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Tesla Motors replaces Model S charging adapters to mitigate fire risk

Tesla Motors announced on Friday that the company is replacing Tesla Model S Universal Mobile Connector’s to mitigate the fire risk in case of overheating. The move follows a garage fire in November involving a Tesla Model S, several reports on the Tesla Motors Club forum of overheating charging connectors, and an over-the-air software fix that adjusts the charging rate if the Model S detects power fluctuations.

The UMC lets Model S owners charge at any power outlet, just by swapping the adapter.
The UMC lets Model S owners charge at any power outlet, just by swapping the adapter.Tesla Motors

The Universal Mobile Connector (UMC) lets a Model S owner plug into nearly any power outlet using a simple adapter. This means Model S owners have the flexibility to charge nearly anywhere they go. Unlike the 120 volt charging cord supplied with other electric cars, the UMC supports charging at high rates depending on the power outlet. For example it includes a NEMA 14-50 adapter, which is an outlet normally used for clothes dryers, and supports 240 volts 50 amps or over 10 kilowatts charge rate. A Model S owner who wants to skip the installation cost for the HIgh Power Wall Connector, could instead opt for the cheaper NEMA 14-50 outlet that may already be in their garage.

The problem is if the power outlet becomes what’s called a “high resistance connection”. Tesla said a “variety of factors such as corrosion, physical damage to receptacles, or inappropriate wiring or installation of electrical outlets can cause higher than normal electrical resistance” and that when charging “higher than normal electrical resistance connections to external energy sources may cause excessive heating of the adapter.”

In November, a garage fire in Irvine California occurred due to a high resistance connection, according to an incident report from the Orange County Fire Authority. At the time Tesla issued this statement:

"Based on our inspection of the site, the car and the logs, we know that this was absolutely not the car, the battery or the charge electronics. There was a fire at the wall socket where the Model S was plugged in, but the car itself was not part of the fire. The cable was fine on the vehicle side; the damage was on the wall side. Our inspection of the car and the battery made clear that neither were the source and were in fact functioning normally after the incident. In addition, a review of the car’s logs showed that the battery had been charging normally, and there were no fluctuations in temperature or malfunctions within the battery or the charge electronics."

Since then the company sent a software update to Model S’s that automatically reduces the charging current by 25% when the car detects unexpected power fluctuations on the charging circuit. By reducing the charging current it will automatically reduce the heat generated and remove the chance of fire, according to Tesla Motors. This requires version 5.8.4 of the Model S software, and Model S owners can verify the software version via the dashboard display.

Today’s announcement take the Model S fire risk mitigation a step further. Tesla’s engineers have designed a new UMC adapter for NEMA 14-50 outlets that contains a thermal fuse. In the case that the circuit breaker on house wiring doesn’t trip, Tesla says “the thermal fuse will prevent current from flowing if the wall socket region heats up for any reason.”

Given the number of melted NEMA 14-50 adapters on the Tesla Motors Club forum, this appears to be a good solution. Tesla Motors is under investigation by the NHTSA for fire related issues, and has informed the NHTSA of this proactive measure.

Source: Tesla Motors