LiFePO4 batteries: how far to charge, how deep to discharge, and why temperature matters
Lithium iron phosphate (LiFePO4) batteries have become the usual choice for small solar systems: they weigh less than lead-acid, accept deep discharges and last thousands of cycles. That long life depends on respecting a few limits.
Voltages of a 12 V battery
A “12 V” battery has four cells in series (4S) at 3.2 V nominal, which makes 12.8 V. These are reference values; always check your manufacturer’s:
| State | Per cell | 4S battery |
|---|---|---|
| Full charge (absorption) | 3.45–3.60 V | 13.8–14.4 V |
| Float (optional) | 3.35–3.40 V | 13.4–13.6 V |
| Nominal | 3.20 V | 12.8 V |
| Low-voltage cut-off | 2.50–2.80 V | 10.0–11.2 V |
The voltage curve is very flat between 20 % and 90 % charge, so resting voltage says little about the state of charge. A shunt monitor, which counts the amp-hours going in and out, is far more reliable.
Depth of discharge
You can use almost all of the capacity, but every deep cycle ages the battery a little more. Using about 80 % per cycle is a good balance.
Example. A 100 Ah battery at 12.8 V stores 1.28 kWh. At 80 % depth it delivers about 1.0 kWh per cycle: enough for an efficient fridge for most of a day, or for LED lighting and a laptop over several evenings.
Temperature: the limit people forget
Discharging in the cold is fine, but charging below 0 °C plates metallic lithium on the anode and damages the cell for good. If the battery will sit somewhere cold, such as a house in the highlands, it needs a BMS with low-temperature charge cut-off or a battery with a built-in heater.
Heat matters too: above roughly 45 °C the battery ages faster. Keep it out of the sun and away from an unventilated inverter.
If it will be stored
To store it for several weeks, leave it at around 50 % charge, disconnected from every load, somewhere cool and dry. Self-discharge is low, but anything left connected, including the monitor itself, drains it slowly.
Quick checklist
- Set the solar charger to the manufacturer’s voltages, not a lead-acid profile.
- Aim to use about 80 % of the capacity per cycle.
- Never charge below 0 °C without BMS protection.
- Read the state of charge from a shunt, not from voltage.