Float charging is the final stage of lead-acid battery charging — a reduced voltage applied indefinitely to maintain a full charge without overcharging. It is essential for standby and backup battery applications.
>The Three Stages of Lead-Acid Charging
- Bulk/Absorb: Maximum constant current until voltage reaches absorb level. Battery recovers 80% of capacity rapidly.
- Absorb (topping): Constant voltage, current tapers. Fills last 20% of capacity. Critical for full charge.
- Float: Reduced voltage maintains 100% SOC indefinitely. Compensates for self-discharge.
Correct Float Voltage Settings
- 12V flooded: 13.5-13.8V
- 12V AGM: 13.5-13.8V
- 12V GEL: 13.5-13.6V
- 12V OPzV: 13.5-13.8V (verify with spec sheet)
- 2V cell: 2.25-2.30V per cell
Always verify with your specific battery manufacturer’s specification sheet.
Effects of Wrong Float Voltage
Too high: Accelerated grid corrosion, water loss, gassing, reduced life. Signs: excessive topping-up requirement, warm battery surface.
Too low: Gradual sulfation from chronic undercharging. Signs: battery never reaching full voltage, progressive capacity loss.
Float vs Equalize
- Float: Continuous maintenance charge. Always on.
- Equalize: Periodic controlled overcharge (for flooded batteries sole). Typically monthly. Corrects cell imbalances.