Check your BMS spec this week

One rule, and getting it wrong permanently damages an expensive battery: do not charge LiFePO₄ below freezing unless it has a built-in heater and explicitly permits it.

Why: charging a cold lithium cell causes metallic lithium to plate on the anode instead of intercalating properly. That plating is permanent — it reduces capacity forever and, in bad cases, creates internal structures that are genuinely dangerous.

The asymmetry: discharging in the cold is generally fine. It's specifically charging that's destructive. So the danger window is a cold sunny morning when your panels wake up and push current into a frozen bank.

Three real solutions: 1. Self-heating batteries — draw a little power to warm cells before accepting charge 2. Keep the bank in heated space — inside the insulated envelope, not an exterior bay 3. A BMS with low-temp charge cutoff — blocks it automatically, which turns a rule you must remember into a rule the system enforces