APS Details Cause of Battery Fire and Explosion, Proposes Safety Fixes

Utility Arizona Public Service has completed its exhaustive study of the most high-profile U.S. grid battery fire.

The company filed its report[1] Monday with the Arizona Corporation Commission, which regulates the utility. The report, produced in collaboration with DNV GL, lays out new safety requirements to prevent dangerous failures at current and future battery installations.

APS planned to massively increase its battery fleet to store solar power for use in the evenings, but put the buildout on hold after the setback last spring. A lithium-ion battery container near Phoenix caught fire in April 2019, and after first responders opened the door, it exploded, sending several of them to the hospital.

The fire ignited in just one of the 27 racks of batteries in the McMicken facility, and did not spread, GTM previously reported[2]. That suggests the layout of the racks effectively isolated them from each other. But the aerosol fire suppressant was powerless to stop a buildup of explosive gas, which combusted when the first responders opened the door and let oxygen into the container.

The explosion revealed that lithium-ion batteries can be dangerous, even in the hands of experienced professionals like APS, storage vendor Fluence, and battery manufacturer LG Chem. The new report offers a chance to identify what went wrong and ensure that future constructions eliminate the failure pathways, said Scott Bordenkircher, director of technology integration and innovation at APS.

“Conversations and learnings around an event like this are hypercritical, because that’s how you get the information out that needs to be considered in the next generation,” he said in a Monday interview.

Immediate changes

The findings include two major categories of recommendation: physical changes to battery system design to eliminate the gaseous buildup scenario, and changes to how utilities train first responders to deal with emergencies at battery plants.

The report breaks down the event into five crucial steps:

  1. It appears that a single lithium-ion cell developed dendrites, or accretions of material that grew from one electrode to the other, which caused a short circuit. This caused the cell to heat up and catch fire.
  2. As that fire spread to neighboring cells, the aerosol fire suppressant proved incapable of stopping such a powerful conflagration.
  3. The placement of pouch batteries in close proximity within modules meant that the fire spread quickly from cell to cell. There was no physical barrier to prevent that spread, although the layout of the facility prevented spread beyond the vertical rack that caught fire.
  4. As hundreds of cells burned up, they released explosive gas, which built up in the container without a means of escape. The container lacked sensors to count this gaseous accumulation.
  5. The lack of sensors or remote viewing limited operational awareness for the first responders, who also had limited training on this type of scenario.

Dendrites are a well-known failure mode for batteries, but it was unexpected that they would appear on a battery that had only operated for about two years.

“Over a long period of time it may be kind of expected, but nowhere near the severity we saw in this short period of time at this system,” Bordenkircher said.

As this event makes clear, even top tier integrators buying batteries from top tier suppliers can end up with flawed cells.

“Our focus is on when it fails, how do you mitigate it,” Bordenkircher said.

To do so, APS will require remote sensing and ventilation, so that if a malfunction ever leads to releasing dangerous gases, the operators can identify them and flush them from an enclosure without exposing any people to risk. This has not been standard practice in the storage industry, although New York City authorities required it for systems installed there.

Other upgrades include cooling systems or barriers between cells to prevent fires from spreading, and inclusion of more intensive fire suppression in case a gaseous clean agent fails to stop a fire.

These steps add materials and labor to a battery project, so they will likely increase costs. But there’s no way around that, Bordenkircher said.

“You’re not going to sell it, regardless of cost, unless it’s safe,” he noted. “The communities and the regulators are going to mandate it, and it’s the right thing to do.”

On the training side, APS will conduct broader regional education around fire response at its battery sites. It had trained local responders in the city where a battery was located, but the hazardous materials team that arrived at McMicken drew personnel from the surrounding area. Battery response training needs to cover anyone who could arrive at a battery fire, and the trainings should be followed with regular refresher briefings, Bordenkircher said.

But the earlier emergency response plans did not contemplate an explosive scenario, which left the responders unprepared for what happened when they decided to open the door.

Actions to restart battery construction

APS shut down its other battery facilities from Fluence—one at Festival Ranch, which is a twin of the McMicken system, and a larger one at the desert community of Punkin Center. Before those reopen, they will need to be retrofitted with ventilation systems.

Going forward, new battery plants will have to meet these requirements as well. When McMicken happened, APS was about to finalize the first contracts in a planned 850 megawatt battery buildout[3] to pair the utility’s large-scale solar fleet with batteries.

Those plans have been on hold since then, but APS maintained that it remains committed to battery technology. The utility is already working with the contract winners to see if they can update their projects to meet the new safety standards emerging from the investigation. Those standards will be baked into new requests for proposals going forward. 

APS only has the power to enforce these standards in its own territory, but Bordenkircher implored people elsewhere to consider how the McMicken failure scenario would play out in their own battery plants.

“There are many other containerized batteries installed across the world,” he said. “I would sure hope that everyone will take a look at the system they own and operate and do a risk assessment based on these findings.”

The report is accessible on APS’ website[4], and the company has planned outreach to several industry groups in the next few days.

In January, the company committed to eliminating carbon emissions[5] from power production. The ability to store power in batteries will play a crucial role in delivering evening peak power as decarbonization proceeds.

“You can’t get there with intermittent reources that only generate during daylight,” Bordenkircher said. “We know [storage] is the answer to how you get to a clean energy commitment like that.”

In other words, the rollout of the battery fleet has been delayed by the fire and its aftermath, but the endpoint has not changed.


  1. ^ report (www.aps.com)
  2. ^ reported (www.greentechmedia.com)
  3. ^ 850 megawatt battery buildout (www.greentechmedia.com)
  4. ^ APS’ website (www.aps.com)
  5. ^ eliminating carbon emissions (www.greentechmedia.com)

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