Choosing the wrong lithium battery size is one of the most common mistakes made by 4WD and caravan owners building an off-grid power system.

Too little capacity and you could find yourself running out of power when you’re miles from the nearest campsite, with a fridge full of food, devices needing a charge and no easy way to top the battery back up. Go too far the other way, and you’re carrying extra weight, spending more money and taking up valuable space on capacity you may never actually use.

Getting the balance right comes down to understanding how much power your setup actually consumes, how you travel, and how often you can recharge. A weekend warrior who drives every day will have very different requirements to someone spending a week parked up off-grid with minimal engine run time.

To help break down what size lithium battery you really need, we spoke to Custom Lithium (↗), experts in off-grid power systems, to explain how to calculate your energy requirements, choose the right capacity and build a system suited to your adventures.

Q: Where do I start when sizing a lithium battery system?

Custom Lithium: Start by listing everything you power. For each device, note the power draw in amps or watts and estimate how long you use it each day. Typical loads include:

  • 12V compressor fridge: 3 to 5 amps, runs 24 hours (less on compressor cycle).
  • Diesel heater fan: 0.5 to 1 amp.
  • LED lighting: 2 to 3 amps for a few hours each evening.
  • 12V water pump: 3 to 5 amps in short bursts.
  • Device charging: Can add up, especially with multiple phones or tablets.
  • Inverter loads: Laptops draw 3 to 5 amps; high-power items like aircon, coffee machines or induction cooktops can draw 80 to 100+ amps.

Q: How do I figure out daily consumption?

Custom Lithium: Multiply each device’s amp draw by the hours used to get daily amp-hours (Ah). Add everything together for your total daily consumption. As a rough guide, a moderate touring setup might use 80 to 100Ah per day.

Q: How does my camping style affect battery size?

Custom Lithium: It matters whether you stay at camp for a week without driving, or if you’re on the road every day. Time at camp versus driving frequency is a major factor in deciding battery size and the right charger.

  • Extended stops: You rely mostly on solar and minimal driving, so you’ll need more battery capacity.
  • Daily driving: You’ll be topping up your battery frequently, so smaller capacity may be fine.

Q: What about charging sources?

Custom Lithium: If you free camp for long periods with minimal driving, you’ll want extra battery capacity and a higher-current DC-DC charger to recharge quickly on the road. Your recharging setup affects how much battery you need. For example:

  • Solar: 200W solar with 5-6 peak sun hours generates about 60 to 70Ah/day.
  • DC-DC charger: Adds power when driving.

Q: How does lithium compare to AGM batteries?

Custom Lithium: Lithium batteries, like ours, are usable to 100-per-cent of their rated capacity. That means a 100Ah Custom Lithium battery really gives 100Ah usable, unlike AGM where you’d typically only use about 50 per cent without shortening its life. This often means you need less rated capacity than with AGM.

Q: Should I add a buffer?

Custom Lithium: Yes. Aim for daily consumption to be 50 to 60 per cent of total usable capacity. This gives you at least two days of autonomy for cloudy days, longer stops, or unexpected loads. Too much capacity, however, adds unnecessary weight and cost.

Q: What’s a typical setup size?

Custom Lithium: For basic touring setups, a single battery of 200Ah is usually sufficient. Moderate setups typically require 310Ah, while serious off-grid setups or extended free camping benefit from 400Ah or more.

If you’re unsure, get in contact with Custom Lithium and they’ll provide a clear recommendation on the right battery size.

MORE Custom Lithium hub page!