It’s a huge difference. And with that much density comes a much higher risk of fire, of a failure.

Josh: What would you recommend? What should people be thinking about or doing when they design for these, when they build lithium systems, given that the codes need more time to evolve, given that we’re learning and we’re on this trajectory?

And when we talk about storage, we just immediately talk about lithium. What are the things you think we should be doing as an industry or us, SepiSolar, as the licensed engineers that are sealing and complying with codes and following through on these safety precautions?

What should people like us and others be doing?

Richard: I think it’s all about awareness.

Numerous projects I’ve worked in the past, it was very much a learning experience for mostly everyone involved, especially on the part of fire departments. They’re very concerned with lithium batteries being installed.

And it’s their job to mitigate, to ensure fire safety. So they get very involved in the permitting process when it comes to lithium batteries. And so I think getting fire departments involved and educated on the technology and the safeties that are involved is really going to help go a long way with getting everybody as a whole, and really enforcing it as part of the permitting process and the safety inspection process.

Josh: I like the suggestion of collaborating with the officials who know a thing or two about fire.

For example, some of the things that the solar industry is not good at—historically—have been things like software.

The construction industry, it’s a very hardware-driven industry. We’re putting big, heavy machinery and we’re doing very high-risk operations and activities on buildings and structures. So it’s very risky work.

And if we’re not aware of the issues, it’s like the twelve-step program, you know? The first step toward recovery is admitting you have a problem.

First, you’ve got to be aware of it, and then you can address it. And by collaborating with the experts who know a thing or two about all this, we’ll get smarter as a whole.

That seems to make a lot of sense. And again, the essence of risk management: maybe we don’t have all the answers, but by being aware of what the risks are, we can surface sometimes highly nuanced issues and work with our customers to decide what the best plan is for that.

Richard: Yeah, absolutely.

Josh: Makes sense. Well, thanks so much for joining us.

We’re going to have many more conversations like these about not just battery safety, but also we’re going to talk about battery performance and the state and evolution of the market with various technologies and different benefits and costs associated with each of those.

Really excited to talk more about that. Please join us at You can read much more on our blog about information we’ve published along these lines, as well as following us on Twitter and LinkedIn.

Looking forward to talking more about this with you soon.


August 5, 2019

Lithium battery safety is not rocket science. Manufacturers with a robust set of production data can show customers success rates for their batteries and the conditions that cause batteries to fail. The problem is that very little safety data is accessible to most buyers or the public.

Buyers will always have to decide for themselves how much risk they are willing to tolerate. Some source batteries from a selective group of original equipment manufacturers (OEMs) and pay a premium to avert risks associated with the lowest-priced batteries. But many buyers are operating in the dark, lacking the safety data they would need to make an informed decision.

Consequently, the energy storage industry in its brief history has already witnessed dangerous and damaging lithium battery safety incidents, including the April 19 fire at Arizona Public Service’s McMicken Energy Storage facility. Other notable incidents include a lithium battery fire and subsequent battery malfunction that led the Federal Aviation Administration in 2013 to ground Boeing’s entire 787 Dreamliner fleet. The next lithium battery fire can happen almost anywhere, anytime.

To safeguard against fire risks, ask lithium battery makers the questions about cell production and testing in this post. Battery buyers don’t have to wait for technology development or new regulations. They can bring about a new safety standard by demanding better safety data and buying lithium batteries only from OEMs that make the data available.

Questions to ask about cell production

Some online shoppers go to commerce platforms like Kickstarter for innovative products and products that may be available at a significant discount from an upstart manufacturer. When sourcing lithium batteries, you want to take the opposite approach. Instead of pursuing innovative products, look for proven products that have a long track record of consistent production. Instead of hunting for discounts from unknown suppliers, expect to pay fair value for a product that has completed a rigorous safety analysis and achieved an exceptionally low failure rate.

How many battery cells and battery packs does your supplier produce each year?

One lithium cell represents one data point. The more cells you produce, the more data you have. As such, the highest-volume producers have the most data on performance, thermal runaway, and failure.

For this reason, an OEM producing 10 million cells per year should have a better understanding of cell safety and performance. Large-volume manufacturers have probably seen every possible failure occur many times. By the same token, a small-volume manufacturer needs more time to analyze and understand cell failure. Buying cells from small-volume manufacturers may carry more risk.

