Comment by refurb | Hacker News Reader

Comment by refurb | Hacker News Reader

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The reviews from Tesla owners who have their own experience with the car in cold weather is interesting.

Can anyone tell me why the car discharges when plugged into a 120V supply? It's mentioned in one of the links above and in the NYT article.

I'm aware that you can't supply much current with a standard 120V supply, but why would it discharge? I would imagine at a minimum you'd have a very slow charge rate, but from these two anecdotes, it appears in cold weather it still slowly loses charge when plugged into 120V.

Is the car constantly heating the battery? I could see that being the issue as engine block heaters draw a ton of current and they take a long time to heat up an engine block. I'm assuming the battery has a lot more thermal mass than a 300 lb engine block.

It's not discharging because it's plugged into 120V. It's discharging faster from the cold than the 120V is charging, so there's a net loss.

I thought the cold caused batteries to slow down on discharge rate. The LiPo batteries I use for RC planes keep a charge much longer if they are refrigerated. They have a horrible discharge rate when cold though.

I speculate that it is a interaction of several things:

1) The current firmware assumes there is "unlimited" energy available when charging, so it can keep the battery warm with little impact on charging rate (this is not a bad assumption for everything other than 120V/15A charging).

2) The current firmware assumes that the battery needs to be kept warm when charging (this is not a bad assumption for everything other than 120V/15A charging, but may not be correct for low-power charging).

3) The power required to keep the battery warm in near-zero (Fahrenheit) temperatures is nearly the same as the maximum continuous power available from a 120V/15A outlet (1.44kW, because the maximum continuous load is only 80% of the maximum load), at least initially.

The firmware probably needs to be tweaked for cold-weather, low-power charging. The firmware needs to know how to estimate state of charge when the battery is colder than normal. It could probably prioritize charging ahead of heating until the battery reaches a higher state of charge.

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