Solar & Battery Electrical Calculator for Campervans | VanPowerCalc

Q: Can I mix different battery brands in parallel?

Strongly not recommended. Different internal resistances cause uneven current sharing — one battery works harder and ages faster. Same brand, same model, same age, same capacity. If you already have a 100Ah Renogy and want to add capacity, sell it and buy two matched batteries.

Q: What cable size for parallel battery connections?

25mm² (4 AWG) minimum for 200Ah systems, 35mm² (2 AWG) for 300Ah+. The interconnect cables carry the full current from one battery to the bus bar. Undersized cables create a resistance imbalance that defeats the purpose of paralleling.

Q: Do I need a fuse between parallel batteries?

Yes — one fuse on each battery's positive cable before they meet at the bus bar. If one battery develops an internal short, the fuse prevents the other from dumping its entire capacity into the fault. 60A ANL for 100Ah batteries, 100A for 200Ah.

Q: How many batteries can I parallel?

Practically, 2-4 batteries in a van. Beyond 4, current imbalances become hard to manage without active balancing. For very large banks (600Ah+), consider fewer larger batteries rather than many small ones.

Q: Can I parallel LiFePO4 and AGM?

Never. Different charge voltages (14.6V vs 14.4V), different internal resistance, different discharge curves. One will overcharge while the other undercharges. This can cause thermal runaway in AGM or BMS lockout on LiFePO4. Same chemistry only.

Leo

Fact-checked on Apr 4, 2026

Adding a Second Battery to Your Van: Parallel Wiring Done Right

Adding a second battery to double your capacity? Parallel wiring guide with fuse sizing, cable specs, and the mistakes that kill batteries.

4 min read

|

Your electrical appliances

Quick Add

Tap a preset above to start

No appliances added yet.

Daily Total

0

WH / Day

Tap a preset above to start

System Config

System Voltage

Average Sun Hours

VANPOWERCALC.COMYOUR ENERGY PROFILE

YOUR ENERGY PROFILE.

This document contains the sizing of your future electrical installation, calculated based on your appliances.

Minimum Daily Consumption

0 WH / DAY

Inventory:

SYSTEM SIZINGPage 2

Battery

To guarantee 0WH without damaging your bank (80% max discharge):

0 AH

LiFePO4 12V

Solar

Minimum power required to recharge your consumption:

0 W

via MPPT

220V AC

Maximum power (with 25% safety margin).

0 W

No Need

Safety & CablingPage 3

12V Cable Sizing Guide

Use this professional reference table to select the correct gauge (mm²) for your cables. For 12V in a van, the maximum tolerated voltage drop is 3%. Always use multi-stranded flexible automotive wire.

Current (A)	Round trip < 2m	Round trip 4m	Round trip 6m
5A (LEDs, USB)	1.5 mm²	2.5 mm²	4 mm²
10A (Fridge, Pump)	2.5 mm²	4 mm²	6 mm²
20A (Heater)	4 mm²	10 mm²	10 mm²
50A (DC/DC Booster)	10 mm²	16 mm²	25 mm²
100A (Inverter)	25 mm²	35 mm²	50 mm²

Fuse Sizing

The fuse protects the wire, not the appliance. Always place it as close to the power source as possible (battery or busbar).

Wire 1.5 mm² → Max fuse 10A
Wire 2.5 mm² → Max fuse 20A
Wire 4 mm² → Max fuse 30A
Wire 6 mm² → Max fuse 40A
Wire 10 mm² → Max fuse 60A

SCHÉMA ÉLECTRIQUE

PANNEAUX SOLAIRES

0W

RÉGULATEUR MPPT

BATTERIE AUXILIAIRE

0 Ah

Lithium LiFePO4

FUSIBLE

BOÎTE À FUSIBLES 12V

Pompe, Leds, Frigo...

CONVERTISSEUR 220V

NON REQUI

PRO GEARPage 4

SHOPPING LIST

Where to find this equipment? Here is the community-approved selection.

