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DRAFT
Self-heated batteries
This means warming only happens when charging is present. This may or may not be what you want.
DIY warming solutions
This means warming happens whether or not charging is present. This may or may not be what you want.
With “soft” charging like a small solar setup it's a wash. a matter of preference and cost.
In some specific use cases one approach might be better-suited than the other.
With robust charging (alternator and/or bigger solar) there can be charging opportunities lost with self-heated batteries in freezing conditions.
Imagine
* freezing overnights followed by a half-hour drive in the morning with a 30A DC-DC. * a battery with warming pad that consumed 60Wh overnight to stay above freezing, vs * a self-heating battery that is below freezing in the morning
The externally warmed bank accepts current immediately and takes 15Ah of recharge. So after recouping the 60Wh used overnight it's net +132Wh.
The self-heated battery shunts 60w to the heater (leaving 10A of charging unused) until the warmth equalizes up to the temp sensors. Let's say this takes 20 minutes. So now we have only 10mins of actual charging and the bank gets 5Ah of recharge, net +64Wh.
With 50A charging it's 260Wh net vs 107Wh net. .
If one was charging by alternator only and only driving to/from work while stealthing in freezing weather it's conceivable the self-heated batt would get *never get charged at all* because it wouldn't have time to get out of the heating phase.
There is no one-size-fits-all solution but outside cost considerations, the essential questions are