Wiring Batteries in Series vs Parallel for RVs and Vans
Adding a second battery to your van is one of the most common upgrades. Understanding series vs parallel wiring prevents expensive mistakes.
The core concept
Series wiring: Connect + of battery 1 to − of battery 2. Take your output from the free + and free −.
- Result: Voltage doubles (12V + 12V = 24V), capacity stays the same (100Ah)
- Use when: Building a 24V or 48V system
Parallel wiring: Connect + to + and − to −. Take your output from either battery's terminals (or from a bus bar).
- Result: Capacity doubles (100Ah + 100Ah = 200Ah), voltage stays the same (12V)
- Use when: Expanding a 12V system
For 12V van builds: always parallel
A 12V van build adding a second battery wires in parallel. Both positives connect together (to the positive bus bar), both negatives connect together (to the negative bus bar via the shunt).
Battery 1: + ──── [fuse] ──── Positive bus bar
Battery 1: − ──── Shunt ──── Negative bus bar
Battery 2: + ──── [fuse] ──── Positive bus bar
Battery 2: − ──── Shunt ──── Negative bus bar
Each battery should have its own fuse between its positive terminal and the bus bar. This protects against one battery faulting and dumping current into the other through the bus bar.
Balancing parallel batteries
When batteries are connected in parallel, they self-equalize — the higher-voltage battery charges the lower-voltage one until they match. This is normal and harmless with LiFePO4 when the batteries are:
- Same brand and model
- Same age (or both new)
- Same state of charge when connected
Before connecting two batteries in parallel for the first time: Charge both to 100% with your normal charger, disconnect the charger, wait 1 hour, then connect them. The equalization current will be small since they're at similar voltage.
Series-parallel for 24V systems
Some builders want more capacity and higher voltage. For a 24V, 200Ah bank using 12V 100Ah batteries:
- Wire two batteries in series → 24V, 100Ah
- Wire another two batteries in series → 24V, 100Ah
- Connect the two series pairs in parallel → 24V, 200Ah
This creates a 2S2P (2 series, 2 parallel) configuration.
Common mistakes
Unequal cable lengths: If one battery is connected with a 12" positive cable and the other with a 6" cable, resistance differs and one battery carries more current. Use equal-length cables on both batteries.
Fusing only one battery: Each battery needs its own fuse on the positive terminal. Without individual fuses, a fault in one battery can blow current through the other battery's case.
Connecting batteries at different states of charge: Connecting a full battery to a depleted one causes a large equalization current — potentially tripping BMS units on both batteries. Always match state of charge before connecting.