What changes have been made to the battery cell and battery pack production process?

A consistent manufacturing process yields predictable safety and performance results. It’s plain to see that different cell materials bring about different cells. But different production equipment can affect safety characteristics just as much. Even if the materials and equipment stay the same, a manufacturer that relocates production may alter a host of environmental conditions and other variables that affect results. Changes in relative humidity, temperature range, and impurities in the air can impact safety characteristics of lithium cells. Differences in quality control and other processes introduced by a new manufacturing technician crew can also have an effect. All these changes should be understood and quantified in a prudent lithium battery safety analysis.

How does your supplier handle material acceptance and storage?

Even if everything goes right during the manufacturing process, pre-production material acceptance and storage can affect lithium battery safety. About five years ago, a global supplier of solar inverters experienced a series of product failures after electronic circuit boards had been stored in the wrong warehouse and exposed to moisture. Once product assembly was complete and the inverters were energized, a short circuit on the boards caused a fire and led to quite a bit of property damage. While moisture can also affect battery cell safety, so can other environmental conditions, such as air impurities and particulate matter.

It’s not easy to perform a safety analysis that identifies failure points for battery cells. To test how moisture affects safety, you would have to take identical cells and store one of the cell materials in an environment that gets wetter in small increments until you find a statistically relevant number of failures. Then you would have to repeat the process with incremental changes in temperature, dust, and other variables. Testing would require a lot of cells, a lot of minor changes in cell processing, a lot of time, and a lot of analysis. And it would all have to be done without vastly increasing cell production costs.

What are the failure rates for your supplier’s battery cells and battery packs?

In the absence of industry-wide standards, contractors seeking assurances about product safety have their work cut out for them. First, they have to request failure rates and analysis from each of their suppliers. Then the manufacturers must provide the data. Next comes the subjective test. If the contractor feels comfortable with the risk, he or she can decide if the battery quality is adequate. Different contractors have different tolerance levels for quality. A contractor who installs one small system per year may not place a great deal of emphasis on quality. The chances of failure are small. However, contractors installing many large systems must pay more attention to quality. Their businesses depend on the successful operation of a much larger population of cells.

Consider an OEM with a 99.98 percent success rate for battery cells in the first three years of operation. That translates to a 0.02 percent failure rate. If a contractor installs 1,000,000 cells per year, the contractor can expect 600 cells to fail. [Multiply failure rate (0.0002) x annual production (1,000,000) x number of years (3).] This might be an unacceptable level of risk. On the other hand, if a contractor installs 10,000 cells per year, the contractor can expect 6 cells to fail. This level of risk might be no big deal, so long as those cell failures don’t propagate to the entire pack or the entire storage system.

Questions to ask about cell testing

We all know not to leave a fireplace unattended or a gas oven running when nobody’s home. We understand that doing so introduces a serious risk of fire. But how many people know the temperature threshold that is likely to cause a lithium battery to catch fire or explode? Before procuring lithium batteries, especially those that will be sited at a building where people live or work, be sure to understand the conditions that create lithium battery safety hazards. Safety hazards that start in a single battery cell can quickly spread to the battery pack and the entire energy storage system.

What are your supplier’s battery cell thermal runaway characteristics?

It’s important to understand how a battery cell responds to the conditions that can initiate a fire or an explosion. There are many ways to test lithium cells for these conditions. Some examples are the top nail test, where a nail of standard size is driven with standard force into the top of the battery, and the side nail test, where the same procedure is carried out with a battery lying on its side.

Other tests include the fast heat test, where a battery inside a control chamber is exposed to a rapid temperature increase; the slow heat test, where a battery is exposed to a slow temperature increase, and the overcharge test, where a fully charged battery stays connected to a power source and is continually charged.

What is the probability of thermal runaway for your battery cells?

