Must Read: The best battery management when cold.

[Tacket]

I've been noodling over this issue for a couple winters now with the Leaf and now with the S as well. Cold is your friend in terms of long term life of the battery but hard on the short term capacity (reduces floating voltage/slows ionic flow across the cells). I leave the car plugged in whenever in the garage with the slider typically set to 50%. based on what I've read and my experience with quite a few other batteries, the combination of a 50% charge and cold should help extend the long term life of the battery. If I need to maximize range I time the topping up charge to end just prior to leaving, to warm the battery internally. I let the battery management do the rest. your battery will age and loose some range gradually no matter what you do, don't worry about it too much.

My practice is similar. Will be nice once/if we can schedule charges from the mobile app.

 

[tezco]

I suspect that smaller batteries are more prone to cold discharge damage since the current draw per cell is higher. Tesla goes one more step further than my Leaf by limiting current in and out via the output and reg limits that pop up when the battery is cold.

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Cotton, Thanks for the data, this is great information.
The only question I have is how you could tell from the data when the battery became warm?

It is informed supposition combined with looking at the output of many control systems in my career. If you look at the combination of the 5 half-hour cycles, you start to see what looks like an exponential approach to a final value. What would heat the cabin to 90˚, but a warm battery? If the heater control circuit is using a simple proportional control loop to heat the large mass of the battery, then that will give you an exponential approach to the final value. This looks like an exponential time constant of an hour or so. If that is the case, the temp will get to within 37% (1/e) of the final value in an hour and within 13% (1/e^2) in two hours.

In my 50˚ garage, it looks like it is approaching a steady value of about 1.25 kW. That is probably what it takes to keep the battery warm in my garage.

Also note that when the cabin temp was set to 65˚, the power draw was a lot less. If there is battery heating happening (I believe that it is), then it looks like the battery is warmed more with higher cabin set points. Also note that the cabin temp is allowed to go higher than the cabin set point. If I turn on climate control in the summer, then the MS turns on the AC to cool the cabin. In this case in my 50˚ garage, the car appears to make no attempt to cool a 90˚ cabin with the set point at 79˚.

Cabin pre-heat (or AC cooling) in the Leaf is set to 78°, regardless of the setting on the console. Perhaps the Tesla has a similar setup. I'm not sure that the current draw patterns definitively prove battery heating. Mario’s data (#10) are supportive of active termal management below freezing, though.

Perhaps the great majority (or all) of the energy is just going to the cabin. As it heats up, the cabin heater and fan speed may taper and thus the dip in current draw. Do you see a difference in Acceleration and Regen limits when you pre-heat at 50°? I certainly see limits when the battery is cold soaked near freezing, but I haven’t paid enough attention to see how warm it has to be before they don’t show. It may be that active thermal management setpoint and current draw/regen limits have the same temp setting.