AGM or lithium battery for my off-grid system?
Same amp-hour label, very different batteries. The usable-capacity and cycle-life math that settles which chemistry goes in your bank.
Side by side
| AGM (sealed lead-acid) | LiFePO4 (lithium iron phosphate) | |
|---|---|---|
| Upfront cost, 100 Ah @ 12 V | ~$200–280 | ~$400–600 |
| Usable capacity (DoD) | 50% → 50 Ah | 80–90% → 80–90 Ah |
| Cycle life (to 80% health) | 400–600 cycles | 3,000–5,000 cycles |
| Cost per usable kWh over life | ~$0.80–1.20 | ~$0.30–0.50 |
| Weight, 100 Ah @ 12 V | ~63 lb (30 kg) | ~26 lb (12 kg) |
| Charge efficiency | ~80–85% | ~98% |
| Charging below 32°F | Allowed (derated) | Blocked by BMS without a low-temp heater |
| Peak current | Handles motor surges well | 100 Ah models often limited to ~100 A BMS — check inverter surge |
The break-even math
A 24 V 400 Ah AGM bank (~$2,400) delivering 4.8 kWh usable lasts 400–600 cycles — 1.5–2 years of daily cycling. Over the same period one 24 V 400 Ah LiFePO4 bank (~$3,200–4,000) delivers 7.7 kWh usable and is still healthy at year 8–10. Replacing AGM twice before lithium reaches half-life is the part the price tag hides. If the system cycles only weekly (weekend cabin), the crossover stretches past a decade and AGM's lower upfront cost can genuinely win.
Where AGM still makes sense
Unheated spaces below freezing: lithium BMS units block charging below 32°F unless the battery has a self-heating pad — a cold shed or seasonal cabin can kill a lithium bank's winter usefulness. Backup that rarely cycles: an outage bank that sits full gets its life from calendar age, not cycles, so AGM's cycle-life disadvantage barely applies. High surge loads on a budget: lead-acid shrugs off motor startup spikes that trip a small lithium BMS — though the fix is simply a bigger lithium model.
Sizing consequences
Same cabin, 2.4 kWh usable overnight: AGM needs 2×200 Ah at 24 V (~126 lb); LiFePO4 needs one 100–130 Ah battery (~30 lb) — and a slightly smaller array, since ~15–20% more of what the panels produce actually reaches the bank. Run both chemistries through the battery bank sizer and the solar size calculator to see the array difference on your own load list.
FAQ
Can I mix AGM and lithium in the same bank?
No. Different charge profiles (AGM absorbs at ~14.4 V, LiFePO4 at ~14.2 V with different current behavior) mean one chemistry always charges wrong. Replace whole banks at once, or keep them on separate controllers.
Do I need a special charge controller or inverter for lithium?
Any MPPT controller with an adjustable or LiFePO4 profile works — set absorption ~14.2–14.6 V and float low or off. The one real gotcha is surge: a well pump or big tool startup can exceed a 100 Ah lithium battery's BMS limit, so parallel batteries or a higher-BMS model is standard for pump systems.