Parallel Battery Banks for Campervans: Wiring Guide for Multiple LiFePO4 Batteries
As your power needs grow, a single battery may not be enough. Adding a second battery in parallel doubles your capacity — 200Ah instead of 100Ah, 2,560Wh instead of 1,280Wh. But wiring batteries in parallel is not as simple as connecting positive to positive and negative to negative.
I added a second battery to my bank six months after the initial build. The first attempt failed — the batteries never balanced and one did all the work while the other sat idle. The problem was the cable lengths. When I fixed the wiring to ensure equal resistance, both batteries shared the load perfectly.
This guide covers how to wire parallel battery banks correctly for campervans.
Why Parallel?
| Configuration | Voltage | Capacity | Use |
|---|---|---|---|
| Single battery | 12V | 100Ah | Small systems, minimal budget |
| 2 × parallel | 12V | 200Ah | Most full-time van lifers |
| 3 × parallel | 12V | 300Ah | Heavy users, electric cooking |
| 4 × parallel | 12V | 400Ah | High-power builds, winter resilience |
| 2 × series (24V) | 24V | 100Ah | Large inverters, reduced cable size |
Parallel wiring keeps the voltage at 12V and adds the Ah ratings. Series wiring doubles the voltage but keeps the Ah the same. For campervans, parallel is the standard because all 12V equipment (lights, fridge, diesel heater) expects 12V.
The Balancing Problem
Batteries in parallel naturally share current based on their internal resistance. The battery with lower internal resistance takes more current. If the wiring is not balanced, one battery does 60% of the work while the other does 40%.
The problem worsens as the batteries age and their internal resistance diverges. One battery degrades faster, works harder, degrades even faster — a vicious cycle.
Balanced vs Unbalanced Wiring
Bad (diagonal connection):
┌── Battery 1 ──┐
│ │
Load +┘ └── Load -
│ │
└── Battery 2 ──┘
The positive cable runs to Battery 1. The negative cable runs to Battery 2. The path through Battery 1 is shorter (lower resistance) than the path through Battery 2. Battery 1 does more work.
Good (equal cable lengths):
┌── Battery 1 ──┐
│ │
+ ←──┐ │ │ ┌──→ −
│ └── Battery 2 ──┘ │
│ │
Positive busbar Negative busbar
Positive from the load connected to a busbar. Negative from the load connected to a busbar. Each battery connected to the busbar with equal-length cables of the same gauge. Every battery sees the same cable resistance.
Wiring Configuration
The Correct Way: Busbar + Equal Cables
- Mount a positive busbar and a negative busbar (250A rated for most campervan banks)
- Connect the load (inverter, fusebox, DC-DC charger) positive to the positive busbar
- Connect the load negative to the negative busbar
- Connect each battery positive to the positive busbar
- Connect each battery negative to the negative busbar
- All battery cables must be:
- Same length (within 5% tolerance)
- Same wire gauge
- Same terminal type
- Same route (similar temperature = similar resistance)
Cable Lengths
For a 2-battery bank: both positive cables 500mm, both negative cables 500mm.
For a 3-battery bank: all three positive cables 500mm, all three negative cables 500mm.
For a 4-battery bank: this gets tricky. The physical layout rarely allows all cables to be the same length. Use the "diagonal" busbar approach:
+ Busbar − Busbar
│ │
├── 500mm ── Battery 1 ── 500mm ──┤
├── 500mm ── Battery 2 ── 500mm ──┤
├── 500mm ── Battery 3 ── 500mm ──┤
└── 500mm ── Battery 4 ── 500mm ──┘
Cable and Fuse Sizing
Cable Size
| Bank Size | Max Current (Inverter) | Cable to Busbar | Cable from Busbar to Load |
|---|---|---|---|
| 200Ah (2 × 100Ah) | 100A (1,200W inverter) | 16mm² | 25mm² |
| 300Ah (3 × 100Ah) | 150A (1,800W inverter) | 25mm² | 35mm² |
| 400Ah (4 × 100Ah) | 200A (2,400W inverter) | 35mm² | 50mm² |
Fuse Each Battery
Each battery cable from the battery to the positive busbar must be fused individually. The fuse protects the individual cable.
| Battery Cable | Fuse Rating | Why |
|---|---|---|
| 16mm² | 80A | Protects the cable, not the battery |
| 25mm² | 120A | Protects the cable |
| 35mm² | 150A | Protects the cable |
Do not fuse the load cable from the busbar — the busbar is effectively the battery's main terminal, and the load circuit is protected by its own fuse at the distribution board.
Connecting Batteries in Parallel
Step-by-Step
-
Pre-charge all batteries: Before connecting, charge each battery to the same voltage. Ideally, all batteries should be within 0.1V of each other. Significant voltage differences cause massive inrush current when connected.
