Power
Battery chemistry voltage reference
Pick a chemistry and a cell count to get the pack level voltages: full charge, nominal, storage, and where to stop discharging.
Parallel cells raise capacity, not voltage, and do not change these figures.
Per cell figures
| Chemistry | Charge | Nominal | Cut off | Storage |
|---|---|---|---|---|
| Li-ion / LiPo | 4.2 V | 3.7 V | 3 V | 3.8 V |
| LiFePO4 | 3.65 V | 3.2 V | 2.5 V | 3.3 V |
| NiMH | 1.45 V | 1.2 V | 1 V | 1.2 V |
| Lead acid | 2.4 V | 2 V | 1.75 V | 2.13 V |
3S Li-ion / LiPo pack
11.1V nominal
Full at 12.6 V, empty at 9 V. That is 3.6 V of swing for a voltage based gauge to work with.
Charge to
12.6V
Nominal
11.1V
Stop discharge at
9V
Store at
11.4V
Usable swing
3.6V
Caution
Do not charge lithium below 0 °C
Charging a lithium cell below freezing plates metallic lithium on the anode instead of intercalating it. The capacity loss is permanent and the plating can eventually short the cell internally. A solar node left outdoors needs a temperature sensor on the pack and a charger that will refuse to run when it is cold. LiFePO4 handles the outdoor temperature swing better than LiPo overall, which makes it the safer default for a node in the field, but this rule applies to it too.
Caution
These are cell level figures, not your cell
Every number here is the per cell figure multiplied by 3. Real cells differ: some Li-ion chemistries charge to 4.35 V, some high drain cells cut off at 2.8 V rather than 3.0 V. Confirm against the datasheet of the cell you bought and set the BMS to that, not to this.
Note
Worth knowing
A pack left at 4.2 V per cell for weeks ages fast. Charge to storage voltage if it is going in a drawer.
Note
Worth knowing
Do not charge below 0 °C. Charging a cold lithium cell plates lithium metal on the anode and the damage does not reverse.
Why this matters
Every one of these numbers is a place a pack gets damaged. Charging a lithium cell below freezing plates metal on the anode and the damage does not reverse. Leaving a LiPo sitting at full charge for a month ages it measurably. Running a lead acid battery flat sulphates the plates and permanently loses capacity. None of these announce themselves at the time; the pack just has less capacity than it did, and nobody connects it to what happened weeks earlier.
Assumptions and sources
- Per cell figures multiplied by the series cell count. Parallel cells raise capacity, not voltage, and do not change any number here.
- These are typical figures for each chemistry, not for your specific cell. Some Li-ion types charge to 4.35 V, and high drain cells often cut off lower than 3.0 V.
- The NiMH charge voltage is approximate because NiMH charging terminates on the small voltage drop at full charge rather than at a fixed voltage. Charging a NiMH pack to a set voltage is the wrong approach.
- Lead acid needs two figures: an absorb voltage to charge at, and a lower float voltage to hold at indefinitely.
- Always confirm against the datasheet of the cell you bought and set the BMS to that.