LiFePO4 vs lead-acid (AGM and flooded) for solar
The battery chemistry decides how big a bank has to be, how much it weighs, how it charges and how long it lasts. Here is how lithium iron phosphate (LiFePO4) and lead-acid — flooded and AGM — compare, using figures from Victron, Trojan and Crown documents rather than marketing claims.
Side by side
| LiFePO4 | Lead-acid (flooded, AGM, gel) | |
|---|---|---|
| Usable capacity used for sizing | About 80% (Victron rates cycle life at 80% DoD) | About 50% (Trojan recommends 20–50% discharge) |
| Usable energy, 12 V 100 Ah | About 1,024 Wh (12.8 V × 100 Ah × 80%) | About 600 Wh (12 V × 100 Ah × 50%) |
| Cycle life | 2,500 cycles at 80% DoD, 5,000 at 50% (Victron, to 80% capacity) | Depends heavily on type and depth — check the data sheet; ageing speeds up sharply past about 80% DoD (Victron) |
| Round-trip efficiency | 92% (Victron) | About 75% full cycles, about 89% in partial-charge use (Victron) |
| Weight, group 24 size | 27.6 lb for 100 Ah (Crown V12-100) | 50 lb for 90 Ah flooded (Crown 24HDC) |
| Capacity at high current | Holds up well within its rated current | Falls: Trojan 27TMX gives 85 Ah over 5 h vs 105 Ah over 20 h |
| Charging temperature | Only +5 °C to +50 °C (Victron) | Wider; flooded −20 °C to +45 °C (Trojan), keep above 60% charge below freezing |
| Needs a management system | Yes — Victron requires an approved BMS | No |
| Maintenance | None | Flooded: check water; AGM/gel: none |
| Self-discharge | Low — check the data sheet | 5–15% a month (Trojan) |
Usable capacity is the big difference
Both chemistries are sold in amp-hours, but you can use far more of a lithium battery. Victron rates its LiFePO4 batteries for 2,500 cycles at 80% depth of discharge; Trojan generally recommends discharging lead-acid only 20–50% of its rating for long life, even though it can be cycled to 80%. So a 100 Ah LiFePO4 battery does roughly the work of 170 Ah of lead-acid. For the arithmetic, see how many watt-hours in a 100Ah battery; for what it means in hours, how long a 100Ah battery runs a fridge and how long a 100Ah battery lasts with a 200W load.
Heavy loads and inverters
Lead-acid loses capacity as current rises. Trojan’s group 27 data sheet shows 105 Ah over 20 hours but only 85 Ah over 5 hours; an inverter running a microwave discharges far faster than that. LiFePO4 holds up better — Victron rates its LiFePO4 capacity at a 1C discharge, 100 A for a 100 Ah battery — but has a different limit — the battery management system’s current rating. Victron recommends no more than 100 A continuous from its 12.8 V 100 Ah battery, so big 12 V inverters need several batteries in parallel either way: see what size battery bank for a 3000W inverter.
Cold weather
- Charging LiFePO4 below freezing damages it. Victron allows charging only between +5 °C and +50 °C (discharging from −20 °C). In cold climates, lithium banks live indoors, in heated compartments, or use self-heating models and chargers that stop when the BMS says so.
- Both lose capacity in the cold. Victron says its LiFePO4 capacity is about 20% lower at 0 °C and about half at −20 °C.
- Lead-acid can freeze when flat. Trojan gives about 16 °F (−9 °C) as the freezing point at 40% charge — keep lead-acid charged in winter.
Charging differences
LiFePO4 accepts charge efficiently. Lead-acid needs a long finish: the last part of the charge runs at falling current, and Trojan warns that operating in a partial state of charge causes sulfation and early failure. In a solar system that means lead-acid needs enough panel to reach full charge regularly. Use charger settings from the battery’s data sheet — Victron gives 14.2 V charge and 13.5 V float for its 12.8 V batteries; Trojan’s flooded 27TMX uses 14.82 V bulk and 13.50 V float. How long a 100W solar panel takes to charge a 100Ah battery compares the two.
Safety
- Lead-acid gives off hydrogen when charging. Trojan: don’t install or charge batteries in a sealed or unventilated compartment.
- Lithium needs its BMS. Victron’s manual warns that overcharge or deep discharge can make a lithium battery unsafe, and requires an approved battery management system and safety relay.
- Both can deliver huge short-circuit currents. Cover terminals, don’t lay tools on top, and remove rings and watches when working on them.
- Fuses and wire come from NEC/ABYC ampacity tables and the equipment manual — have a qualified installer check them.
More on handling and charging safely is in the battery safety guide.
Which to choose
- LiFePO4 when weight, space and daily cycling matter — vans, boats, RVs that boondock, off-grid cabins used most days — and when you can keep it above freezing while charging.
- AGM for occasional use, cold storage, or as a lower-cost upgrade over flooded with no maintenance.
- Flooded for budget banks that are checked and watered, and in places where weight doesn’t matter.
Frequently asked questions
Is LiFePO4 better than AGM for solar?
For daily cycling, usually: you can use about 80% of it versus about 50% of AGM, it is roughly half the weight for the same capacity, and Victron rates it for 2,500 cycles at 80% depth of discharge. AGM is simpler in freezing conditions and cheaper up front.
Can I charge a LiFePO4 battery below freezing?
Not unless the battery is designed for it. Victron allows charging only between +5 °C and +50 °C; self-heating batteries warm themselves first.
Can I mix lithium and lead-acid batteries?
Not in the same bank. They charge at different voltages and share load unevenly; mixing them can shorten the life of both.
How much lighter is a lithium battery?
In Crown’s data, a group 24 LiFePO4 battery (100 Ah) weighs 27.6 lb against 50 lb for its group 24 flooded deep cycle battery (90 Ah).
Sources
- Victron Energy — Lithium Battery Smart manual, technical data (opens in a new tab) — LiFePO4 12.8 V: cycle life 2,500 at 80% DoD, 3,000 at 70%, 5,000 at 50% (to 80% of capacity); 92% round-trip efficiency; 12.8 V 100 Ah: recommended continuous discharge ≤100 A (1C), maximum 200 A; capacity about 20% lower at 0 °C; charge only between +5 °C and +50 °C.
- Victron Energy — Lithium Battery Smart manual, safety precautions (opens in a new tab) — Terminals are always live; avoid short circuits, very deep discharges and excessive currents; charge only from 5 °C to 50 °C; use with an approved BMS.
- Victron Energy — Energy Unlimited (book) (opens in a new tab) — Lead-acid charge efficiency 80–95% (85% suggested as a starting value); battery energy efficiency about 75% for full cycles, about 89% in partial state of charge; inverter efficiencies above 90%.
- Trojan Battery — Technical support FAQs (opens in a new tab) — “We generally recommend a discharge of 20 to 50% of the battery’s rated capacity even though the battery is capable of being cycled to 80%”; 20-hour and 100-hour ratings; freezing points by state of charge.
- Trojan Battery — 27TMX data sheet (opens in a new tab) — Group 27 flooded: 85 Ah (5-hr), 97 Ah (10-hr), 105 Ah (20-hr), 117 Ah (100-hr); state of charge vs open-circuit voltage table; charger settings.
- Crown Battery — Deep Cycle Battery Product Specifications (effective Oct 1, 2024) (opens in a new tab) — 20-hour capacities by BCI group for flooded, AGM and lithium 12 V batteries.