What actually gets cold
Air temperature is a proxy — what matters is the battery's internal temperature, which lags behind ambient air, especially inside an insulated bag or a shanty. A battery that's been sitting in a cold truck overnight is colder than the air reading on your phone suggests.
Discharge vs. charge are different problems
LiFePO4 batteries discharge reasonably well in the cold — capacity drops, but they keep working down to well below freezing. Charging is the real hazard: pushing current into a lithium cell that's below freezing can cause permanent internal damage. Lead-acid and AGM batteries tolerate a cold charge far better, which is the main practical advantage they still hold.
Weight and runtime tradeoff
Lithium's big advantage — usable capacity at a fraction of the weight — still holds in the cold, even with the derating. A 20Ah lithium battery that's lost 15% of its capacity to cold is still lighter and gives more usable runtime than a same-rated AGM battery that's lost 25%+ of its own capacity in the same conditions.
Practical takeaway
If you're hauling electronics out on a sled or ATV, lithium's weight savings usually outweigh the cold derating. The discipline that matters is charging indoors, not on the tailgate — run the numbers for your setup with the ice power calculator.