With test results in hand, you can make reasonable predictions about how a battery will perform according to design specifications. Graph 1 shows how increased temperature leads to thermal runaway. While all five cells exhibit similar power generation as temperature increases, there is a notable difference in how close each cell comes to the failure point represented by the horizontal red line at 160°.

how increased temperature leads to battery cell thermal runaway

Graph 1

Graph 2 shows how constant temperature over time leads to thermal runaway. The battery cell depicted by this graph remains at very low risk of thermal runaway when temperature is held constant at 159°. But a 1° increase in constant temperature vastly increases the probability of thermal runaway. A 2° increase makes thermal runaway a near certainty.

how constant temperature over time leads to battery cell thermal runaway

Graph 2

One of the challenges when characterizing lithium cell failure is calculating at what temperature and over what duration a cell fails. Because the answer is different for each cell, we need to see how different the answers are. What if one cell failed after two hours at 60°, another cell failed after 5 hours at 190°, and a third cell failed after 3 hours at 250°? This data would be difficult to characterize. It seems like almost every temperature is dangerous and could lead to cell failure.

Now what if the data looked more like this? Cell 1 fails after two hours at 160°, Cell 2 fails after two hours at 161°, and Cell 3 fails after 1.5 hours at 162°. This data suggests that thermal runaway is consistent and predictable. If we can find consistent results, we know when failures occur and how to prevent failure by designing systems for lithium battery safety.

How does thermal runaway spread from cell to cell?

This is really a two-part question. For starters, let’s look at how thermal energy from a failed lithium cell gets distributed across neighboring cells. Do all neighboring cells get the same amount of energy from the failed cell? Does one cell get all the energy while the others get none? Do two cells get 90 percent of the energy? Next, let’s look at how much stress an initiator cell applies on neighboring cells. If a failed cell exposes neighboring cells to temperatures up to 120°, the risk of cell-to-cell propagation is low. The risk is much higher if a cell failure has a magnitude of 180°. If energy is distributed unevenly, we would want to know the magnitude of stress for each of the neighboring cells.

What are the conditions that lead an entire battery pack to catch fire?

If cell-to-cell propagation extends to one or two neighboring cells and stops, failure of the whole battery pack or battery module is unlikely. If cell-to-cell propagation extends to hundreds of neighboring cells, it’s far more likely that the entire pack will burn. By understanding when battery packs catch on fire and start heating up the neighboring packs, the system designer can plan for fire detection and suppression systems as required by NFPA 855 and UL 9540A to kick in as a last line of defense.

What are the conditions that lead an entire energy storage system to catch fire?

If a fire containment system fails to contain a fire within the energy storage system enclosure, the charging infrastructure for the batteries may also catch on fire. (Think of a car catching on fire while being pumped with fuel at the gas station. Fire can spread to the gas pump, then the entire gas station.) Once the charging infrastructure is on fire, the entire property, including its occupants, are at risk. In sum, one battery cell failure can lead to the destruction of an entire building and the loss of life.

Demand lithium battery safety data

A safe battery is a well-documented battery. Test data helps engineers, system integrators, system owners, and regulators make smart, effective business decisions. While data doesn’t eliminate risk, it does inform us of the risk. Therefore, data empowers us to make decisions on how to manage, contain, suppress, mitigate, or ignore it. By putting appropriate measures in place, we can reduce risk to an industry-acceptable level. Without test data, a battery might operate safely today, but we don’t know why. Then if conditions change and the battery is no longer safe, we won’t know how to mitigate the risk.

The industry can expect a steady supply of safe lithium batteries as soon as buyers make purchasing decisions conditional on access to safety data. There are many safe batteries on the market. But there are many more cheap, risky batteries on the market. The simple solution is to buy safe batteries—which might mean accepting a higher price.

Contractors should request data from OEMs or look for third-party evaluations from independent engineering (IE) reports or independent test laboratories. The data is not publicly available, or hard to find, which is a serious problem. UL, the “gold standard” in product safety, even has trouble gaining access to this sort of data. So one requirement in the UL 9540 standard is to capture thermal runaway data, even for batteries that pass all the tests. In other words, when a lithium battery goes for the UL 9540 test, the test lab will force the battery into thermal runaway and then document the results to help characterize lithium cell failure.


June 10, 2019 1

A recent fire at a utility-owned energy storage facility near Phoenix, Arizona has implications for everyone who is standardizing around lithium-ion batteries to design storage systems. Since lithium represents about 95 percent of the market, this is a topic of near-universal interest.

