What Could Go Wrong for the Tesla Car Battery at the Supercharger

A Tesla Model S electric car caught fire during charging at a Supercharger in Norway, on January 1. The driver left for a store nearby while waiting, and a few minutes later, the car started to burn. Fortunately nobody got injured. Tesla said it is “undergoing a full investigation”.

It seems like every time this happened to Tesla, it could make the news (an unparalleled treat for a car company) – Let’s count: The collide with a “large metallic object” near Seattle, the crash in Mexico, the tow hitch hit in Tennessee, the wall adapter in Irvine, the unplugged fire in Toronto and this Supercharger one. Nevertheless, electric cars are still arguably safer than conventional cars in terms of e.g. number of fires per billion miles driven.

Tesla receives such a great deal of attention (and concern) partly due to the fact that its cars represent the fast growing trend to go electric. And the new thing about the electric cars is that they use batteries.

A lithium-ion battery has a few chemicals in it, among which there are 1) some fuel that can burn – the organic solvents and 2) some oxidizer (think about the air) that the fuel can burn with – the cathode material. Yes, you are right. We are trying to complete the fire triangle. So when there is too much heat, the battery can catch fire.

Please don’t panic yet. For one thing, lithium-ion batteries have been around for 25 years. You cannot live without them, literally. Your phone, your laptop, your iPad… And the Earth is still rotating just fine.

We just need to make them behave. And they usually do under normal use and conditions. Things can go wrong however under abuse conditions, such as physical deformation (crush and penetration), external short circuit, overdischarge, overcharge and external heat. Batteries can get shorted and/or generate a lot of heat. In extreme cases, temperature can start to climb up self-sustainably, so called thermal runaway.

One more thing: lithium dendrite growth from the anode to the cathode inside the battery stack. The battery makers are pursuing higher energy density, so the car can run longer and one can talk on the phone for more hours. (Please see the Battery Status Tracker on energy density on our homepage)

One design approach is to stuff as much electrode materials in a battery as it allows. As a result, the anode can start to have trouble digesting lithium ions from the cathode, especially during fast charging. Lithium can therefore be plated on the surface of the anode. Dendrite can grow, reach the cathode side and cause internal short circuit and heat generation.

The Tesla supercharger can charge the 85 kWh battery from 10% to 80% in 40 mins. It is not that fast, considering Nissan Leaf can charge to 80% in 30 mins and Microvast LpCo battery can be fully charged in 15 mins (Please see the Battery Status Tracker on fast charging on our homepage). However, we are talking about a 120 kW charger charging a big 85 kWh battery. The process itself can generate quite some heat from the resistive heating already. (BTW, Electric cars usually have battery cooling system on board). On top of that, if some cell(s) falls into the abuse category and too much heat is generated/sustained, there is enough fuel and oxidizer to complete the fire triangle.

So, as one can imagine, it is a crucial but difficult task to manage batteries in electric cars within normal use and conditions, because they are big (hundreds and thousands of times bigger than the batteries in a phone, in terms of the energy inside among other things) and they consists of tens to thousands of smaller cells which tend to have slightly different characters (but we really need them to be as identical and as under normal use as possible).

Electric cars are doing alright, for now. Nissan Leaf and Tesla Model S both crossed 1 billion-mile mark in June 2015.

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