HOW TO SIZE YOUR LITHIUM BATTERY SYSTEM - A PRACTICAL GUIDE FOR 4WD AND CARAVAN OWNERS
One of the most common mistakes people make when upgrading to lithium is choosing the wrong battery size - either going too small and running out of power, or over-speccing and paying for capacity they'll never use.
This guide walks you through exactly how to size your system correctly.
Step 1 - List everything you power
Start by writing down every electrical load in your setup. For each one, note the power draw in amps or watts, and estimate how many hours per day you use it. Common loads in a touring setup include:
A 12V compressor fridge typically draws 3–5 amps and runs 24 hours a day, although less on the compressor cycle.
A diesel heater fan draws around 0.5–1 amp. LED lighting might total 2–3 amps across a few hours each evening.
A 12V water pump draws 3–5 amps for short bursts.
Phone and device charging can really put a dent in it over a few hours (especially with the kids and their iPads).
An inverter running a laptop draws 3–5 amps. If you're running air conditioning, coffee machines or an induction cooktop these draw significantly more - upwards of 80–100+ amps - and need to be factored carefully.
Step 2 - Calculate your daily consumption
Multiply each load's amp draw by the hours it runs per day to get amp-hours (Ah) consumed. Add them all up for your total daily consumption. As a rough example, a moderate touring setup might total 80-100Ah per day.
Step 3 - How long do setup camp for without driving?
Whether you like to setup camp for 1-2 weeks, or hit the road every day. This can impact how much battery you need, because your system will be getting charged up accordingly.
If you setup camp for 1-2 weeks, you're relying heavily on solar only, with minimal driving.
Whereas if you drive most days while on holidays - I get it, you're the adventuring type - you'll be putting far more charge back into your battery every day.
This factor of time becomes a large decider on what size battery to get, and what size charger to pair it with.
Step 4 - Factor in your charging sources
How are you recharging? If you have 200W of solar and you're getting 5–6 peak sun hours per day in summer, you're generating roughly 60–70Ah per day from solar alone. Add in DC-DC charging from driving and you're likely covering a typical daily consumption with margin. If you're free camping for extended periods with limited driving, you'll want more battery capacity as a buffer, and a higher current DCDC charger to get power back in quickly for those short times that you are driving.
Step 5 - Apply the LiFePO4 advantage
Remember that Custom Lithium batteries are usable to 100% of rated capacity. A 100Ah Custom Lithium battery gives you 100Ah of genuinely usable power. This means you often need less rated capacity than you would with AGM, where you'd be sizing for double your actual daily consumption to protect the battery.
Step 6 - Build in a buffer
A good rule of thumb is to size your battery so your daily consumption represents no more than 50–60% of your total usable capacity. This gives you two days of autonomy without charging as a minimum - a practical safety margin for cloudy days, longer stops and unexpected loads. The more usable capacity the better, but if you go too far - you may be adding unnecessary spend, and weight to your vehicle!
For most basic touring setups, a single 100Ah or 150Ah Custom Lithium battery will be genuinely sufficient. Moderate setups would see 200Ah to 310Ah and more serious off-grid setups with high loads or extended free camping typically benefit from 400Ah or more.
Still not sure? Ask us.
At Custom Lithium, helping you get the sizing right is part of what we do. Tell us your setup, your loads and how you use your vehicle - and we'll give you a straight answer on exactly what you need. No upsell, no guesswork. Just the right battery for your adventure.