12V 6-way Fuse Box

Mandatory protection

Digital Multimeter

Test your connections

Heavy Duty Crimping Tool

For perfect lugs

Heat Shrink Tubing

Insulation and safety

GENERATED BY VANPOWERCALC.COM

Doubling your battery bank by adding a second battery in parallel is the most common upgrade in van life. Two 100Ah batteries in parallel give you 200Ah at the same 12V — twice the capacity, same voltage. But wire it wrong and you'll kill both batteries within a year.

The golden rule of parallel batteries: same brand, same model, same age, same capacity. Mixing a new battery with a 2-year-old one forces the new battery to constantly compensate for the older one's higher internal resistance. The new battery works harder, heats up more, and degrades to match the old one within months. If you can't buy two identical batteries at the same time, don't parallel them.

Solar panels series vs parallel comparison guide for campervans

Expert tip

Before connecting two batteries in parallel, charge both to exactly the same voltage independently. If battery A is at 13.2V and battery B is at 12.8V, connecting them causes a massive inrush current as they equalize — potentially damaging the BMS or welding your terminal connections. Charge both to 13.3V, then connect.

Example calculation for your setup

Results based on a typical use case

Appliance	Power	Usage/day	Wh/day
Compression fridge	45W	24h	1080
LED lighting	20W	4h	80
Water pump	30W	0.5h	15
Phone charging	15W	2h	30
Daily consumption			1205 Wh

Recommended LiFePO4 battery

151 Ah

(80% depth of discharge)

Recommended AGM battery

241 Ah

(50% depth of discharge)

Recommended solar panels

402 Wc

4h average sun hours

Recommended inverter

No 220V appliances — no inverter needed

Adjust these values with the calculator below

About this tool

Wiring matters more than people think. Connect both batteries with equal-length cables to a common bus bar — not daisy-chained (battery 1 → battery 2 → load). Daisy-chaining means battery 1 handles more current because it's closer to the load, leading to uneven wear. Equal-length cables ensure both batteries share the load 50/50. Use 25mm² (4 AWG) minimum for the interconnects on 200Ah systems, with a fuse on each battery's positive cable before they merge at the bus bar. I use 60A ANL fuses on each 100Ah battery — rated to protect the interconnect cable, not the battery capacity. One more thing: if you're paralleling LiFePO4 batteries with separate BMS units, make sure both BMS chips are compatible with parallel operation. Some cheaper BMS units fight each other — one cuts off while the other tries to compensate, creating a voltage oscillation that can trip both. SOK and BattleBorn are explicitly rated for parallel. Check the spec sheet for any others.

ByLeo—Electrical Engineer & VASP Builder

Published March 31, 2026

Frequently asked questions

Can I mix different battery brands in parallel?▼

Strongly not recommended. Different internal resistances cause uneven current sharing — one battery works harder and ages faster. Same brand, same model, same age, same capacity. If you already have a 100Ah Renogy and want to add capacity, sell it and buy two matched batteries.

What cable size for parallel battery connections?▼

25mm² (4 AWG) minimum for 200Ah systems, 35mm² (2 AWG) for 300Ah+. The interconnect cables carry the full current from one battery to the bus bar. Undersized cables create a resistance imbalance that defeats the purpose of paralleling.

Do I need a fuse between parallel batteries?▼

Yes — one fuse on each battery's positive cable before they meet at the bus bar. If one battery develops an internal short, the fuse prevents the other from dumping its entire capacity into the fault. 60A ANL for 100Ah batteries, 100A for 200Ah.

How many batteries can I parallel?▼

Practically, 2-4 batteries in a van. Beyond 4, current imbalances become hard to manage without active balancing. For very large banks (600Ah+), consider fewer larger batteries rather than many small ones.

Can I parallel LiFePO4 and AGM?▼

Never. Different charge voltages (14.6V vs 14.4V), different internal resistance, different discharge curves. One will overcharge while the other undercharges. This can cause thermal runaway in AGM or BMS lockout on LiFePO4. Same chemistry only.

Advertising

Independent and free thanks to your clicks

Links marked with * are affiliate links. If a purchase is made through them, I receive a commission at no extra cost to you. The editorial selection and product evaluation are not influenced by commission rates. Your click helps fund this free tool.