A second battery for camping & 4x4
A second battery lets you run your fridge, lights and chargers while you're camping or off-road – and still be sure your car will start in the morning. Here's how it works and how to choose the right size.
Why two batteries?
A starting battery and a leisure battery are built for two very different jobs, so one can't do the work of the other.
Starting batteryThe one under your bonnet
- Gives a big burst of power for a few seconds to start the engine.
- It isn't made to be run flat. Draining it completely a few times can damage it for good. Why batteries go flat
- If you run a fridge off it overnight, the car may not start the next morning.
Leisure batteryDeep-cycle – the second battery
- Gives a steady supply of power for hours, to run fridges, lights and chargers.
- It's built to be drained and recharged over and over again.
- It's also the right choice for caravans and winches.
How a dual-battery setup works
This is how a correctly installed dual-battery system should work. While you drive, the alternator charges both batteries. When you stop, they are kept apart so your camping gear only uses the second battery.
The key part is the isolator (a split-charge relay or a DC-DC charger). While the engine runs, it joins the two batteries so both get charged. When you switch off, it separates them – so your fridge can never flatten the battery you need to start the car.
While the engine is off, nothing charges. The leisure battery runs your gear and recharges as soon as you drive again – or from a solar panel or a mains charger at the campsite.
Camp battery flat when you pack up? As soon as you start the drive home, it tries to grab as much charge as it can, all at once. The isolator joins it to your starting battery, so the whole system is put under strain.
- Alternator: works flat out and gets hot. Do this often and it can wear out early.
- Starting battery: is joined to the flat one, so it gets pulled down too.
- Camp battery: charges unevenly and often isn't full by the time you arrive.
Best fix: charge a flat camp battery at home or with solar first – then let the car keep it topped up while you drive. A battery left flat for days can be damaged for good, so we're happy to test it for free.
Ask us
Quick answers
Typical sizes for 12 V setups. Find yours, or use the calculator below.
| Your setup | 1 day | 2 days | 3 days |
|---|---|---|---|
| Fridge only60 W fridge | 80 Ah | 200 Ah | 2 × 120 Ah |
| Fridge, lights & phoneWeekend camping | 100 Ah | 200 Ah | 2 × 150 Ah |
| + laptopon an inverter | 120 Ah | 2 × 120 Ah | 2 × 200 Ah |
| Caravanfridge, lights, TV, pump, kettle | 200 Ah | 2 × 200 Ah | 3 × 200 Ah |
Days without charging. Green = a battery pack (batteries joined together). Same maths as the calculator, with no extra buffer added.
A 100 Ah lead-acid battery gives about 50 Ah. Lithium gives about 80%.
24 V and up use 12 V batteries joined in series.
Two or three batteries working together cover bigger needs.
Anything on 230 V uses about 15% extra to convert the power.
Battery life: looked after, up to about 4 years. Normal care, about 2 years. Run it flat and it can be damaged.
Rather just tell us? WhatsApp us what you’ll run and for how long. We’ll size it for you.
Want an exact answer for your gear?Use the calculator ↓
What size battery do I need?
Answer four quick questions and we’ll work it out for you.
What will you run?
Tap everything you’ll use on the trip.
Change watts or hours
How many days without charging?
Days with no driving, solar or mains to top it up.
What voltage is your system?
Check the label on your inverter, charger or battery box.
Which type of battery?
Not sure? Most people choose lead-acid / AGM.
Pick one and watch the result below change.
How we worked it out
You’ll use about 0 Ah per day
- In the battery–
- You can use–
- Kept in reserve to protect the battery–
A battery's size is not what you can use. Draining it below this shortens its life.
How we work it out
1. Daily use: watts × hours = watt-hours (Wh) for everything you tick. Items on 230 V get 15% extra for the inverter. Then Wh ÷ your system voltage (12 V for most setups) = amp-hours (Ah) per day.
2. Battery size: Ah per day × days without charging, then ÷ 0.5 for lead-acid, AGM or gel (you can safely use about half), or ÷ 0.8 for lithium (about 80%). Rounded up to the next common battery size. If that is more than one battery can give, we suggest a pack.
3. Running times use the part you can safely use (about half for lead-acid, AGM or gel, about 80% for lithium). They assume no charging. In real life, driving or a solar panel will top it up.
Already have a battery? Check it
How long will my battery last?
These voltages are for 12 V lead-acid, AGM and gel batteries, measured with a voltmeter when nothing is switched on.
Below 10.5 V, or left flat, damage starts. One deep drain can finish the battery.
Battery University figures, typical, varies by brand. In real life a battery also ages with time, so these are not years.
WhatsApp us this result
Good to know
A few simple things make a big difference to how well your second battery works and how long it lasts.
- Use thick cables and tight connections. Thin wire wastes charge, so the second battery never fills up properly.
- Fit a fuse close to each battery to protect the wiring if something goes wrong.
- Clamp the battery down securely. Vibration on gravel roads will damage a battery that isn't held firmly.
- Charge it fully after every trip with a proper charger. If it's standing, top it up every 3 months (lead-acid/AGM monthly is best), or keep it on a smart maintainer.
- Don't connect a flat second battery straight to your starting battery. It will drain the starting battery as well, and the car may not start.
- Don't use a normal car battery as the second battery. It won't last long when used this way, and the warranty doesn't cover it. Battery types explained
Planning a dual-battery setup?
Tell us about your vehicle and what you'd like to run, and we'll recommend the right leisure battery for your trips.
WhatsApp us
General guidance only. Have a dual-battery system installed by a qualified auto electrician.