How to Wire Campervan Batteries in Parallel (Step-by-Step)

Wiring batteries in parallel is the standard way to increase your campervan battery bank capacity while keeping the voltage at 12V. Whether you are adding a second battery to double your capacity or building a larger bank from the start, getting the parallel wiring right is essential for safety, battery longevity, and even charging. This guide walks you through the full process for both LiFePO4 and AGM batteries in a UK campervan context. For a complete overview of battery types and sizing, see our campervan battery guide.

Before you start connecting batteries, it is worth understanding exactly what parallel wiring does and does not do. Parallel wiring increases amp-hour capacity while keeping voltage the same — two 12V 200Ah batteries in parallel give you 12V at 400Ah. This is the configuration most campervan builds use. For essential safety practices including fusing, cable sizing, and connection standards, review our campervan wiring safety guide.

Parallel vs Series: Which Do You Need?

In a campervan, you almost always want parallel wiring:

• Parallel: Voltage stays the same, capacity adds up. Two 12V 200Ah batteries = 12V 400Ah.
• Series: Voltage adds up, capacity stays the same. Two 12V 200Ah batteries = 24V 200Ah.

Most campervans run a 12V system, so parallel is the correct choice. The only exception is if you are building a 24V system, which is uncommon in UK van conversions.

What You Need

Tools

• Wire strippers rated for your cable gauge
• Hydraulic or hex crimp tool (not a basic ratchet crimper — you need proper compression for high-current lugs)
• Torque wrench or spanner for terminal bolts
• Multimeter
• Heat gun for adhesive-lined heat shrink

Materials

• Battery interconnect cables (see sizing section below)
• Ring terminal lugs (tinned copper, correctly sized for your cable and terminal bolt)
• Adhesive-lined heat shrink
• Class T or MRBF fuse for each battery (recommended)
• Battery terminal covers or boots

Step 1: Choose Matching Batteries

This is non-negotiable. Batteries wired in parallel must be:

• Same chemistry (both LiFePO4 or both AGM — never mix)
• Same capacity (both 200Ah, for example)
• Same brand and model (ideally from the same production batch)
• Same age (new batteries only — do not add a new battery to an old one)

Mismatched batteries cause uneven charging and discharging. The stronger battery does more work, which degrades it faster and creates a cycle that ruins both batteries prematurely.

Step 2: Match State of Charge Before Connecting

Before physically connecting your batteries in parallel, they must be at the same state of charge — ideally both fully charged. If you connect two batteries with different voltages, a large equalisation current will flow between them. This can:

• Trip the BMS on LiFePO4 batteries
• Damage terminals and cables
• Create sparking at the connection point

How to match charge:

1. Fully charge each battery individually using a suitable charger.
2. Measure the resting voltage of each with a multimeter (leave them disconnected for 30 minutes after charging).
3. Voltages should be within 0.1V of each other.
4. If they differ by more than 0.2V, charge the lower one until they match.

Step 3: Size Your Interconnect Cables

The cables connecting your batteries in parallel must be sized for the maximum current they could carry. This is determined by your largest load or your charging current — whichever is higher.

General sizing guide for interconnect cables:

For most campervan systems with a 2000-3000W inverter, 50mm² cable is the standard choice for battery interconnects. This handles up to 200A comfortably with minimal voltage drop.

Step 4: Wire Using the Diagonal Method

The diagonal method (also called cross-diagonal) is the correct way to connect parallel batteries to your system. It ensures both batteries share the load equally.

How it works:

1. Connect Battery 1 positive to Battery 2 positive (interconnect cable).
2. Connect Battery 1 negative to Battery 2 negative (interconnect cable).
3. Connect your main positive feed (to fuse box/inverter) from Battery 1 positive terminal.
4. Connect your main negative feed (to bus bar/shunt) from Battery 2 negative terminal.

The key is that your main system cables connect to opposite ends of the parallel bank — positive from one battery, negative from the other. This means current travels through both batteries equally, regardless of which one is physically closer to the loads.

Common Mistake: Same-Side Connection

If you connect both the main positive and main negative to the same battery, that battery handles more current and wears out faster. The diagonal method eliminates this problem.

Step 5: Fuse Each Battery Individually

Each battery in a parallel bank should have its own fuse on the positive cable, as close to the battery terminal as possible. This protects against a short circuit in one battery or its interconnect wiring from dragging unlimited current from the other battery.

Recommended fuse types:

• Class T fuse in a fuse holder — rated for DC and high interrupt capacity
• MRBF terminal fuse — bolts directly to the battery terminal for maximum protection

Size the fuse based on the maximum current the battery should deliver. For a 200Ah LiFePO4 with a 200A BMS, a 200A or 250A Class T fuse is appropriate.

