Lithium (LiFePO4) vs AGM Batteries for Overlanding: Capacity, Weight and Cost Compared

If you’re adding an auxiliary battery to run a fridge, lights and devices, the first decision is chemistry: a traditional AGM (absorbent glass mat) lead-acid battery, or lithium iron phosphate (LiFePO4). Lithium costs more up front, but the gap shrinks a lot once you compare usable capacity, weight and lifespan rather than sticker price.

Quick comparison

	AGM	LiFePO4
Typical usable capacity of a 100Ah battery	About 50Ah (staying above 50% charge)	About 80–95Ah
Typical weight, 100Ah	About 28–32kg	About 10–13kg
Typical cycle life	A few hundred cycles at deep discharge	Several thousand cycles
Voltage under load	Drops steadily as it discharges	Stays fairly flat until nearly empty
Charging in freezing weather	Fine	Needs a low-temperature cut-off or built-in heating
Up-front cost	Lower	Higher

Figures vary by brand and model, so check the spec sheet of the specific battery you’re considering.

1. Usable capacity

Both batteries can say 100Ah on the label, but you can’t use all of it in both. AGM batteries last much longer if you avoid regularly discharging them below about 50%, so a 100Ah AGM realistically gives you around 50Ah. A 100Ah LiFePO4 can be used down to 80% or more of its capacity without much effect on its life. In practice, one lithium battery gives you close to double the usable energy of an AGM with the same rating.

2. Weight

A 100Ah AGM typically weighs around 28–32kg, and a 100Ah LiFePO4 around 10–13kg, so swapping one for one saves roughly 15–20kg. The bigger saving comes when you compare equal usable energy. To match the usable capacity of one 100Ah lithium battery you’d need about 200Ah of AGM, which is two batteries and around 60kg. Against that, the lithium battery saves something like 45kg. On a vehicle where payload is already tight, that matters.

3. Lifespan and cost over time

Quality LiFePO4 batteries are commonly rated for several thousand cycles. AGM batteries used deeply every day may only last a few hundred. If you camp often, an AGM may need replacing several times over the life of one lithium battery, which closes much of the price gap. If you only camp a few weekends a year, an AGM will often age out from time before it wears out from cycles, and the cheaper option can make more sense.

4. Charging

Lithium accepts high charge current until it’s nearly full, while AGM charging slows down a lot in the last part of the charge. That means lithium makes better use of short drives and limited solar. It also needs the right charge profile, so make sure your DC-DC charger and solar regulator have a lithium setting. Many smart alternators also need a DC-DC charger to charge an auxiliary battery properly.

5. Cold weather

LiFePO4’s main weakness is cold. Most should not be charged when the cells are below about 0°C (32°F). If you camp in freezing conditions, look for a battery with low-temperature charge protection or built-in heating, or mount the battery inside the cabin where it stays warmer.

Which should you choose?

• Occasional weekend trips, tight budget: AGM is still a reasonable choice.
• Frequent or long trips, weight-sensitive vehicle, or a fridge running 24/7: LiFePO4 is usually the better long-term buy.

Once you’ve picked a chemistry, the next step is working out how big a battery you need. Our LiFePO4 sizing guide walks through the maths, and our look at REDARC’s REDWorks panels covers one way to handle the charging side.

Figures in this article are typical values from manufacturer specifications. Always check the data sheet for the battery you buy.

Featured photo: Jackery Power Station on Unsplash.