Especially for me. My experience with lithium-ion batteries goes beyond the storage system engineering and design work we do here at SepiSolar. Ten years ago, not long after founding SepiSolar, I helped launch Green Charge Networks, an early leader in lithium battery deployments. That’s where I saw technology, product configuration, permitting, performance, and operational risks associated with lithium batteries begin to materialize.

The industry is moving fast to push lithium battery deployment to new heights, but we still cannot easily quantify risks. Nor the costs.

We at SepiSolar are technology agnostic. Our commitment is to openly consider the costs and benefits of all commercially viable design options. Given how many projects appear to be treating lithium as the only commercially viable technology, I encourage developers to reevaluate lithium—particularly after the recent fire in Arizona—and consider flow batteries as an alternative that can be deployed at lower cost, greater speed, and superior safety.

Arizona storage system fire

Reported facts about the fire at the McMicken Energy Storage facility are limited. Based on local media reports, we know that firefighters responded to an incident on April 19. While inspecting the 2 MW / 2 MWh battery system, eight firefighters suffered injuries in an explosion. The cause is unknown. All of the firefighters with the most serious injuries were in stable condition in the days following the blast.

The system owner, Arizona Public Service, switched off other energy storage projects in the aftermath of the fire but, as Greentech Media has reported, APS is not wavering on plans to deploy 850 MW of battery storage by 2025.

The entire industry will be paying close attention when investigators reveal their findings about the root cause of the fire and the ensuing sequence of events. Even now, however, developers can size up the inherent risks that all projects using lithium-ion batteries should address.

Lithium battery risks

Eight years ago, when the US Department of Energy awarded Green Charge Networks a $12 million grant to deploy lithium battery storage systems, I was bullish on the technology. If anyone was a believer in lithium, it was me. Then, one by one, the following risks came to light.


Every time you cycle a battery, capacity and efficiency drops bit by bit. Performance on day 30 will not be the same as on day 1. How well does your financial model sold to your client calculate degradation along the performance line?

Thermal runaway

It’s critical to make sure that a battery operates according to its specification. This means when you integrate lithium batteries at a facility, the function of the HVAC system expands from comfort to safety. Now, when an HVAC system requires a little maintenance, it’s not just an O&M concern, but a safety risk. A battery that begins to operate outside of its normal temperature range can experience thermal runaway.

Hazardous materials

Lithium-ion batteries use materials that can introduce safety and environmental hazards if not properly contained. The storage industry needs an effective process for salvaging lithium, nickel, cobalt, or manganese. There’s no need to reinvent the wheel. We can borrow best practices from the solar industry, which has recycling for silicon-based solar modules and collection and recycling of cadmium-telluride thin film modules at the end of their operating lifetime.

Warranty claims

Lithium battery vendors have not yet established a track record showing the warranty claims rate. Or the frequency of warranty claims, which reflects the long-term failure rate for systems operating in the field. Early adopters carry the risk that failures may exceed expectations, straining the supplier’s ability to make good on all claims. In fact, engineers at one large lithium battery supplier have published a peer-reviewed scientific paper saying that lithium batteries are degrading faster than expected and proposing a patent to resolve the issue, suggesting that the risks should be taken seriously.

Human rights

Three years ago, the Washington Post published an expose on cobalt mining practices in Congo, where children help populate the workforce that uses hand tools to dig in underground mines, exposing themselves to health and safety hazards. Cobalt is an ingredient in lithium batteries. The Post has also traced the lithium supply chain from parts of Chile, where indigenous communities have struggled to protect the environment and win local economic benefits from the extraction and sale of the lucrative mineral.

Downstream costs

The low upfront cost of lithium batteries is only part of the total cost of ownership, one that excludes downstream costs associated with operations and maintenance of the batteries, the fire detection / suppression system, or the HVAC system that keeps the batteries within their specified temperature range. A failure to perform proactive operations and maintenance could not only increase long-term costs but void the manufacturer’s warranty.

Parasitic load

As industry analysts have gained a deeper understanding of how much storage capacity is needed to keep storage-integrated HVAC systems running, it appears that round-trip efficiency for lithium battery systems may be lower than originally thought. Citing Lazard’s ongoing levelized cost of storage analysis, Greentech Media has reported that parasitic loads could knock down system efficiency by 17 percent or more.

