Tesla Energy - Utility scale

How safe are grid energy storage units? Will they catch fire as Tesla’s cars have?

We have to consider this – because, any failure of a grid connected energy storage system risks the whole project going down in flames.  Just as happened with the handful of electric car fires, an energy storage system fire could become the target of ire by certain political types and poison the adoption of these highly promising technologies.  That was the topic of discussion at a set of panels at the Intersolar US 2015 conference this week.

The panels were geared towards manufacturers who are building stuff, and what they’ll need to know to build safe stuff.  In other words, the safety regulations, and more importantly the UL standards under which to be certified.

Before we get into that, let’s back up a sec and go over the risk.  Lithium-ion battery technology is becoming good enough to perhaps replace other energy storage technologies in the next couple years.  Current preferred energy storage technology is lead-acid batteries for small scale energy storage, and bulk energy storage using compressed air tanks or pumped hydropower storage.  Some other battery technologies are on the market, like “flow” batteries where the electrolyte flows between storage tanks and the electrodes.

On electric cars there’s been a high premium on high energy density so the battery pack is small enough for a car, and to leave cargo and passenger space.  While the 300+ mile range at an affordable price target has yet to be met, doing so will mean increasing energy density while driving down the price.  Given recent carmaker announcements, the target appears to be reachable by late 2016 or perhaps late 2017.

The rapidly falling lithium-ion battery price is making them more attractive for grid energy storage projects.

That’s cool – electric car adoption driving down lithium-ion battery prices making it feasible to use lithium-ion batteries in other large scale energy storage which encourages even more lithium-ion battery production and should cause the price to fall even further.  … But …

Tesla Model S fire near Nashville TN

Tesla Model S fire near Nashville TN

What’s the risk of fire?

Lithium-ion battery packs have caught fire in electric cars and in other circumstances.  That creates a non-zero chance that lithium-ion based grid energy storage systems could also catch fire.  In fact, there have been a few fires, such as the Kahaku wind farm on Oahu which had had a string of failures and lawsuits.

It’s one thing for a car to catch fire – they’re so common that there’s maybe 70-100 per day in the U.S. with essentially no news coverage (unless it’s an electric car, in which case the coverage is blown out of proportion).   But what’ll happen if a 100 mWh energy storage system goes down causing a whole city to go dark?

In other words it’s highly desirable that grid energy storage makers work hard to ensure this doesn’t happen.

The purpose of the session at InterSolar was to raise visibility on this issue, and hopefully instruct manufacturers and installers on mitigating or eliminating the fire risk.

Two people from Underwriters Laboratories (UL) were present to explain the need for certification, the process, and which UL certifications are important.  Generally speaking certification through organizations like UL is very important.

UL’s process involves disclosure of lots design details (don’t worry, it’s all under NDA, and UL’s data protection system is extremely robust) so that UL’s engineers can do testing and potential fault analysis.  They’ll do both functional and destructive testing.

An example of destructive testing a lithium-ion energy storage system is to drive nails into one or more cells in the storage unit.  Presumably the cell will catch fire, but then what happens?  Does the fire propagate to other cells or does the product contain the fire correctly.

  • UL 9540, 1973 – battery systems and BMS
  • UL 1741 – Inverter
  • UL 991, 1998 – Software
  • IEC 60812 for guidance on fault analysis
  • IEC 61052 for fault tree analysis

Additional items they’ll check are how the BMS and the whole system handles over discharge, short circuiting, forced cell imbalance, temperature and operating parameters, and failure analysis of thermal management system (if any).  Then there are physical container tests, like dropping the system, or checking wall mount systems.  Environmental testing looks for leakage if it’s flooded, or effects of a “salt fog” (which can cause corrosion or even bridge circuit lines).

