AGM vs lithium batteries
AGM is cheap up front but delivers roughly half its rated capacity usefully and a few hundred cycles. LiFePO4 costs more, delivers nearly all of it, and lasts thousands.
For house batteries in a van, a boat or a solar setup, this is the main decision. Lithium costs several times more to buy but gives you far more of what you paid for: you can safely use nearly all of its capacity, where an AGM should only be run half flat, and it survives several thousand charges against a few hundred. It is also about a third of the weight. AGM makes sense on a tight budget, in very cold installations, or where you cannot change the charger.
- Verdict:
- Lithium costs more up front and less per year
Side by side
| Specification | AGM | LiFePO4 |
|---|---|---|
| Nominal system voltage | 12 V (6 × 2 V cells) | 12.8 V (4 × 3.2 V cells) |
| Usable capacity | About 50% of rated | 80–100% of rated |
| Cycles | 300–700 | 2000–5000+ |
| Weight | Heavy | About a third of AGM |
| Up-front cost | Low | Several times higher |
| Charging below freezing | Possible | Needs a heater or a BMS that blocks it |
Compare usable capacity, not rated capacity
A 100 Ah AGM gives you about 50 Ah before you start shortening its life. A 100 Ah LiFePO4 gives you 80 to 100. Two batteries with the same number on the label are not the same purchase, and this is the single most common mistake in the comparison.
Cost over the installation's life
Set the price against cycles rather than against the shelf. Several thousand cycles against a few hundred usually reverses the apparent price difference well before either battery is retired.
What can stop you switching
Lithium needs a charger and often an alternator setup that suits it, and it must not be charged below freezing without heating. In an unheated installation in a cold climate, AGM's tolerance can outweigh everything above.
Check before you swap. Batteries with similar dimensions are not always interchangeable. Always verify voltage, chemistry and the requirements printed in your device manual before fitting a replacement.