LiFePO4 vs AGM, honestly
Compare usable capacity, not the number on the label. A 100 Ah lithium battery and a 100 Ah AGM are not the same size of battery, and most price comparisons start from that mistake.
The label tells you what it holds, not what you can use
An amp-hour rating describes what a battery stores, not what you can take out of it every day without shortening its life. This site sizes LiFePO4 at 85% usable and AGM at 50%, the same assumptions the battery calculator shows.
| Per 100 Ah at 12 V | LiFePO4 | AGM |
|---|---|---|
| Usable capacity | About 85 Ah | About 50 Ah |
| Round-trip efficiency | About 96% | About 85% |
| Weight | Roughly a third of AGM | The heavy one |
| Charging below 0 °C | Only with heating or a BMS that blocks it | Yes, slowly |
| Left partly charged for days | Tolerates it | Sulphates and loses capacity |
| Charging from the alternator | Needs a DC-DC charger | Works with a simple relay |
What that does to bank size
Take the van from the battery calculator's worked example: 1,450 Wh a day, two days of autonomy, 12 V, with most of the load running through an inverter.
| Same loads, same autonomy | LiFePO4 | AGM |
|---|---|---|
| Bank needed at 12 V | 318 Ah | 611 Ah |
| Practical bank | 300–400 Ah | 600–700 Ah |
Expect well over 150 kg of AGM against roughly 50 kg of lithium. In a van that weight comes straight off your payload; in a cabin it decides what the floor has to carry.
Cost per kWh you can actually use
The fair comparison is lifetime energy per euro. Four numbers from a datasheet and a price tag give it to you:
Cost per usable kWh = price ÷ (rated kWh × usable share × rated cycles)
Suppose a lithium battery costs three times as much as an AGM of the same rating. It is rated for 3,000 cycles against the AGM's 500, and you use 85% of it against 50%. Over its life it delivers 1.7 × 6, about ten times the energy, so each usable kWh costs under a third as much.
Use the cycle ratings on the datasheets you are actually comparing, and check the depth of discharge each was measured at. A cycle count at 50% depth is not comparable with one at 80%. And cycle life only pays if you cycle: a battery that dies of age first never collects it.
Where AGM still wins
- Cold charging with no heating. LiFePO4 must not be charged below 0 °C unless the cells are heated or the BMS blocks charging. If the bank lives in an unheated space through winter and heating is not an option, lead acid is the simpler answer.
- Standby banks that rarely cycle. A backup bank that sits on float and is drawn down a few times a year will age out long before it uses lithium's cycle life. The extra price buys nothing.
- A small, fixed budget today. AGM costs less per rated amp-hour up front. If the alternative is no bank at all, the cheaper bank wins.
- Simple charging. AGM works with most existing chargers and a plain split-charge relay. Lithium needs compatible charge profiles and, from an alternator, a DC-DC charger.
Where LiFePO4 wins
- Daily cycling. Vans, cabins and anything off-grid most days. This is where the cycle life actually gets used.
- Weight and space. Roughly a third of the weight for the same rating, and a smaller bank for the same usable energy.
- Partial charge. Lithium is fine sitting half full through a cloudy week. Lead acid sulphates if it is not regularly brought back to full.
- Fast charging. It accepts high current for most of the charge, so a short drive or a sunny hour puts more back.
If you switch to lithium, change these too
- Charger profiles. The solar controller, shore charger and inverter-charger all need a lithium or user-defined profile.
- Alternator charging. Fit a DC-DC charger. Because lithium accepts current so readily, a plain relay can overload the alternator.
- Main fuse. A lithium bank can deliver far more fault current than lead acid. Use a fuse rated to interrupt it; DC cable and fuse sizing covers which types.
- Low-temperature protection. Check that the BMS blocks charging below 0 °C, or buy cells with built-in heating.
- Battery monitor. Reset the shunt monitor for the new capacity and chemistry. Lithium's voltage barely moves across most of its charge, so voltage alone says little about how full it is.
Never mix chemistries, ages or capacities in one bank.
Check your own numbers. Switch chemistry in the battery calculator and watch the bank size change for your loads.
This guide gives planning estimates. Check any battery installation against the manufacturer's instructions and the regulations that apply where you are.