Step 6: Make Proper Cable Terminations

Poor crimps are the number one cause of electrical problems in campervan builds. For high-current battery connections:

1. Strip the cable to the correct length for your lug barrel.
2. Use a hydraulic or hex crimp tool — hand ratchet crimpers do not create sufficient compression for cables above 16mm².
3. Apply adhesive-lined heat shrink over the crimp to seal against moisture and vibration.
4. Tug-test the crimp firmly — it should not pull off.
5. Torque terminal bolts to the manufacturer's specification (typically 10-12Nm for M8 studs).

Step 7: Test Before Connecting Loads

Once your parallel bank is wired:

1. Measure voltage across each battery individually — they should read the same.
2. Measure voltage across the main positive and negative outputs — should match the individual battery voltage.
3. Check all connections are tight with no exposed copper.
4. Install terminal covers or boots on all battery terminals.

LiFePO4 Parallel Wiring: Special Considerations

BMS Behaviour

Each LiFePO4 battery has its own BMS. In a parallel bank, the individual BMS units operate independently. If one BMS trips (due to overcurrent or temperature), that battery disconnects while the other continues operating. This means the remaining battery suddenly takes the full system load, which could trip its BMS too.

To minimise this:

• Size your system so that each individual battery can handle the maximum load temporarily
• Monitor both batteries independently via Bluetooth
• If one BMS trips repeatedly, investigate the cause before reconnecting

How Many LiFePO4 Batteries in Parallel?

Most manufacturers recommend a maximum of four LiFePO4 batteries in parallel. Beyond that, BMS coordination becomes unreliable and equalisation currents between batteries increase. For campervan use:

• 2 in parallel is the most common and works reliably
• 3 in parallel works with careful matching and cable sizing
• 4 in parallel is the practical maximum for most brands
• Beyond 4 requires specialist design and is not recommended for DIY builds

AGM Parallel Wiring Considerations

AGM batteries do not have a BMS, so parallel wiring is simpler in some ways but requires more attention to matching:

• AGM batteries must be the same age — internal resistance increases with age, causing uneven loading
• Never add a new AGM to an existing one that has been in service for more than a few months
• AGM batteries are more tolerant of minor voltage differences than LiFePO4, but you should still match state of charge before connecting

Troubleshooting Common Issues

One Battery Draining Faster Than the Other

This usually indicates unequal cable lengths or a poor crimp on one connection. Check all cables are identical length and re-inspect every termination. If the problem persists with verified equal wiring, one battery may be faulty.

BMS Tripping Under Load

If a BMS trips under normal load, check that the total load does not exceed the individual battery's BMS rating. In a 2-battery parallel bank, each battery handles roughly half the total load. If your inverter draws 200A and each BMS is rated for 200A, you have headroom. But if one BMS has slightly lower tolerance, it may trip.

Batteries at Different Voltages After Sitting

A small voltage difference (0.05V) after sitting overnight is normal and will equalise when connected. Differences greater than 0.2V after a full charge suggest one battery has a cell imbalance or degradation. Investigate before continuing to use the bank.

Connecting to Your System

Once your parallel bank is complete, connect it to your system as follows:

1. Main positive from the bank goes to your primary fuse (Class T or ANL), then to your positive bus bar or distribution panel.
2. Main negative from the bank goes to your battery monitor shunt (if fitted), then to your negative bus bar.
3. All loads and charging sources connect to the bus bars — never directly to the battery terminals.

For complete wiring safety standards including cable routing and fuse selection, refer to our campervan wiring safety guide. Use our calculator to verify your battery bank size is right for your daily energy needs.

FAQ

Can I add a third battery to an existing parallel pair?

You can, provided the third battery is identical (same brand, model, and ideally from the same production batch) and all three are at the same state of charge before connecting. You must also add a fuse to the new battery and ensure all interconnect cables are equal length.

Do I need a battery balancer for parallel LiFePO4 batteries?

No. Each LiFePO4 battery has its own BMS that handles internal cell balancing. A battery balancer is needed for series-connected batteries, not parallel.

Can I wire different capacity batteries in parallel?

This is not recommended. A 100Ah and 200Ah battery in parallel will work initially, but the smaller battery will be stressed more during high-current loads because it has higher internal resistance. This shortens its lifespan and eventually degrades both batteries.

What happens if one battery fails in a parallel bank?

If a LiFePO4 battery's BMS disconnects it, the other battery takes the full load. If a battery develops an internal fault, the fuse on its positive cable should blow, isolating it from the bank. The remaining battery continues to power the system at reduced capacity.