Frosty Tesla model S with charging plugged in

Update December 22- the software now notifies you when your battery is heating up before charging! This is very useful since the charge will take longer.

Say you’ve not used the car much over the weekend, it’s not fully charged and you need to make a surprise trip on Monday morning. It’s mild weather and you turn up at a public high power charger a few miles from home, and find the car charges at 20kW. Eh? It should manage at least 100kW! Alternatively, maybe it’s a frosty morning and you plug the car in, only to find that it’s not charging after a few minutes. Maybe there’s something else going on that the car isn’t telling you about…

In general, EV thermal management isn’t something that most people need to be concerned about. However very cold weather can bring surprises and unexpected behaviour from your EV if you’re unaware of the issues involved. The issue is sometimes referred to as “coldgate” and results in low,, or even no, charging at low temperatures.

An EV battery might weigh a few hundred kg, so this will of course take some time and energy to heat up to optimum temperature when it’s cold outside. I’m based in the south of the UK where we get sub-zero temperatures for only a few days at a time in winter. Generally winter temperatures are between 5 and 10C. Occasionally it will snow, but not every year.

Frosty bonnet of Tesla model S

One way that the driver might be aware of a cold battery is the available regen power, since regen is the same as charging the battery. With a cold battery, the Model S has essentially no regen available. As the battery warms it is gradually made available, which can take 40 minutes in average winter weather on a motorway. Here the battery might be actively heated, and will warm due to normal use, depending on the driving style. In very low temperatures and very low states of charge, the max power available will be limited as well.

Logging some data (using an OBD adapter and the “Scan My Tesla” app) revealed some interesting stuff. This is the Max Charge rate available at different cell temperatures. This relates to the regen as well as charging (since it’s effectively the same thing). Note that near zero C, the allowable charge rate is zero. More on this below.

Charging at 10A from cold (1.6 degC):

There are a few distinct phases here.

Initially the allowed charge rate is zero.

Plugging in the 10A “granny charger”, with a nominal 2.4kW available, the battery heater fires up (first peak in the yellow coolant heater outlet curve). Some power is also drawn from the battery (the state of charge dips and some battery current is drawn)

After a few minutes, all of the charger current is used to power heating. This is the second peak in the yellow curve. Still there’s no charging going on. This stops when the battery warms to 6C.

Finally after 20 minutes being plugged in, the coolant heater stops and the battery starts charging at 1.8kW. A few hundred watts is being used for other stuff; computer, pumps etc. I suspect if charging continued for hours in these conditions, the heater might be needed again as cell temperature is gradually falling.

The final peak is related to unplugging and starting the cabin heater, although the car went nowhere, the battery heater and cabin with ancillaries draws 7.8kW.

Throughout, the maximum charge rate allowed is less than 10kW. You can look at this two ways; Tesla have a very conservative setup and are looking after the battery well. Alternatively; Tesla chose the wrong battery type for cold conditions. It’s the right battery type for high power use however, and thermal management allows for the best of both; power delivery and use in any weathers.

In practise, all but the newbie Tesla driver would use the app to pre-condition the battery, and warm the cabin at the same time. I think it’s useful to realise that the battery takes some time to heat up. It would also be useful if Tesla displayed the battery temperature somewhere!

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