LiFePO4 vs AGM Leisure Batteries: Energy and Charging

Compare LiFePO4 and AGM using usable energy, current demand, temperature and compatible charging, not Ah alone.

LiFePO4 vs AGM is the most common battery decision in campervan builds. Both work — but they are different technologies with very different cost profiles over time. Here is the full comparison.

The headline numbers

Specification	AGM 100Ah	LiFePO4 100Ah
Usable capacity (safe DoD)	50Ah (50% DoD)	90Ah (90% DoD)
Cycle life (to 80% capacity)	400–600 cycles	3,000–5,000 cycles
Weight	26–30kg	10–14kg
Charge rate	0.2C (20A for 100Ah)	0.5–1C (50–100A for 100Ah)
Operating temperature (charge)	-20°C to 50°C	0°C to 45°C (BMS blocks cold charging)
Self-discharge	3–5% per month	1–2% per month
Maintenance	Occasional equalisation	None
UK price (100Ah)	£80–160	£200–500

Usable capacity: the most misunderstood comparison

When someone says "100Ah AGM vs 100Ah LiFePO4," the label understates the difference.

An AGM battery should only be discharged to 50% — running it below 50% (below ~12.2V) dramatically reduces its cycle life. So a "100Ah" AGM gives you 50Ah usable.

A LiFePO4 battery can safely discharge to 80–90%. A "100Ah" LiFePO4 gives you 90Ah usable.

To match the usable capacity of one 100Ah LiFePO4, you need two 100Ah AGM batteries. When priced on usable capacity, AGM is often more expensive than it first appears.

Cycle life: the long-term cost difference

This is where LiFePO4 pays for itself.

AGM at 50% DoD: 400–600 cycles before capacity drops to 80%. At 1 cycle per day (typical van life use), this is 1–1.5 years before replacement.

LiFePO4 at 80% DoD: 3,000–5,000 cycles. At 1 cycle per day, this is 8–14 years before replacement.

Real cost over 5 years (100Ah usable):

AGM: 2× 100Ah = £300, replaced 3× over 5 years = **£900 total**
LiFePO4: 1× 100Ah = ~£350, not replaced = £350 total

Over 5 years, LiFePO4 is typically cheaper than AGM when replacement costs are included.

Weight: significant for a moving vehicle

A 200Ah AGM bank (two 100Ah batteries) weighs approximately 55–60kg. The equivalent 200Ah LiFePO4 bank weighs 20–25kg. Saving 30–35kg improves payload, fuel economy, and handling — meaningful for a van living vehicle.

Charging speed: faster solar and alternator recovery

LiFePO4 accepts charge at up to 1C (100A for a 100Ah battery). AGM should not be charged above 0.2C (20A for 100Ah) to maximise cycle life.

In practice, this means:

A 200W solar array can charge a LiFePO4 at its full output
The same array is wasted on AGM because the battery cannot accept it fast enough
LiFePO4 can be recovered from empty to full in 1–2 hours with a large charge source; AGM takes 4–8 hours

When AGM makes sense

Low budget, short term: If you are building a van for 6–12 months and want to minimise upfront cost, AGM is the lower-risk investment.

Infrequent use: A van used for occasional weekend trips that sits stored most of the year — AGM on a maintenance charger works fine and the cycle life is less relevant.

Existing system: If you already have a well-functioning AGM setup that is not yet worn out, there is no need to replace it for the sake of it. Run it until it needs replacement, then switch to LiFePO4.

When LiFePO4 is clearly better

Full-time van life (high cycle count)
Solar-dependent setup (benefits from fast charge acceptance)
Inverter use (LiFePO4 handles high discharge current without voltage sag)
Weight-sensitive builds
Cold storage situations (LiFePO4 stores better over winter)

Compare the complete replacement, not just the battery price

A battery change can also affect charging profiles, low-temperature charge control, mounting, fusing, monitoring and alternator charging. Before comparing AGM and LiFePO4 quotes, list which existing parts can remain and which need manufacturer-confirmed compatibility. The cheapest battery is not automatically the cheapest conversion.

An illustrative equal-energy comparison uses a 12.8V 100Ah lithium battery with an assumed 80% usable fraction: 1,024Wh. A nominal 12V AGM bank with a 50% planning fraction needs about 171Ah to provide the same calculated energy. These fractions are assumptions for comparison, not instructions to discharge either battery to that limit. High current, temperature and ageing change the result.

Can I reuse my old charger after switching chemistry?

Only if its supported profile and limits match the new battery instructions. Check mains, solar and alternator chargers individually; changing one setting in the battery monitor does not reconfigure the chargers.

FAQ

Can I mix LiFePO4 and AGM in the same bank?

No — different voltage profiles make this damaging to both batteries. Use a single chemistry throughout your leisure battery bank.

Do I need to change my solar charge controller if I switch from AGM to LiFePO4?

You need to reconfigure it, not replace it — most modern MPPT controllers have a LiFePO4 preset. Check your specific model.

Is an AGM battery safe to use with an inverter?

Yes — AGM handles inverter loads fine, but the high current draw of a large inverter (1,000W+) at 12V causes more voltage sag on AGM than LiFePO4. A 100Ah AGM may struggle to sustain a 1,500W inverter load at its peak; LiFePO4 of the same nominal capacity handles it more comfortably.

Worked planning check

For an illustrative 1,000Wh usable requirement, a nominal 12V AGM bank at 50% planned depth of discharge needs about 167Ah. A nominal 12.8V lithium bank at an 80% allowance needs about 98Ah. Those allowances are planning assumptions, not universal manufacturer limits.

The figures or dimensions in this example are explicit planning assumptions, not a product test or an installation specification. Replace them with your equipment data and actual measurements.

Planning questions

How much energy is in a 100Ah lithium battery?

At nominal 12.8V, the arithmetic is 1,280Wh. Delivered energy is lower after your reserve, losses and operating limitations; it is not a measured capacity result.

Can I mix battery brands or capacities?

Only with explicit manufacturer approval for the complete arrangement. Chemistry, voltage, age, current sharing and BMS behaviour can make a seemingly simple combination unsuitable.

Can lithium batteries charge in freezing weather?

Follow the exact battery’s permitted temperature range and heating conditions. Do not bypass a protective cutoff or infer charge permission from air temperature alone.

Use the free electrical planner to organise your daily loads and charging assumptions.

Continue planning

See the budget agm starter wiring example for its complete HTML cable and fuse schedule, assumptions and downloadable PDF.