Quick Answer
For most off-grid solar systems, LiFePO4 is the better long-term choice. It delivers 2x the usable capacity per rated amp-hour, lasts 5–8x more cycles, and costs 5–7x less per usable kWh over its lifetime. AGM still wins for freezing outdoor installations without climate control and for tight upfront budgets where purchase price matters more than lifetime cost.
In This Guide
What Is Each Chemistry?
LiFePO4 (Lithium Iron Phosphate)
A lithium-ion chemistry using iron phosphate cathodes. It's the dominant off-grid battery because it's non-flammable (unlike NMC lithium), has a flat discharge curve holding ~13.0V until nearly empty, and includes a built-in BMS protecting against overcharge, over-discharge, shorts, and temperature extremes. Typical 12V nominal, 3.2V per cell in a 4S configuration.
AGM (Absorbent Glass Mat)
A sealed lead-acid battery where electrolyte is absorbed into fiberglass mats between plates. Maintenance-free, spill-proof, and handles deeper cycling better than flooded lead-acid. It's forgiving of imperfect charging and costs less upfront — but is heavier, has shorter cycle life, and cannot be discharged as deeply without permanent capacity loss.
Usable Capacity & Depth of Discharge
Nameplate amp-hours are misleading. What matters is how many you can actually use per cycle without destroying the battery.
| Chemistry | Recommended DoD | Usable from 100Ah | Voltage Under Load |
|---|---|---|---|
| LiFePO4 | 80–100% | 80–100Ah | ~13.0V flat, drops sharply at end |
| AGM | 50% | 50Ah | Sags to ~11.8V at 50% DoD |
This is the single most important number. A 100Ah LiFePO4 gives you the same usable energy as a 200Ah AGM bank — at roughly half the weight and a third the size. If your existing AGM bank is 400Ah, you only need about 200Ah of lithium to match it in real-world daily cycling.
AGM voltage also sags significantly as it discharges. At 50% DoD under moderate load, an AGM may read 11.8V — below the low-voltage cutoff of many inverters. LiFePO4 holds 13.0V+ until nearly empty, so you get all the rated usable capacity without nuisance shutdowns.
Cycle Life: How Long Do They Last?
Numbers below assume recommended DoD: 80% for lithium, 50% for AGM. One cycle = one full discharge and recharge to that depth.
| Chemistry | Cycles to 80% Capacity | Years (Daily Cycling) | Calendar Life |
|---|---|---|---|
| LiFePO4 | 3,500–5,000 | 10–15 years | 10–15 years |
| AGM (quality) | 400–600 | 1.5–2 years | 3–5 years |
Real-world: Off-grid cabin
At one full cycle per day, LiFePO4 retains 80% capacity after 10+ years. AGM cycled to 50% daily needs replacement in under 2 years. Over a 10-year off-grid installation, you'll buy one lithium bank — or four to five AGM banks.
If you only cycle batteries occasionally (backup power a few times per year), calendar aging dominates. AGM sulfates and fails in 3–5 years regardless of use. LiFePO4 calendar life is 10–15 years with 1–3% monthly self-discharge.
Cost Per Usable kWh (Lifetime)
Upfront price is the wrong metric. Levelized cost — price per usable kilowatt-hour delivered over the battery's life — tells the real story.
| Metric | LiFePO4 (12V 100Ah) | AGM (12V 100Ah) |
|---|---|---|
| Typical purchase price | $250–400 | $180–250 |
| Rated capacity | 1,280 Wh | 1,280 Wh |
| Usable capacity per cycle | 1,024 Wh (80% DoD) | 640 Wh (50% DoD) |
| Lifetime cycles | 3,500 | 500 |
| Lifetime usable kWh | 3,584 kWh | 320 kWh |
| Cost per usable kWh | $0.08–0.11 | $0.56–0.78 |
Lithium costs 5–7x less per usable kWh over its lifetime. The "AGM is cheaper" argument only holds if you're installing batteries for a one-time event or expect to sell within 2 years.
There's also a hidden cost: because you can only use 50% of AGM nameplate capacity, you need twice the amp-hours for the same daily usable energy. A system needing 200Ah usable per day requires 250Ah of LiFePO4 or 400Ah of AGM — and the AGM bank weighs twice as much and still needs replacement in 2 years.
Temperature Behavior & Cold-Weather Solutions
Temperature is where AGM still has a clear advantage, particularly for unconditioned outdoor installations.
| Condition | LiFePO4 | AGM |
|---|---|---|
| Charge temperature | 32°F to 113°F (0°C to 45°C) | -4°F to 122°F (-20°C to 50°C) |
| Discharge temperature | -4°F to 140°F (-20°C to 60°C) | -4°F to 122°F (-20°C to 50°C) |
| Cold-charge risk | Permanent plating damage below 32°F | Charges safely below freezing |
| High-temp degradation | Minimal below 113°F | Loses ~50% life per 15°F above 77°F |
| Self-heating available | Yes (premium models) | N/A |
The Freezing Problem (and Three Solutions)
Charging LiFePO4 below 32°F causes lithium plating on the anode — permanent capacity loss. This is physics, not a manufacturing defect.
- Self-heating batteries — BMS diverts charge current to an internal heating pad before allowing current to cells. Adds ~$50–80 per battery.
- Insulated, conditioned enclosure — Insulated box with a small thermostatically controlled heater.
- Programmable charge controller — Temperature sensor + controller configured to block charging below 32°F. Battery can still discharge.
If your batteries live in an unconditioned shed in Vermont and you don't want to deal with any of this, AGM is the right call.
Charging Requirements & Controller Compatibility
The two chemistries have fundamentally different charging needs — a profile set for one will damage the other.