Advantages of flow batteries

In recent years, I have had many opportunities to compare lithium batteries and vanadium flow batteries side by side, while designing storage systems at SepiSolar and performing battery tests in partnership with Nextracker. The battery test, ongoing since 2017, consists of over two dozen battery types, including 5 lithium batteries, 6 flow batteries and 2 flywheels, plus an ultracapacitor, an advanced lead-acid battery, a copper-zinc battery, and a nickel-iron battery.

Through firsthand experience, one key observation at this point is that the market currently has two leaders in the race to achieve lowest total cost of ownership: lithium batteries and vanadium flow batteries. Vanadium flow batteries have earned a place on the leaderboard based on advantages in cost, performance, installation speed, safety, and design simplicity.


Please note, first of all, that battery costs vary based on storage system design and use case. The battery cost for a commercial system used principally for demand-charge reduction will be different than the battery cost for a grid-scale storage project designed for transmission and distribution deferral.

That said, Nextracker has shown that vanadium flow batteries can yield a lower total cost of ownership than lithium batteries due to significantly lower O&M costs over 20 years.


Nextracker has also demonstrated a competitive installation process with vanadium flow batteries. Installation of Nextracker’s NX Flow, a solar-plus-storage solution using Avalon Battery’s vanadium flow battery, requires less installation time and fewer materials than a central storage system due to being shipped “wet.” This means it’s full of electrolyte from the factory. It’s the first battery in the world to demonstrate this feature.

The battery is pre-commissioned and integrated with a 3-port string inverter at the factory. All battery-to-inverter wiring is complete on arrival. Before installation, the construction crew drives piles and installs cross rails to set up a mounting platform. Then the crew places the battery with a forklift and bolts the battery to the platform. Finally, the crew connects DC and AC wiring from the solar array to the inverter. Here is a 3-minute demonstration.


All plated batteries, including lithium batteries, have inherent safety risks. If you take the positive and negative sides and create a short circuit, the wire can get so hot that it explodes. Firefighters have reported on fires in electric vehicles that get damaged in a car crash, get towed, and catch fire days later.

Vanadium flow batteries have three key safety advantages. First, you can turn a vanadium flow battery off, preventing the device from charging or discharging altogether, and with zero voltage on both the positive and negative terminals. Second, the temperature rise in a flow battery is limited. Even if you short the battery on the chemical side or the fluid side, the temperature rises briefly and then drops, and the battery can be placed back into service immediately with no downtime to speak of. It’s the most boring test you’ll ever see. Finally, there are no flammable, toxic, nor hazardous materials or components. Check out this white paper on energy storage system safety from retired San Jose Fire Captain Matthew Paiss to learn more.

Battery design

Flow batteries are simple by design. They consist of two chemical solutions, one with positively charged ions, another with negatively charged ions. When connected to a generator (actually, a reversible fuel cell) the battery charges by pulling ions from the positive solution and pushing them into the negative solution. When you switch the battery to discharge, the ion flow goes in reverse and generates an electric current. The “secret sauce” of vanadium flow batteries is that the entire electro-chemical reaction happens in a purely aqueous state, which translates to “no degradation,” which translates to “lowest LCOS.”

Trust and visionary thinking

As a licensed engineering firm, SepiSolar’s first obligation is to follow national and jurisdictional codes and standards. The value of our design work depends on our ability to optimize the best products and technologies for the right applications that maximize benefits and minimize costs, all while providing structural and electrical engineering stamps in all 50 states. Beyond that, SepiSolar follows a set of core values that promotes trust and integrity, and encourages visionary thinking.

When customers approach us to design lithium batteries for residential and commercial applications, we do it. When customers ask us to advise them on the tradeoffs between battery technologies, we do that as well, covering all the topics raised here.

Our commitment to promoting trust and visionary thinking compels us to discuss openly the risks (and, therefore, costs) of lithium batteries, especially in the aftermath of the Arizona storage system fire. While we hope the industry can mitigate all the risks, many have not yet been fully addressed. Meanwhile, we owe it to the Arizona firefighters who suffered injury to engage in an open discussion about lithium batteries.

