Chemistry
| LFP | NMC | AGM / Flooded | |
|---|---|---|---|
| Usable DoD | 90–100% | 85–90% | 50% |
| Cycles | 6,000–10,000 | 3,000–5,000 | 500–1,200 |
| Thermal runaway | Very unlikely | Possible | No (hydrogen gas instead) |
| Cold charging | Blocked below 0°C | Blocked below 0°C | Reduced |
| Cost / usable kWh | Low over life | Medium | Highest over life |
LFP has won residential storage on safety and cycle life. NMC still shows up where energy density per cubic foot matters. Lead-acid survives in small off-grid and telecom because it is cheap to buy and the customer does not price the replacement.
C-rate: the number people forget
Capacity tells you how much energy. C-rate tells you how fast you can take it out. A 14 kWh bank rated 0.5C delivers 7 kW continuous — which is fine until the well pump and the AC start together. Size the bank for energy and power, both.
BMS and comms
- Closed loop — battery and inverter talk over CAN or RS485. The BMS tells the inverter real SOC, cell temps, and charge limits. Always do this when the pair supports it.
- Open loop — inverter guesses SOC from voltage. Voltage on a flat LFP curve is nearly useless between 20% and 80%. Expect wrong SOC readings and premature cutoffs.
Temperature
Charging LFP below freezing plates lithium metal on the anode and permanently kills capacity. A good BMS blocks it. A cheap one does not. If the bank lives in an unconditioned garage in the north, either buy a heated battery or heat the space — do not just hope.
Parallel strings
Batteries in parallel must be the same chemistry, same capacity, same age, and matched in state of charge before you close the disconnect. Paralleling a new bank to a three-year-old bank means the new one carries the load and dies early.