LiFePO4 Charging
- Bulk/absorption: 14.2–14.6V (3.55–3.65V/cell)
- Float: 13.5–13.8V — or none at all (lithium doesn't need it)
- Absorption time: Short, 15–30 min. Stop when current drops.
- Equalization: Never equalize lithium.
- Efficiency: ~98% — nearly all energy goes into the battery.
- Charge rate: Typically 0.5C (50A for 100Ah). Some accept 1C.
AGM Charging
- Bulk: 14.4–14.7V at 0.2–0.3C (20–30A per 100Ah)
- Absorption: 2–4 hours until current drops to ~1% of C.
- Float: 13.5–13.8V — must maintain to prevent sulfation.
- Equalization: 15.0–15.5V every 6 months recommended.
- Efficiency: ~85% — 15% of input energy becomes heat.
- Charge rate: Limited to ~0.2C. Large banks take all day.
Charge Controller Compatibility
Most modern MPPT controllers (Victron SmartSolar, Midnite Classic, OutBack FM, etc.) have presets for both chemistries. If you're using a budget PWM controller with only "lead-acid" presets, do not use it with lithium — the absorption voltage is close enough to work, but float and equalization behavior will damage the battery. Budget another $100–200 for a lithium-compatible charge controller.
Which Use Case Calls for Which Battery?
Full-time off-grid home or cabin
Daily cycling for years. Lithium's cycle life and flat voltage curve make it the obvious choice. Higher upfront cost is recovered within 2–3 years through avoided AGM replacements.
RV, van, or skoolie conversion
Weight matters and space is tight. A single 100Ah LiFePO4 (28 lbs) replaces two 100Ah AGMs (130 lbs). The BMS handles alternator charging safely with a DC-DC charger. Flat discharge curve means your inverter won't cut out early.
Grid-tied backup with daily cycling
If cycling daily for time-of-use arbitrage or self-consumption, lithium pays for itself. If the battery only cycles during outages (a few times per year), the math is closer — see the standby backup case below.
Unconditioned outdoor shed in cold climate
If batteries regularly sit below freezing and you don't want to install heating, AGM is simpler and safer. Charges without damage down to -4°F.
Tight upfront budget, short-term project
If you need batteries for a 1–2 year project or are building a system you'll sell soon, AGM's lower purchase price wins. Just factor the replacement cost into any long-term plan.
Standby backup (rarely cycled, climate-controlled)
For a grid-tied backup battery that cycles a handful of times per year in a basement or garage, AGM's calendar-life limitation (3–5 years) still applies — but the lithium cost advantage shrinks because you won't reach its cycle-life potential. Either chemistry works here; lithium's longer calendar life may still justify the premium.
Solamp Catalog Recommendations
Here's how Solamp's battery lineup maps to the use cases discussed above.
| Use Case | Recommended Chemistry | Solamp Category |
|---|---|---|
| Off-grid cabin / home | LiFePO4 | LiFePO4 Batteries |
| RV / van / mobile | LiFePO4 | LiFePO4 Batteries |
| Grid-tied home backup (daily cycling) | LiFePO4 | 48V LiFePO4 Rack Batteries |
| Cold outdoor install (unconditioned) | AGM | Sealed AGM Batteries |
| Budget / short-term project | AGM | Sealed AGM Batteries |
| Standby backup (rare cycles) | Either | All Batteries |
Pro tip: Not sure what size battery bank you need? Start with our battery sizing calculator to determine your daily watt-hour requirement, then use the table above to choose your chemistry.
Size Your Battery Bank
Enter your daily energy usage and system voltage for a personalized battery bank recommendation with chemistry options.
Open Battery Sizing CalculatorFrequently Asked Questions
Which is cheaper over time: LiFePO4 or AGM?
LiFePO4 is cheaper over time despite a higher upfront cost. At $0.08–0.11 per usable kWh vs $0.56–0.78 for AGM, lithium costs 5–7x less over its life. A 100Ah LiFePO4 battery delivers 80–100Ah usable each cycle for 3,500–5,000 cycles. An equivalent AGM bank needs 200Ah to deliver the same usable energy and lasts only 400–600 cycles.
Can I use LiFePO4 batteries in freezing temperatures?
Yes, but only with low-temperature charging protection. You can discharge LiFePO4 down to -4°F (-20°C), but charging below 32°F (0°C) causes permanent cell damage. Most quality LiFePO4 batteries have a built-in BMS that blocks cold charging. For unconditioned outdoor installations in cold climates, AGM remains simpler — it charges safely down to -4°F.
How many years do LiFePO4 and AGM batteries actually last?
LiFePO4 lasts 10–15 years (3,500+ cycles to 80% capacity). AGM lasts 2–5 years (400–600 cycles to 50% DoD). Calendar aging also favors lithium — AGM sulfates and loses capacity after 3–5 years regardless of cycle count, while LiFePO4 retains capacity for a decade or more.
Can I mix LiFePO4 and AGM batteries in the same bank?
No. Different voltage curves, charge profiles, and internal resistance mean the lithium battery does all the work while the AGM sits partially charged — accelerating sulfation. Series connections unbalance the string. Replace the entire bank when upgrading.
What size LiFePO4 battery replaces a 200Ah AGM bank?
A 100Ah LiFePO4 provides roughly the same usable energy as a 200Ah AGM bank (80–100Ah usable vs 100Ah usable at 50% DoD). Verify the LiFePO4 battery's continuous and surge discharge ratings if your AGM bank was sized for surge current like starting a well pump. Most 100Ah LiFePO4 batteries are rated for 100A continuous and 200A surge.