Our customers are remarkably entrepreneurial. We expect that contractors will quickly adapt to market changes by delivering storage solutions that balance cost, speed, and safety.

Please contact us if you want to learn more about engineering and design for storage systems using flow batteries.


February 1, 2019

When I saw this article about LG lithium-ion energy storage fires in Korea, I couldn’t help but think of the fires that PG&E is being held responsible for in California. Those fires have ultimately lead PG&E into bankruptcy and will inevitably increase energy costs to ratepayers.

It’s amazing how something as seemingly simple as a campfire, power line, or a 18650 lithium cell—about the size of a lipstick container–can cause so much damage to California, one of the wealthiest states in the world and PG&E, the largest utility in the state, and, of course to the loss of lives and homes.

Some of these hazards defy logic or at least expectations. When SepiSolar was providing technical due diligence and engineering review services to NRG Home Solar from 2014 – 2016, we came across residential projects on the East coast that had unexpected dangers. For example, there was a solar PV system installed on top of the garage where snow had piled up on the PV system. Some rain had turned that snow into a giant slab of hardened ice. When the ice slipped off the solar array, it crushed the car parked in the driveway–not dented, dinged, or scratched. It completely totaled the car. The homeowner told us “that’s exactly where my children play in the summertime.”

Having just become a father at the end of December 2018, I think it’s fair to say that safety cannot, should not, and will not ever be taken for granted on my watch.

Risks vs Benefits

I don’t mean to suggest that we ought to over-design, over-engineer, over-regulate, over-install, or somehow bullet-proof every single component or assembly in a traditional solar or storage system.  That’s like saying “Since car accidents kill people, let’s require everyone to drive army-grade tanks down the street.” That line of thinking effectively kills an industry and becomes a zero-sum game. Instead, I would pose that taking risks is a part of life and is healthy for us, since taking risks and stepping outside our comfort zones is exactly how we grow, learn, and evolve.

The goal is to take calculated risks, or, alternatively, educated risks. What’s a calculated risk? It’s a risk that you’re aware you’re taking. The difference between educated risks and blind or reckless risks is awareness.

We then need to weigh those risks against the benefits in order to make effective decisions. After those decisions are made, we need to be ready to revisit them again soon because the learning process never stops. Assumptions will need to be revised, data recalculated, risks revisited, benefits re-weighed, and decisions re-evaluated. This is how we evolve and approach an ever-safer future, together.

So, let’s build some awareness, shall we? Let’s have a data-driven discussion about the fire risks associated with energy storage systems, and let’s turn our blind risks into calculated ones. Having helped build Green Charge Networks into a nationwide energy storage integrator (acquired by Engie in 2015), engineered solar and battery systems for over 10+ years, and having worked with utilities, UL, code officials, etc. on safety standards, I think I might have a thing or two to say about this subject.

Evaluate the Energy Storage Technology

To minimize risks in energy storage, perhaps the most obvious approach is to work with a technology that inherently works with chemicals and materials that have no fire risk associated with them. This is particularly difficult with batteries because when almost any battery is short-circuited, they instantly become a fire hazard. But that’s the nature of batteries – they can produce insanely high amounts of current, since the resistance in the battery circuit is governed by however fast (or slow) the chemicals involved can react with each other, allowing the free flow of electrons to accumulate. Of course, these chemicals are designed to react with each other in order to release electric charge. So, fire hazard is almost inherent in any battery (with at least 1 exception).

I love this side-by-side technology comparison authored by Fire Captain Matthew Paiss, a 22-year veteran of the San Jose Fire Department. Captain Paiss is the Fire Department’s subject matter expert on energy storage and is the IAFF primary representative to NFPA 70 (National Electrical Code) and NFPA 855 (Energy Storage System Standards), which has been incorporated into UL standards such as UL 9540. It was surprising and gratifying to know that there’s at least 1 technology that rises above the rest when it comes to safety.

Codes & Standards

There are a ton of uber-smart tradesmen, engineers, officials, and subject matter experts who love to wordsmith and craft codes and technical language (God love them!) in order to impose a minimal, universal set of health and safety standards designed to protect personal property and life. Some of these codes go all the way back to 1897, as is the case with the National Electrical Code, when electricity was thought of as a liquid! (Check out Leyden jars.)

