'Hanging by a thread': Leaked emails tell the story of a design flaw discovered in 2012 in Tesla's Model S battery that could lead to breakdowns and firesLinette Lopez

Flickr/ Maurizio Pesce


Leaked emails from 2012 reveal that Tesla knew its Model S battery had a design flaw that could lead to break downs and fires, but it sold the cars anyway. It's unclear when the design flaw was fixed.

This revelation comes as Tesla is dealing with customer complaints of manufacturing defects in its new Model Y crossover vehicles, including loose seatbelts and back seats.

The National Highway Transportation Safety Administration also just initiated a probe into faulty Tesla Model S touch screens made from 2012-2015. The investigation covers 63,000 vehicles.

Tesla did not respond to Business Insider's request for comment.

As Tesla's first car, the Model S, was rolling out of the company's Fremont, California factory, e-mails reveal that the company was still grappling with a design flaw that could lead to leaks inside the battery.

Those leaks could then cause the car's battery to short, or leave behind a flammable residue inside the battery, according to experts BI spoke to for this investigation.

The problem was a poorly designed cooling mechanism — internally referred to as a cooling coil or bandolier — which wraps around the battery pushing coolant around it to regulate temperature. According to internal emails and three people familiar with the matter, the end fitting of the cooling coil was made from weak aluminum and tiny pinholes sometimes formed where the male and female parts of the end fitting were supposed to braze together.

The part was tested by third party companies twice. First, a few cooling coils were sent to a test lab called IMR Test Labs in upstate New York in July of 2012. According to the IMR report, which was reviewed by Business Insider, the end fittings on the cooling coils did not meet chemical requirements for a regulation strength aluminum alloy. A source close to the matter said the results were shared with Tesla, but the Model S cars kept rolling out of the factory. According to Tesla's 2012 third-quarter earnings report, the company delivered more than 250 Model S sedans.


Jason Schug, a Vice President at Ricardo Strategic Consulting, has done teardowns of Tesla's Model S and X vehicles, which share the same battery. He told Business Insider that if coolant leaked into a battery module it could render the battery useless.

"When we disassembled the Tesla Model X, a technician accidentally spilled coolant in the battery pack and it sat there for a long time," Schug told Business Insider. "There was no immediate danger, but when we removed the battery modules quite a while later we found a lot of corrosion on the battery cells and it was bad enough that some of the cells were leaking electrolyte. If this were to happen in the field and go unnoticed, it could result in bricking the battery."

"Bricking the battery" means that the battery would go dead.

If you work at Tesla and want to share your experience, or own a Tesla and want to talk about it email me at llopez@businessinsider.com.

Find the problem now, fix it later

In August 2012, the part was tested again. Tesla sent the part to Exponent, an engineering and scientific consulting firm. According to internal emails reviewed by Business Insider, Tesla was concerned because the end fittings on the cooling coils were just not staying together and as such were a source of leakage. One Tesla employee described them as "hanging by a thread" in August 2012, according to internal emails viewed by Business Insider.

The engineer who handled this at Exponent was a man named Scott Lieberman. He is now at LPI Inc. Lieberman declined to speak with Business Insider about this story. In internal emails between him and Tesla viewed by Business Insider, however, his opinion of the part was clear. He found defects — specifically, tiny pinholes which could cause leaks — on the tested materials after limited testing.

Tesla continued to find leaking coils in various stages of production through the end of 2012, according to documents reviewed by Business Insider. Some were found late enough on the production line to be described as a "critical quality issue," or were found to have leaked liquid into the battery pack, according to internal emails sent in October 2012, which were viewed by Business Insider. At this point the problem had been flagged for senior management, documents indicate.

In another email sent in late September 2012, employees said that workers on the production line sometimes had to use a hammer to get the end fittings to stay together. Tesla continued to find leaking coils on the production line through November 2012, according to emails, and eventually senior management was informed of the problem. It's unclear when the company changed the part's design, though. Tesla did not respond to Business Insider's request for comment.


A former employee, who left the company in 2014, said that employees sometimes forced end fittings together as Tesla rushed to make production goals.

"We did find [leaks in] a few vehicles," the former employee said. "I don't know exactly how many, but again that is what I would consider normal for a company that decides to launch with a limited amount of R&D in the hopes of: 'We will launch with this, and we will put inspections in place that we catch it [leaks] at the plant.'"

Check out BI's full investigation into the Model S battery design issues.

If you work at Tesla and want to share your experience, or own a Tesla and want to talk about it email me at llopez@businessinsider.com.

Mark Matousek also contributed to the reporting of this story.

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