• LiFePO4 cannot be charged when the cells are below ~freezing.
• some kind of warming is required before charging
  • self-heated: easier
  • DIY: more control, more charging under certain conditions, usually cheaper
• in practice, warming is combined with other forms of protection: BMS low temp charging shutoff, solar or DC-DC low temp charging cutoff, etc.¹⁾

Self-heated batteries

1. sense ~freezing temps
2. shut off charging²⁾
3. allow incoming power only to the internal heaters, typically ~60w
4. when cell temps are >freezing charging is re-enabled
5. optional: heating may continue until some warmer setpoint is reached, typically 40F-50F when this feature is utilized

This means warming only happens when charging is present. This may or may not be what you want.

DIY warming solutions

1. use an external temperature controller and temperature probe
2. use the lowest-wattage warming mat practical for conditions³⁾
3. and turn the mat on/off with the controller

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. But in some specific use cases one approach might work better.

When you want a packaged solution.

If you run your bank very low habitually⁴⁾ and the warming load is enough to trigger BMS discharge shutdown. This problem does not occur with self-heated because heating only runs when charging is present.

With robust charging (alternator and/or bigger solar) there can be charging opportunities lost with self-heated batteries in freezing conditions.

Imagine the following scenario

• 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.

You may want more control over the temperature setpoints.

Note from Secessus: “since reading this cell spec sheet data I keep my bank at toasty 50deg F.”

There is no one-size-fits-all solution but outside cost considerations, the essential questions are

• how much charging would I miss out on because the bank is taking only 60w to run the heaters instead of taking everything I can throw at it?; and
• how does this compare to the Wh required to keep the bank above freezing when no charging is present?
• do I need greater configurability than the built-in heater allows?
• do I have the interest or ability to DIY?

If your preferred battery has everything but heating you might want to DIY a heater. It is typically much cheaper than built-in heat and offers more control. It typically involves:

1. a low-wattage warming pad, as one finds for reptile pets, animal comfort pads, etc. RV tank heating pads will work but the wattage is much higher (60w?) and might result in less even heating or overheating.
2. a temperature controller

Both pads and controllers are available in 12vdc and 120vac.

Multiple solutions can be combined into a more failsafe setup.

It may be possible to add low temperature cutoff to a battery whose BMS lacks that feature. It requires the charging source that can be disabled on demand. In this approach the we are reversing the logic so that the default state of the charging source is disabled and it is only enabled when battery temps are warm enough to safely charge.

For belt-and-suspenders you could add a $10 12v temp controller or NO thermal switch in series [with the disabling method]. Stick the probe (or switch) on the bank and only complete the [disabling ] circuit when the ignition is on and measured temp >0C or >2C or whatever you prefer.

• DIY Solar Forum thread on DIY warming