A lead-acid charger must push current into the battery at a limited rate, hold the voltage at a set ceiling until the current tapers off, then drop to a lower voltage that only replaces self-discharge. The sources in this wiki put the ceiling for a 12 V battery between 14.4 and 14.8 V and the holding voltage between 13.2 and 13.8 V, so find the right values for your own cells by measurement rather than copying one figure.
Part of Battery Building and Maintenance
How a charging circuit takes current from a generator or a solar array and returns it to a lead-acid battery without overcharging it.
A Charger Needs A Current Limit, A Voltage Ceiling And A Way To Stop
Charging means pushing current backward through the cell, so the supply voltage must be higher than the battery’s. Rechargeable Batteries puts it as 10 to 20% above battery voltage, and a current of C/10 (one tenth of the amp-hour rating each hour) as a safe rate. Three stages follow (Charging Systems):
- Bulk: constant current, voltage rising as the cell fills.
- Absorption: voltage held at the ceiling while the current tapers. Charging is finished when the current falls to C/50, which for a 100 Ah battery is 2 A.
- Float: a lower voltage that holds the battery full without driving it into gassing.
A battery that is never held at a ceiling stays undercharged; one held there too long boils away its water and corrodes its positive plates.
The Voltage Figures Disagree, So They Must Be Found
For a 12 V battery of six cells, the corpus gives these:
| Source | Charge ceiling | Float |
|---|---|---|
| Carrier | 2.40 V per cell (14.4 V) | 2.25 V per cell (13.5 V) |
| Charging Systems | 14.4–14.8 V | 13.2–13.8 V |
| Rechargeable Batteries | 2.3–2.5 V per cell, never above 2.5 | 2.15–2.20 V per cell |
| Charge Controller (phase 4) | 14.4–14.8 V flooded | 13.2–13.6 V flooded |
Dividing by six cells: 14.4 ÷ 6 = 2.40 V and 14.8 ÷ 6 = 2.47 V per cell for the ceiling; 13.2 ÷ 6 = 2.20 V and 13.8 ÷ 6 = 2.30 V per cell for float. The Charge Controller article also gives lower ceilings for sealed gel and AGM batteries, so a flooded-cell figure is not safe for those. Home-built cells have no maker’s data. The method is to start low, charge, and watch: the voltage at which a cell begins to gas steadily, how fast the water level falls, and whether specific gravity reaches its full-charge reading (Specific Gravity Control). Raise the ceiling only if the cell stays undercharged; lower it if it uses water fast or runs hot. The corpus gives a temperature correction of −5 mV per °C per cell (Charge Controller).
What You Need
- A DC source — a generator, rectifier or solar array
- A current limiter — resistor or regulator (Constant Current Charging)
- A blocking diode — to stop the battery discharging back through the source at night or when the generator stops
- A voltmeter and ammeter — Metering
- A fuse in the line — Fuse Design
- A hydrometer — to confirm charge
Method: A First Charge
Step 1: Check polarity with the meter before connecting (Polarity Check); reversed DC destroys diodes and controllers immediately.
Step 2: Set the current to no more than C/10.
Step 3: Watch the voltage and the cell temperature every 30 to 60 minutes (Charging Systems). A battery heating past 50°C is being overcharged: disconnect.
Step 4: Hold at the ceiling until the current tapers to C/50 and the specific gravity stops rising.
Step 5: Drop to float or disconnect.
Worked Example: Charge Time For A Discharged Battery
A 100 Ah battery that is half empty needs about 50 Ah back. At C/10, the current is 100 ÷ 10 = 10 A, so the bulk phase takes about 50 ÷ 10 = 5 hours if the full 10 A is accepted throughout, which it is not. The tapering absorption stage adds time and the battery needs more ampere-hours than it gave out, because some of the current goes into gas. The corpus gives no efficiency figure; treat 5 hours as the shortest possible. At C/50 the finishing current is 100 ÷ 50 = 2 A.
Common Mistakes
- No current limit — a flat battery draws as much as the source can supply and heats
- A ceiling that is too high — boils the electrolyte and warps plates (Charging Systems)
- No blocking diode — the battery drains through a stopped generator or an unlit panel
- Charging in a closed room — see the safety note
Safety
Charging makes hydrogen, and the bank can deliver a destructive short current
Gassing starts once the plates are full, and the hydrogen is explosive in air from 4% to 75% and rises to the highest point of the room or box. Charge only where the top of the space is ventilated, away from switches and sparks, and make and break connections with the supply off (Hydrogen Gas Hazard). A tool across the terminals of a charged bank welds in place; use insulated tools and remove watches and rings (Terminal and Bus Bar Connections).
What’s Next
- Learn next: Constant Current Charging — the simplest limiter, a resistor
- Combine with: Charge Controllers — automating the voltage ceiling from a solar array