Don't charge lithium below freezing

One rule, and getting it wrong destroys the most expensive component you own.

The asymmetry people miss: discharging in the cold is generally fine. You can run loads all night at 20°F without harm. It's specifically charging that's destructive. So the danger window is a cold sunny morning when your panels wake up and start pushing current into a frozen bank.

Three real solutions: 1. Self-heating batteries — draw a little power to warm cells before accepting charge. Worth the premium in genuine winter. 2. Keep the bank in heated space — inside the insulated envelope rather than an exterior bay. Free if you design for it. 3. A BMS with low-temperature charge cutoff — blocks charging below the threshold automatically. Most quality LiFePO₄ has this. Verify yours does — it turns a rule you must remember into a rule the system enforces.

Other cold realities: capacity drops in cold (it returns as things warm). Solar production collapses — short days, low angle, snow. But panels are more efficient cold, so a clear cold day can outproduce a hot one.