Bottom line, let’s be sure to read and understand the modern codes thoroughly, including NFPA, NEC, UL, among others. Every word, comma, and comment were crafted with the care one would expect of a nationally applicable set of requirements, even if you disagree with many of them. It’s important to follow voltage, current, and sizing requirements, naturally. NEC 706, for instance, was just added to the NEC in the 2017 edition. That’s the first time batteries have been overhauled in the NEC since Article 480 was written back in the early 20th century! Let’s expect this new code section to evolve with the times as more data becomes available and continue to think of these codes as a “minimal” set of safety standards that we can go above-and-beyond as necessary to ensure the safety of the systems we design and build.

Real-time Data

While codes and standards are important, one of their drawbacks is that they are slow to change. Technology and data often evolve faster than codes and policies. Because of this, it’s important to look at the data, stay up-to-date on the latest-and-greatest information available, and dynamically build this data into your systems as it becomes available. Basically, I’m advising you to read. Read articles, publications, journals, media newsletters, and absorb as much as possible to keep up-to-date.

For instance, now that the above Korean article has surfaced about LG battery fires, it’s imperative to find out the root cause failures that led to these hazards. There is much to learn from failure, thereby converting failure into learning opportunities (which perhaps negates the use of the term “failure” in the first place – nothing is a failure, so long as you learn something from it!). We don’t have to wait for new technologies or new codes to come out. Instead, let’s use the data right away in any or all systems that we may be using with LG batteries, or any battery, for that matter.

The first time I thought about the risks associated with batteries was when I heard that Boeing grounded the Dreamliner. Our Co-Founder and CEO of Green Charge Networks at the time was a retired Boeing executive, so this naturally caught our attention. Wikipedia does a decent job summing up that experience, and you can get the full investigative report here.

The general takeaway is that regulatory bodies, manufacturers, and engineers were not “up to snuff” on the risks associated with battery technology. To a great degree, as the above Korean article shows, we are still learning these risks. At our time at Green Charge Networks, we understood that this meant that the safe deployment of battery systems would largely rest on us, since codes, standards, products, and regulations were still too much in their infancy to support us.

Direct Experience and Training

Nothing prepares you for danger, uncertainty, or risk more than education, experience, and training. The more hands-on experience you have with a particular product or technology, the more you will understand its limitations, weaknesses, and risks. Understanding not only what and when a battery undergoes thermal runaway, but also the “how” can really help put battery risks into perspective. What I learn from this is that it’s not just the battery one should be cautious of, but also the environment the battery is in. For example, does the battery have a fire suppression system? Is the battery located near any buildings or structures that have no fire suppression?.

One time I dropped a wrench on an old golf cart battery, and it just so happened that the wrench landed perfectly on both positive and negative terminals simultaneously. It was the first time I saw metal turn bright red, orange, and then white, and eventually melting all over the battery. This was just a regular ol’ lead acid battery, so it was surprising to me that such an old battery could have such a great impact on something as solid and stiff as a wrench. Needless to say, I am very cautious around terminals of batteries, since most batteries cannot be inherently turned “off” (again, with some exceptions).

In a nutshell, if you’re working with lithium batteries, make sure to identify the risks and retire them as much as possible. For instance:

  • HVAC systems for lithium are not just there to support battery performance, but they are safety devices as well. Make sure they’re appropriately sized and adequate for the operating environment the batteries will be in.
  • Lithium batteries that get too hot can result in thermal runaway, and other types of hazards, aside from accelerated degradation of the cell capacities and efficiencies. Fire suppression systems are required with the appropriate cleaning agents.
  • Closely monitoring and isolating cells that are approaching their end-of-life is critical. Battery degradation not only leads to capacity loss, but also battery failure.

There are many other aspects to keep in mind, and nearly all are avoidable if you’re aware of them in the first place.

I strongly believe that lithium-ion battery systems will continue to grow and thrive in our new renewable energy world, but as the Korean article shows, there are risks. As engineers, it’s our responsibility to be aware of these risks, evaluate them, and to find the solutions that will decrease those risk and perhaps even eliminate them with new safety innovations.


