LiFePO4 vs AGM Batteries for Solar: Which Is Better?

For almost any solar system that cycles daily, LiFePO4 (lithium iron phosphate) is the better choice. You can use about 85% or more of its capacity. In addition, it lasts 3,000 to 6,000 cycles. It weighs about a third as much. As a result, it costs less per kWh over its life. However, AGM still makes sense for small backup batteries that you rarely use. It is also good for very cold, unheated locations. Furthermore, it suits tight upfront budgets.

Side-by-side comparison

Feature	LiFePO4	AGM (sealed lead-acid)
Usable depth of discharge	80 – 100% (we plan with 85%)	50% for good life
Cycle life	3,000 – 6,000 cycles	500 – 1,000 cycles at 50% DoD
Weight per usable kWh	roughly 10 – 13 kg	roughly 50 – 60 kg
Round-trip efficiency	95 – 98%	80 – 85%
Voltage under load	Stays flat until nearly empty	Sags as it discharges
Charging below 0 °C	Not allowed without a heater	Allowed, at reduced rate
Maintenance	None	None (but sensitive to undercharging)
Upfront price per kWh	Higher	Lower
Cost per kWh over life	Lower	Higher

Usable capacity is the big difference

A “100 Ah” battery is not 100 Ah of usable energy:

• 12 V 100 Ah LiFePO4: 12.8 V × 100 Ah = 1,280 Wh, of which about 1,090 Wh is usable at 85% DoD.
• 12 V 100 Ah AGM: This battery provides 12 V × 100 Ah. So, it stores 1,200 Wh. However, only about 600 Wh is usable at 50% DoD.

So one lithium battery replaces roughly two AGM batteries of the same rating. Our depth of discharge guide explains why lead-acid should not go below half.

AGM also loses usable capacity at high discharge rates. This is due to the Peukert effect. For example, run a 1,000 W inverter from an AGM bank. You may only get 60% to 70% of the rated Ah. However, lithium delivers close to its full rating.

Cycle life and real cost

Price per cycle matters for solar. This is because you use the battery every day.

A 100 Ah LiFePO4 battery costs about three times more than a 100 Ah AGM battery. However, the lithium battery gives about twice the usable energy per cycle. Also, it provides about five times the cycles. So, over its life, it delivers roughly ten times the energy. As a result, its cost per stored kWh ends up around a third of AGM’s. This is before you count replacement labor and weight savings.

Weight and space

Lead is heavy. For example, a 10 kWh usable AGM bank weighs between 500 and 600 kg. In contrast, an equivalent LiFePO4 bank weighs around 120 kg. This difference alone decides it for RVs, vans, and boats. See our RV boondocking solar setup.

Cold weather

You must not charge LiFePO4 below 0 °C (32 °F). Doing so can permanently damage the cells. Discharging in the cold is fine. Many modern lithium batteries include built-in heaters. They also have low-temperature charge protection. If your batteries live in an unheated shed in a cold climate, read charging LiFePO4 in cold weather.

You can charge an AGM battery in the cold. However, it loses significant capacity. Also, a discharged lead-acid battery can freeze.

Charging differences

• LiFePO4 charges to about 14.2 to 14.6 V. A 12 V battery needs no float stage. Also, it accepts high current. So, it refills quickly from solar.
• AGM batteries need a long absorption stage. This stage is around 14.4 to 14.7 V to reach 100%. Chronic undercharging is common in solar systems during winter. As a result, this causes sulfation and early failure.

Make sure your charge controller has a lithium profile. Also, ensure your inverter-charger has one. You can also use user-adjustable voltages.

When AGM still makes sense

• Consider this a backup battery. It sits on float most of the year. You will use it only a few times annually.
• Very low budgets where the upfront price is the deciding factor.
• These applications require batteries to charge below freezing. No heated option is available. So, consider these factors.

Verdict

For daily-cycling solar, RVs, vans, boats, and off-grid homes, choose LiFePO4. Size it with our battery bank formula. You can also run your numbers in the calculator. It switches between 85% and 50% DoD automatically.

Worked example: upgrading an RV from AGM to lithium

An RV has two 12 V 100 Ah AGM batteries (about 100 Ah usable). The owner replaces them with one 12 V 200 Ah LiFePO4 battery.

	Before (2 × AGM 100 Ah)	After (1 × LiFePO4 200 Ah)
Usable energy	about 1.2 kWh	about 2.2 kWh
Weight	about 60 kg	about 22 kg
Inverter voltage sag at 1,500 W	Significant	Minimal
Recharge from 50% with 400 W solar	About a full sunny day (long absorption)	About 3 to 4 hours

Upgrade checklist:

1. Set your converter/charger to a lithium profile. If you cannot set it, then replace it.
2. Set the solar controller’s battery type to LiFePO4.
3. Add a DC-DC charger for alternator charging.
4. Check the inverter’s low-voltage cutoff suits lithium.
5. Confirm the battery has low-temperature charge protection if you camp in freezing weather.

When each chemistry makes sense

Situation	Better choice
Daily cycling (off-grid, full-time RV, van)	LiFePO4
Weight-sensitive (vans, boats, trailers)	LiFePO4
Standby backup, rarely discharged	AGM can be fine
Very cold, unheated, no heater option	AGM, or self-heating LiFePO4
Lowest upfront cost	AGM / flooded

Common mistakes

• Keeping a lead-acid “equalise” mode enabled with lithium.
• Do not assume lithium needs float charging. It does not. Instead, a float slightly below full is better. No float at all also extends battery life.
• Mixing lithium and AGM in one bank.

FAQ

Can I replace AGM with LiFePO4 directly?

Often yes, but check that your charger, charge controller and any alternator charging have lithium settings. In vehicles, add a DC-DC charger to protect the alternator. See DC-DC charger vs solar charge controller.

Are LiFePO4 batteries safe?

LiFePO4 is the most thermally stable common lithium chemistry. It is far less prone to thermal runaway. This differs from the lithium-ion types used in phones. A good battery management system adds further protection. See battery management system.

Is gel better than AGM?

Gel tolerates deep discharge slightly better. However, it needs lower charge voltages. It also handles high currents poorly. For solar, LiFePO4 outperforms both.

Is lithium worth the extra cost?

For batteries cycled daily, usually yes. You get a lower cost per stored kWh over its life. Also, it has less weight and better performance.

Will a LiFePO4 battery work with my old charger?

Only if the charger’s voltages suit lithium (about 14.2 to 14.6 V absorption, no equalisation, no desulfation pulse). Many older chargers do not.

Related guides

• How Many Batteries Do I Need for a 5,000-Watt Inverter?
• Days of Autonomy: How Much Battery Backup Do You Need?
• How Many Batteries Do I Need for a 3,000W Inverter?
• Can You Mix Old and New Batteries in a Solar Bank?

Sources and further reading

• Battery University: BU-205 Types of Lithium-ion: how lithium chemistries, including LiFePO4, compare.
• Battery University: BU-409b Charging Lithium Iron Phosphate: LiFePO4 charging voltages and behaviour.
• Battery University: BU-415 How and When to Charge: charging habits for lead-acid and lithium batteries.