LFP, NMC, and AGM: which battery chemistry for which job

A lithium iron phosphate (LFP) battery and a nickel-manganese-cobalt (NMC) battery look similar on a marketing page. They are both lithium-ion. They both come in tidy boxes. The performance numbers in the spec sheet are close enough that a buyer might call it a tie.

In the real world, they are not the same battery. LFP and NMC behave differently when stored hot, when cycled hard, when discharged below freezing, and when something goes wrong inside the cell. AGM, the older lead-acid technology, sits in a third category with its own use cases.

This guide is a working comparison of the three for the question that matters most: which one belongs in which job.

What each chemistry actually is

LFP (LiFePO₄, lithium iron phosphate). A lithium-ion variant where the cathode is iron phosphate. Trade name "LiFePO4" or "lithium iron phosphate." Lower energy density than NMC, but much better thermal stability, longer cycle life, and a flatter voltage curve.

NMC (NCM, nickel-manganese-cobalt). A lithium-ion variant where the cathode is a mix of nickel, manganese, and cobalt. Higher energy density than LFP. Used in most laptops, most EVs (until 2022), and most older portable power stations.

AGM (absorbed glass mat). A sealed lead-acid battery where the electrolyte is held in a glass mat instead of free-flooded liquid. Lower energy density than either lithium chemistry, but proven in vehicles for forty years and far cheaper per kWh of installed capacity. Doesn't outgas in normal use.

The four numbers that decide the job

Cycle life, energy density, thermal envelope, and shelf life. Each chemistry occupies a distinct spot in this matrix, and the spot tells you what jobs it suits.

The energy-density gap is what gets people excited about NMC. The cycle-life and thermal-stability gap is why they switch to LFP after a year or two.

When LFP is right

LFP is the right answer for a backup-power battery that lives somewhere stable and cycles regularly.

The case for it:

• Cycle life. A 3,000-cycle battery lasts ten years if you cycle it daily. Most backup systems do not cycle daily, so the calendar life dominates. Even there, LFP is rated for ten years of shelf life with periodic top-up.
• Thermal stability. LFP is very hard to push into thermal runaway. The decomposition temperature of the iron-phosphate cathode is above 500°F. NMC's cathode starts decomposing around 400°F. Translated: an LFP cell punctured by a nail vents and gets hot; an NMC cell punctured by a nail can ignite. For a battery in your home, this matters.
• Depth of discharge. You can run an LFP bank to 90-100% depth without significant degradation. AGM degrades fast below 50% depth. NMC can run to 80-90% but sees more cycle-life degradation than LFP at the same depth.
• Voltage curve. Flatter than NMC. The pack delivers near-nominal voltage across most of its discharge, which means inverter efficiency stays high and the "battery getting low" point arrives suddenly rather than gradually. For backup loads, this is mostly a virtue.

Pick LFP if:

• The system lives indoors in a temperate space (basement, garage with insulation, utility closet).
• You cycle it daily or weekly (solar-paired home backup, off-grid cabin).
• You want a 10-year calendar life with minimal babysitting.
• Fire risk in your home is not a tradeoff you want to accept.

Don't pick LFP if:

• Weight is the primary constraint and the unit is portable. NMC is lighter for the same energy.
• The system has to charge below freezing without a heater. LFP cannot accept charge below 32°F. Discharge below freezing is fine.
• Budget is the deciding factor and cycle count is low. AGM may be a better fit at the price.

When NMC is right

NMC remains the right answer when energy density per pound is the constraint that decides everything else. The handful of jobs where this is true:

• Portable power stations under 1 kWh. A 500 Wh portable unit is meaningfully smaller and lighter in NMC than in LFP. For a 72-hour bag chassis, this matters.
• Older inventory. Some portable units shipped with NMC before LFP became dominant in 2023. They work fine; they just have shorter cycle life and worse thermal characteristics.

The case against NMC for stationary backup is straightforward. The thermal runaway risk is real, and a stationary battery does not need the energy density. Pay the size penalty for LFP, gain 3x the cycle life and a much better fire profile.

Pick NMC if:

• The unit is portable (under 1 kWh, in a bag).
• Weight per watt-hour is the constraint.
• The unit is not stored long-term in heat.