July 26, 2018 0

“Where are we on the energy storage adoption curve?” That’s one of the questions a participant asked at our latest monthly Ask SepiSolar Anything “Car Talk for Solar” webinar series. It’s the only place on the web where solar professionals can ask SepiSolar’s solar+storage engineer experts anything about a particular renewable energy topic.

For July, our topic was to Ask SepiSolar Anything… about battery storage technologies. That is, about the technical aspects of different battery chemistries on the market today.

To offer a broad view than the typical Lithium-ion energy storage landscape, SepiSolar’s CEO Josh Weiner invited Matt Harper, the Chief Product Officer and Co-Founder of Avalon Battery, a Vanadium flow battery company based in Fremont, California.

Together, Josh and Matt tag-teamed on the answers, offering various perspectives and case study examples of their years of experience in developing and modeling energy storage for various applications.

From the live audience at Intersolar and on the web, we received the following questions. Jump to the topic that most interests you in the time stamp at the end of the question:

  • Overview of Energy Storage Technology Landscape 00:01
  • Matt Harper briefly describes Avalon Battery and its Vanadium flow battery technology 01:40
  • Question 1: Please give a concrete example of a project that has integrated electric vehicles with solar and storage in some kind of a project or a use case 07:00
  • Question 2: There are a lot of flow battery technologies out there. Zinc Iron, Zinc Bromide, Iron-Iron, etc. Why Vanadium? What makes it so special? 12:48
  • Question 3: If you have a grid-tied inverter already installed, how would you integrate a battery into that, perhaps aftermarket? 17:55
  • Question 4: What are the HVAC requirements for Avalon and other flow batteries? 21:27
  • Question 5: Does the efficiency level of Vanadium Batteries change at cold temperatures? 27:30
  • Question 6: I have an existing PV array that uses SMA inverters, two strings with a total of 6kW. Can it work with a flow battery? 29:20
  • Question 7: Where are we on the energy storage adoption curve? i.e., Utility vs DG projections? 45:51
  • Question 8: When will batteries—of all kinds—be cheap enough and available enough to combine with solar, wind, etc and replace a baseload natural gas plant. That is when will utilities have a choice between a baseload natural gas plant and a solar+storage plant? 47:04

Have a question about selling solar+storage for commercial projects? Then tune in next month for Ask SepiSolar Anything about Selling C&I Energy Storage. Click the button below to sign up!


June 26, 2018 3

If you’re not already installing solar+energy storage for your customers, you soon will be. But which energy storage technology is best? What’s the price per kWh? How long will the battery technology last? Are Lithium-based batteries really the best? How expensive are flow batteries? How about flywheels?

You can ask any of these questions–or anything else–and get an answer from SepiSolar’s CEO and storage expert, Josh Weiner. Also answering your questions will be our special battery technology guest, Matt Harper, Chief Product Officer of Avalon Battery.

Before Avalon, Matt was a former VP of Products and Marketing at Prudent Energy and has 9 years developing energy storage technologies. He holds 7 US patents.

In addition to being CEO of SepiSolar, Josh has been designing solar+storage systems since 2004 and was one of the early co-founders of Green Charge Networks (recently acquired by Engie.). He now consults with developers, as well as various storage manufacturers.

Over the years, Josh and Matt have separately studied many different battery technologies, and they’re both excited to be sharing their objective knowledge and opinions, answering any of your battery technology questions LIVE at the EES stage at Intersolar or on our usual web platform at the same time.

Join us for Ask SepiSolar Anything – Live from Intersolar!

Topic: Ask SepiSolar Anything about energy storage technologies. Josh will be answering questions with our special guest, Matt Harper, Chief Product Officer of Avalon Battery.

When: Thursday, July 12, at 1 pm Pacific.

Where: Sign up to get a link to watch via the web or be in the audience at Intersolar. Get all the info and a reminder here:

P.S. If you or your company are on Twitter and want to meet other solar people behind the solar brands on Twitter, RSVP for the 8th Annual Intersolar Tweetup, which @SepiSolar is sponsoring. Space is limited so get a ticket before Intersolar!

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