Second battery · split charge relay · D+
Dual battery wiring diagram with a split charge relay
A second battery runs the fridge, the lights and the inverter at camp without touching the battery that starts the engine. A relay connects the two only while the alternator is charging; when the engine stops it opens, and the starting battery is left alone.
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The circuit
- Starting battery + → fuse → split charge relay 30; relay 87 → fuse → second battery +.
- Both fuses go right at their batteries: the cable can be fed from either end.
- Relay coil 86 → alternator D+ (it is live only while the engine runs); 85 → ground.
- Second battery − → body or the main ground with a cable as thick as the + cable.
- Accessories (fridge, lights, inverter) run from the second battery only.
Typical loads on 12 V
| Load | Power | Wire length | Current | Fuse | Wire |
|---|---|---|---|---|---|
| Parking lights | 10 W | 4 m | 0.83 A | 2 A | 0.35 mm² (22 AWG) |
| Headlight bulb H7 | 55 W | 3 m | 4.58 A | 7.5 A | 0.75 mm² (18 AWG) |
| Pair of headlights | 110 W | 3 m | 9.17 A | 15 A | 1.5 mm² (16 AWG) |
| Horn | 60 W | 2 m | 5 A | 7.5 A | 0.5 mm² (20 AWG) |
| Wiper motor | 120 W | 2 m | 10 A | 15 A | 1 mm² (17 AWG) |
| Radiator fan | 250 W | 2 m | 20.83 A | 30 A | 4 mm² (12 AWG) |
| LED light bar | 300 W | 4 m | 25 A | 35 A | 6 mm² (10 AWG) |
| Audio amplifier | 1000 W | 5 m | 83.33 A | 125 A | 35 mm² (2 AWG) |
Cable and fuses
The charge current into a flat second battery can reach 40 A and more. For 40 A over 3 m the calculator gives a 50 A fuse at each battery and 10 mm² cable. Longer runs — to a canopy or a trailer — need a thicker cable to keep the drop down, otherwise the second battery never fully charges.
Relay, VSR or DC-DC charger?
| Relay on D+ | Voltage-sensing relay (VSR) | DC-DC charger | |
|---|---|---|---|
| Joins when | Engine runs | Voltage above ~13.3 V | Always charges with a set profile |
| Smart alternators | May not charge well | May drop out | Yes |
| Lithium second battery | No | No | Yes |
| Cost | Lowest | Low | Higher |
See also: Inverter to car battery · Alternator terminals D+, IG, S
Watch it charge in the simulator
- Run the live demo: two batteries, a split charge relay on D+, fuses at both ends and a fridge on the second battery.
- Ignition on, engine off: the relay stays open — the simulator notes the coil gets only a few volts through the charge lamp, not enough to pull in. The fridge runs from the second battery alone.
- Set the engine speed: D+ rises, the relay pulls in and current flows into the second battery (shown as charging).
- Click the link cable to see the charge current and its voltage drop.
Questions
Can I just connect the two batteries in parallel?
Then the fridge flattens both and the car will not start. The relay is what keeps the starting battery separate.
Do the batteries need to be the same?
With a plain relay, both should be lead-acid of similar type. A lithium or very different second battery needs a DC-DC charger.
Why fuse both ends?
A short anywhere on the link cable is fed by both batteries; a fuse at each one protects the cable from both sides.
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