Float charging holds a full lead-acid battery at a voltage low enough that it gasses little, so the small current that flows only replaces self-discharge. The corpus’s float figures run from 2.15 to 2.30 V per cell, 12.9 to 13.8 V for a 12 V battery, and they differ with battery type and temperature, so find the right value for your cells by watching specific gravity, water use and float current over several weeks.
Part of Battery Building and Maintenance
Holding a charged lead-acid battery full without overcharging it, for standby loads and as the end stage of a solar or generator charge.
Float Is The Voltage At Which Charging Just Balances Self-Discharge
A fully charged battery slowly loses charge by itself; the rechargeable-batteries article gives lead-acid self-discharge as 3 to 5% a month. A float voltage supplies exactly that loss and no more, which the carrier describes as maintaining full charge “without overcharging”: current drops to near zero when the battery is full. Too high a float gasses the electrolyte and corrodes the positive plates; too low lets the battery slowly sulfate (Lead-Acid Chemistry).
The corpus quotes several floats for a 12 V battery:
| Source | Float |
|---|---|
| Carrier | 2.25 V per cell, 13.5 V |
| Charging Systems | 13.2–13.8 V |
| Charge Controller (phase 4), flooded | 13.2–13.6 V |
| Same, AGM | 13.2–13.4 V |
| Same, gel | 13.0–13.2 V |
| Rechargeable Batteries | 2.15–2.20 V per cell (12.9–13.2 V); trickle 2.17 V per cell, 13.0 V |
Per cell, 13.2 ÷ 6 = 2.20 V and 13.8 ÷ 6 = 2.30 V, and 13.5 ÷ 6 = 2.25 V, so the full spread of the corpus is about 2.15 to 2.30 V per cell. The Charge Controller article advises conservative values when the battery type is unknown, because undercharging only loses capacity temporarily where overcharging destroys the battery.
What You Need
- A regulated source at the float voltage — a controller (Charge Controllers) or a regulated supply
- A voltmeter and a milliammeter — Metering
- A hydrometer — Specific Gravity Control
- A thermometer — for temperature compensation
- A log book — weekly records
Method: Find A Float Voltage For Your Cells
Step 1: Charge fully at the absorption ceiling until specific gravity stops rising (Charging Circuits).
Step 2: Start at the conservative end, about 2.20 V per cell, 13.2 V for a 12 V battery.
Step 3: Record weekly: float current, specific gravity of each cell, cell voltages, water added and temperature.
Step 4: Adjust in small steps. If specific gravity creeps below its full-charge reading or cell voltages diverge, the float is too low; raise it a little. If water use is high or the battery warms, it is too high; lower it.
Step 5: Compensate for temperature. The Charge Controller article gives −5 mV per °C per cell against a 20°C reference: warmer batteries need a lower float, colder ones a higher.
Worked Example: The Current And The Temperature Shift
For a 100 Ah battery with 3 to 5% self-discharge per month: 3% of 100 Ah is 3 Ah and 5% is 5 Ah. A month is 30 × 24 = 720 hours, so the average float current that replaces the loss is 3 ÷ 720 = 0.0042 A to 5 ÷ 720 = 0.0069 A, about 4 to 7 mA. A float current of amperes means the voltage is far too high or the battery has an internal fault.
Temperature: six cells at −5 mV per °C each, over a 10°C rise, change the setting by 6 × 0.005 × 10 = 0.30 V. A 13.5 V float at 20°C becomes 13.5 − 0.3 = 13.2 V at 30°C, and the Charge Controller article’s table shows the same 0.3 V shift for its absorption voltage between 20°C and 30°C.
Common Mistakes
- Floating a solar-cycled battery as if it were standby — a battery that discharges every night is not floating; it cycles
- Copying a gel-cell float into a flooded cell or the reverse — the table above shows they differ
- Ignoring temperature — a battery in a hot room floated at a cold-room voltage is slowly boiled
- Judging by voltage alone — specific gravity and water use say more
Safety
A battery on float still makes some hydrogen
A float that is a little too high gasses continuously and invisibly, and the hydrogen collects at the top of any closed box or ceiling space, where a relay click or a loose terminal can ignite it in air between 4% and 75%. Keep float-charged banks in a space ventilated at the top, not in a sealed cupboard, and check the water level often (Hydrogen Gas Hazard).
What’s Next
- Learn next: Equalization Charging — the periodic overcharge that complements float
- Combine with: Shunt Regulator Design — a simple way to hold a float from a solar array