Don't pick NMC if:

• The battery is stationary, in a home or shop.
• The bank is sized for daily cycling. Cycle life is the limit, and LFP wins.
• The storage location ever sees above 95°F. NMC degrades faster in heat than LFP and the thermal-runaway risk also rises.

When AGM is right

AGM is the right answer for systems that need cheap installed capacity, do not cycle often, and live in cars, trailers, or unconditioned spaces.

The case for it:

• Cost. Three to five times cheaper per installed kWh than LFP. For a system that needs 4 kWh of capacity for a starter, AGM is half the price.
• Cold tolerance. AGM accepts charge below freezing. The discharge envelope is wider. A vehicle battery in a New England winter is invariably AGM or flooded lead-acid because the alternatives have charge-temperature limits.
• Forgiving with chargers. AGM works with almost any charger, including older PWM solar setups, alternators, and basic chargers from twenty years ago. LFP requires a chemistry-aware charger; the wrong one cuts cycle life in half.

The case against:

• Cycle life. 200-500 cycles to 80% capacity is a fraction of LFP. If you cycle it daily, it is dead in a year.
• Depth-of-discharge limit. Going below 50% DoD damages the battery. The "real" capacity of a 100 Ah AGM is 50 Ah of usable energy. LFP at the same nameplate gets you 90 Ah usable.
• Weight. Five to seven times heavier than LFP for the same usable capacity. For stationary use, this does not matter. For anything portable, it does.

Pick AGM if:

• The bank is for a vehicle, trailer, or boat where cold tolerance and cost matter most.
• Cycle count is low (a winter generator that runs ten times a year).
• You have a 12V system and 12V is not getting replaced soon.

Don't pick AGM if:

• The system cycles regularly. The cycle-life gap to LFP overwhelms the upfront cost difference within two years.
• Floor space is constrained. AGM at the same usable capacity is several times bigger and heavier.
• You want to run loads to "empty" without maintenance windows.

The thermal runaway question, in plain terms

The reason this guide treats LFP as the default for stationary home backup: LFP cells are hard to ignite from internal failure. NMC cells are not.

In testing (UL 9540A, the standard for stationary energy storage), LFP packs vent and smolder on internal short. NMC packs of similar size will, depending on the specific cell chemistry and pack design, propagate flame from cell to cell, vent flammable electrolyte, and produce sustained fire that fire suppression systems struggle with.

For a portable unit you might lose to fire and replace for $500, NMC is fine. For a 10 kWh wall-mounted home battery in a finished basement, the math changes.

What to actually buy

For a home backup or off-grid solar bank that lives indoors and cycles daily or weekly:

• LFP. No exceptions worth listing here. The cycle life and the fire profile both win.

For a portable power station for a 72-hour bag or weekend trip:

• LFP if the unit is over 500 Wh. Modern LFP portables are good and the weight penalty is acceptable.
• NMC for very small units (under 500 Wh) where pocket-size matters. Acceptable trade-off.

For a vehicle or trailer auxiliary battery, especially one that lives outdoors year-round:

• AGM. The cold-charging envelope and price-per-kWh are decisive. LFP works in the same role only if you add a heater pad and a chemistry-aware DC-DC charger, which adds cost and complexity that AGM does not need.

If you are sizing a backup power station for a home outage scenario right now, LFP is the answer for any unit you can afford. The real input the battery is paired with is the solar that recharges it; the math for winter solar input determines how long the bank actually carries you in a multi-day outage.

What to do this weekend

Three things, in order:

1. Identify the chemistry of every battery you currently own. Power station, car, drill, laptop. If you cannot tell, look for "LiFePO4" or "Lithium Iron Phosphate" on the label (LFP), "Li-ion" without iron-phosphate marking (probably NMC), or "AGM" / "lead acid" on the case.
2. For any LFP or NMC battery stored above 95°F (a hot car, a south-facing balcony, an attic), move it. Heat is the cycle-life killer that everyone underestimates.
3. For any AGM battery in a backup role, check resting voltage with a multimeter. 12.6V or above means full and healthy. 12.4V or below means it is sulfating; charge it. AGMs that sit below 12.0V for more than a few weeks are usually dead.

Battery health is a maintenance habit. The chemistry only decides which maintenance habit you need.