A resistor in series with a charging source limits the current to I = (V_source − V_battery) ÷ R, so a 20 V source charging a 12 V battery at 5 A needs a 1.6 Ω resistor that dissipates 40 W. This is only an approximation of constant current: the current falls as the battery voltage rises and never reaches zero, so the charge must be ended by measurement unless the source voltage is chosen close to the battery’s charging ceiling.
Part of Battery Building and Maintenance
A resistor-limited charger for a lead-acid battery, with the sizing arithmetic and its limits.
The Resistor Takes The Voltage Difference
Whatever voltage the source has above the battery’s has to drop somewhere, and a series resistor takes it. By Ohm’s law (Ohm’s Law) the resistance for a wanted current is R = (V_source − V_battery) ÷ I_charge, and the resistor must dissipate P = I² × R. The carrier’s example is a 12 V battery charged from a 20 V source at 5 A: R = (20 − 12) ÷ 5 = 1.6 Ω, and the power is 5² × 1.6 = 40 W, which the carrier writes as 5 A × 8 V = 40 W.
The 12 V figure is the battery at its lowest. During charging its voltage rises to the ceiling of 14.4–14.8 V (Charging Circuits), and the same resistor then passes less current.
What You Need
- A DC source above the battery’s charging ceiling
- A resistor of the right value and power rating — wire-wound or improvised (Handmade Resistors explains the nichrome method); rate it well above the computed watts
- A fuse and an ammeter, and a voltmeter across the battery
- A blocking diode if the source can stop (see Charging Circuits)
Method: Size And Run A Resistor Charger
Step 1: Choose the current. Use C/10 for normal charging (Rechargeable Batteries calls it safe); for a 50 Ah battery that is 5 A. For plate formation use C/20 (Plate Curing and Formation).
Step 2: Compute R and P from the formulas above.
Step 3: Build the resistor to carry the power without overheating: a 40 W resistor needs a ceramic former and room to shed heat.
Step 4: Connect and measure. Read the actual current; adjust the resistor if it is off.
Step 5: Watch the battery — voltage, temperature (disconnect if past 50°C) and specific gravity — and stop the charge by hand.
Worked Example: What Happens As The Battery Fills
Source 20 V, R = 1.6 Ω. At the start (battery 12 V) the current is (20 − 12) ÷ 1.6 = 5 A. At 14.4 V it is (20 − 14.4) ÷ 1.6 = 3.5 A, and the resistor dissipates 3.5² × 1.6 = 19.6 W. At 14.8 V the current is (20 − 14.8) ÷ 1.6 = 3.25 A. For a 50 Ah battery the finishing current of C/50 is 50 ÷ 50 = 1 A, so 3.25 A at the top is more than three times what a fully charged battery should be taking: this charger overcharges unless somebody stops it.
Efficiency at 14.4 V: the battery receives 14.4 × 3.5 = 50.4 W, the source supplies 20 × 3.5 = 70 W, so 50.4 ÷ 70 = 72% of the power reaches the battery and the rest is heat.
A better match: a 16 V source and R = (16 − 12) ÷ 5 = 0.8 Ω. The resistor now dissipates 5² × 0.8 = 20 W. At 14.4 V the current is (16 − 14.4) ÷ 0.8 = 2 A, and at 14.8 V it is (16 − 14.8) ÷ 0.8 = 1.5 A. That tapers toward the finishing current much better.
Common Mistakes
- Sizing R for 12 V and forgetting the rise — the current at the top of the charge is not the starting current
- Leaving it unattended — the current never stops; stop it by a timer, a meter or a controller (Float Charging)
- An under-rated resistor — 40 W in a small resistor burns it
- A wasteful source — a source far above the battery wastes the difference as heat
Safety
The resistor is a heater and the battery a hydrogen source
A 40 W resistor is hot enough to ignite paper and burn skin, and the battery beside it gives off hydrogen once it starts gassing at the top of the charge, which a spark or hot surface can ignite between 4% and 75% of the air. Mount the resistor in free air away from the battery’s top, use insulated connections, and never charge a battery in a closed box (Hydrogen Gas Hazard).
What’s Next
- Learn next: Float Charging — holding a full battery full
- Combine with: Equalization Charging — a deliberate periodic overcharge