One manufacturer who was in the room complained about the tests required of their product which weren’t appropriate because they chose lithium-iron-phosphate cells.  It’s well known that different lithium-ion chemistries present different risks, and that LFP battery cells are the safest around.  Yet, this manufacturer claims the requirements they face were designed around an earlier generation of lithium-ion batteries that definitely were more dangerous.  For example the UN regulations for shipping lithium-ion batteries don’t distinguish between the chemistries, and therefore this manufacturer had to jump through extra unnecessary hoops to satisfy those regulations when their LFP systems were safe, according to that manufacturer.

The UL representatives said that, in regards to UL certification, that the testing and the requirements is adjusted based on battery chemistry.  For example if a battery system doesn’t require cooling, the standard doesn’t require it to be present.

Electrical Code

The next issue is whether the systems are installed safely, and what building and electrical codes apply.

That is, any electrical system installation must be approved by local building inspectors.  One thing the inspectors will look for is UL certification, but there are also electrical code requirements.

California’s 2013 electrical code is based on the NEC 2011 code book.  That code book simply doesn’t cover newfangled systems like grid energy storage systems.  According to the experts present, it’s the NEC 2017 code book (currently under development) which will cover these systems, and if California follows it’s typical practice that code won’t be adopted until 2021 or so.

Until then the recommended process to follow for gizmos not yet covered by the current electric code is to go to the NEC 2017 code being developed now, and refer to the appropriate sections.  The local inspectors will be able to write up a document giving approval based on the future code.


The technology is moving faster than electrical code and other standards bodies can keep up.   Therefore at the current time pretty much any energy storage product installation should be seen as an experiment.  That’s because the standards and best practices haven’t been written yet for these products.

What are the risk factors for lithium-ion battery systems?  How are those risks mitigated on a stationary system?  (the mitigation thoughts are primarily mine – some of this was discussed in the meeting)

  • Intrusion, puncture:  Make sure the enclosure is strong, and then protect the enclosure further with barriers.  The internal design can minimize the spread of fire if a fire does start, for example by channeling heat/fire away from the pack.  The Tesla Model S was designed to channel fire out the front of the car, hence the picture above shows the front completely destroyed but the passenger compartment stayed intact.
  • Flooding: Ensure the enclosures and cabling are water tight.  Siting is also important – don’t install this stuff where it’ll be flooded.  Remember that the Fukushima nuclear disaster occurred because critical equipment got flooded and failed.
  • Over charging, under charging:  The BMS and other systems are to protect against this, and also keep the pack balanced.
  • Discharge rate:  Size the system so it can supply the required power level without damage to the battery pack.
  • Ambient heat:  A canopy over the energy storage system can provide much-needed shade.  Some manufacturers will design in a cooling system, or design in enough insulation so the units can bake in the sun without damaging internal components.  Some battery technologies are less affected by ambient heat.
  • External fire:  Ensure the battery system is physically separated from other things which could catch fire.   You’ll notice that Tesla’s image above suggest separating each unit by a few feet, perhaps to limit propagation if one catches fire.  There’s some external fire conditions which simply cannot be avoided.

Firefighters will sometimes let something burn itself out if

  • The fire doesn’t present risk to others, or other buildings
  • The fire is too intense or otherwise the firefighters don’t have correct equipment – for example, typical firefighter gear doesn’t protect against electric shock
  • Fighting the fire would present a risk to firefighters lives
  • The equipment on fire is already a loss

About David Herron

David Herron is a writer and software engineer living in Silicon Valley. He primarily writes about electric vehicles, clean energy systems, climate change, peak oil and related issues. When not writing he indulges in software projects and is sometimes employed as a software engineer. David has written for sites like PlugInCars and TorqueNews, and worked for companies like Sun Microsystems and Yahoo.
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About David Herron

David Herron is a writer and software engineer living in Silicon Valley. He primarily writes about electric vehicles, clean energy systems, climate change, peak oil and related issues. When not writing he indulges in software projects and is sometimes employed as a software engineer. David has written for sites like PlugInCars and TorqueNews, and worked for companies like Sun Microsystems and Yahoo.

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