LiFePO4 vs NMC: what the chemistry decides, in years and in pounds

The ratings, side by side

Across the current units, LiFePO4 units are rated between 2,000 and 6,000 cycles (rated to 70% or 80% of original capacity), and NMC units at figures their makers do not state. Energy density, computed from capacity and weight, has a median of 37.1 Wh per lb for LiFePO4 and 45.1 Wh per lb for NMC. Price per Wh: median $0.63 for LiFePO4, $0.95 for NMC.

Cycle life, and how to read a cycle claim

A cycle is one full discharge and recharge. A rating of "3,000 cycles to 80%" says that after 3,000 full cycles the maker expects the battery to hold at least 80% of its original watt-hours; it does not stop working at cycle 3,001, it keeps fading. The share the rating is to matters as much as the count: a claim rated to 70% is a weaker promise wearing a larger number, and the table shows both parts wherever the maker states them. Partial cycles count in proportion: two half discharges are roughly one cycle.

Turn the count into years by counting your cycles. A van or off-grid office that drains and refills the unit every day runs about 365 cycles a year: an NMC unit rated 500 cycles reaches its 80% point in well under two years, a LiFePO4 unit rated 3,000 in about eight. An outage kit cycled once a month runs 12 cycles a year and neither chemistry's count is the limit; calendar ageing and how it is stored are. A weekend camper at 30 to 50 cycles a year sits in between, where NMC's rating lasts a decade and the choice can be made on weight.

Weight and size

NMC's higher energy per pound is why it survives in compact units meant to be carried, and why the lightest unit in a class is sometimes NMC. The medians above put the gap at roughly 21% per pound across the tracked table, though the inverter, case and ports weigh the same whatever the cells, so the gap narrows on small units and widens on large ones. For a unit that lives in a closet or a van, weight barely matters; for one that goes on a hike, it may decide.

Heat, safety and storage

LiFePO4 cells are more thermally stable than NMC: their makers rate them for higher operating and charging temperatures, and the chemistry is less prone to thermal runaway. Both chemistries lose capacity when stored full and hot; makers' manuals typically say to store at 50% to 80% charge in a cool place and top up every few months. Those manual instructions, not the chemistry, are the biggest lever on how long a rarely used unit lasts.

Storing it, and leaving it plugged in

Both chemistries age fastest when stored full and hot. Makers' manuals say to store at 50% to 80% charge in a cool place and top up every three to six months; a LiFePO4 unit kept that way loses little in a year. Leaving a unit plugged in all the time is what the UPS mode is for: the units that state a switchover time are designed to sit on the wall and hold near full, and their chargers stop at full rather than trickle. Where a maker offers a charge limit in its app (80% or 90%), use it for a unit that lives plugged in. The one thing to avoid with either chemistry is months at 0%.

What it costs

LiFePO4's median price per Wh ($0.63) against NMC's ($0.95) shows that the chemistry premium has largely disappeared from the mainstream: nearly every current mid-size and large unit is LiFePO4 at a mainstream price, and NMC survives mainly in lightweight compacts and in older designs still on sale. An older NMC unit sold at a LiFePO4 price is the one case to look at twice, and the chemistry column in every table makes it visible.

The decision

Daily or near-daily use (van, off-grid work, a UPS that cycles): LiFePO4, and the higher cycle rating the better. Weekend use with a real weight constraint: either, chosen on pounds and price. An outage kit used a few times a year: either, stored per the manual. In every case, buy on fit first (the sizing calculator) and chemistry second; a chemistry that lasts a decade is no use in a unit whose inverter cannot start the fridge.

Units whose pages do not name a chemistry

Anker SOLIX C1000 Gen 2, Jackery Explorer 300 ("Li-ion (unspecified)"), Zendure SuperBase V6400 ("Li-ion (unspecified)"). A chemistry a maker does not state is left blank, never guessed from the cycle count or the weight.