My solar battery won't charge to 100%: causes and fixes

Your battery stops at 80-90% and never finishes charging to 100%. One of the most common problems, and almost always has a cheap fix. Here are the 5 causes by probability.

1. Cell imbalance (LiFePO4)

One of the 8 cells (24V) or 16 cells (48V) charges before the others. BMS cuts the whole battery when that cell hits max, leaving the rest at 80-85%. Fix: external balancer (Heltec, Neey) for 3-5 days.

2. Misconfigured absorption voltage

Controller doesn't reach correct voltage. 24V LiFePO4 needs 28.4 V to top off. Set to 27.6 V (typical for lead-acid), you stay at 90% always. Check MPPT menu and set the real chemistry.

3. Sulfation (lead-acid)

AGM or flooded lead left discharged too long: sulfate crystals coat plates. Usable capacity drops 30-50%. Fix: equalization cycle at higher voltage (15.5 V per cell) for 2-4 hours. If no luck, replace.

4. Bad temperature sensor

Some controllers compensate voltage for temperature. If sensor reads wrong (loose wire, damaged probe), wrong compensation applied. Verify charge voltage is correct at room temp.

5. Insufficient absorption time

Controller hits voltage but cuts off too early. Increase absorption time to 2-4 hours in menu. Especially useful in LiFePO4 with passive BMS (needs time to balance).

When to replace

If after ruling out the above the battery still caps at 80%, it has lost real usable capacity. LiFePO4 6+ years heavy use or lead 4+ years: natural lifespan ended. Replace.

On BMS read LiFePO4 BMS explained.