-
Power down: Disconnect all loads and chargers from the batteries.
-
Connect the busbars: Mount and connect the positive and negative busbars.
-
Connect battery cables: Starting with the battery closest to the busbar, connect each battery's positive cable to the positive busbar, then the negative cable to the negative busbar. Use a torque wrench — 8–10 Nm for M8 terminals.
-
Connect the main positive fuse: Install the main fuse between the positive busbar and the load. 200A for a 200Ah bank, 300A for a 300Ah bank.
-
Check voltage: Measure voltage across the busbars. It should read the same as each individual battery (within 0.05V).
-
Connect the load: Connect the inverter, fusebox, and DC-DC charger to the busbars or the main fuse.
First Charge
- The batteries will balance during the first charge cycle
- Monitor the bank temperature for the first few hours (warm busbars or battery terminals indicate a poor connection)
- After the first full charge to 14.2V, check that all batteries reach the same voltage within 20 minutes
Mixing Old and New Batteries
Do not mix batteries of different ages, brands, or capacities in the same bank. The older battery has higher internal resistance and will be under-utilised. The newer battery will do more work and degrade faster.
| Scenario | OK? | Why |
|---|---|---|
| Same brand, same age, same capacity | ✓ Yes | Perfect match |
| Same brand, different age (1 year gap) | ⚠️ Short-term OK | Expect reduced lifespan for the older battery |
| Different brand, same capacity | ✗ No | Different internal resistance, BMS behaviour |
| Different capacity (100Ah + 200Ah) | ✗ No | The 100Ah battery limits the bank |
| Different chemistry (AGM + LiFePO4) | ✗ Never | Different charge voltages, dangerous |
If you must add a battery to an existing bank: Buy the same make and model as the original. Charge both to the same voltage before connecting. Monitor the bank balance over the first month.
BMS Considerations in Parallel
Each battery has its own BMS. In a parallel bank:
- All BMS units see the same bank voltage (good)
- Current splits between batteries based on internal resistance (as discussed)
- If one BMS trips on over-current or under-voltage, the other batteries carry the full load — which may trip their BMS too (cascading failure)
- When the BMS resets (after charge is applied), all BMS units should reset simultaneously
Cascading failure scenario:
- Inverter draws 200A (100A per battery in a 2-battery bank)
- One BMS trips at 110A (over-current)
- The other battery now carries 200A — its BMS trips at 110A
- Both batteries are disconnected — you are completely without power
- Neither BMS resets because there is no charge source (the solar controller sees 0V at the bank)
Solution: Size each battery's BMS for the total bank's peak load, not the individual battery's share. If your inverter draws 200A peak, each battery should have a 200A BMS, not a 100A BMS. Then if one BMS trips, the other handles the full 200A.
Monitoring a Parallel Bank
A single battery monitor (Victron SmartShunt) installed on the main negative cable from the busbar monitors the entire bank. It does not need to monitor individual batteries.
What to watch:
- Total bank voltage
- Total current in/out
- State of charge of the whole bank
- If one battery develops a fault, the bank's behaviour changes (faster discharge, lower voltage under load)
- Check individual battery voltages monthly (by disconnecting and measuring) — a 0.2V+ difference indicates an issue
FAQ
Q: How many batteries can I connect in parallel? A: Most LiFePO4 BMS units allow up to 4 batteries in parallel. Check the manufacturer's specification. Beyond 4, the risk of imbalance and cascading BMS trips increases significantly.
Q: Can I add a second battery to an existing single battery? A: Yes, if it is the same make, model, and age. Buy the same battery you already have. Pre-charge both to the same voltage before connecting.
Q: Do I need to fuse each battery separately in a parallel bank? A: Yes. Each cable from each battery to the positive busbar must have its own fuse. This protects the cable if a battery develops an internal short.
Q: Will my batteries last longer in parallel? A: Yes and no. Each battery is cycled less frequently (a 200Ah bank with 200W of solar cycles through 100Ah per day, each battery cycles through 50Ah — half the depth of discharge). Lower depth of discharge = longer lifespan.
Q: Can I mix battery chemistries in parallel? A: Never. Different chemistries have different charge voltages (LiFePO4: 14.2V, lead-acid: 14.8V) and different internal resistances. One battery will be overcharged or undercharged. Both will degrade or fail.
Q: How do I know if my parallel bank is unbalanced? A: Measure each battery's voltage while the bank is under a significant load (inverter running a microwave). If one battery reads 0.2V+ lower than the others, it has higher internal resistance and is not contributing equally. Check your cable lengths and connections.
