Calculator KB-02
Battery Pack Calculator
Calculate the voltage, capacity, and energy of a pack made from individual cells, and sort used cells into groups.
Result
| Group | Cells | Total |
|---|
In brief: Calculate an S×P battery pack using 18650, 21700, and LiFePO4 cells: voltage, capacity, energy, current, and weight. Arrange cells of different capacities into parallel groups. Example: Pack: 4S3P · 14.4 V · 9 Ah.
Calculation example
| Cells | 18650 Li-ion |
|---|---|
| Series, S | 4 pcs |
| Parallel, P | 3 pcs |
| Cell capacity | 3000 mAh |
| Cell current | 10 A |
| Rating | 3.6 V |
| Charge to | 4.2 V |
| Discharge to | 3 V |
| Cell weight | 48 g |
| Pack | 4S3P · 14.4 V · 9 Ah |
|---|---|
| Nominal voltage | 14.4 V |
| Full charge / discharge | 16.8 / 12 V |
| Capacity | 9 Ah |
| Energy | 130 Wh |
| Maximum pack current | 30 A |
| Power | 432 W |
| Cell weight | 576 g |
How it is calculated
C = P · Ccells
E = S · P · Ucells · Ccells
Series connection adds voltage; parallel connection adds capacity and allowable current. A pack made up of groups works only as long as its weakest group does, so it’s worth balancing the group capacities: the calculator sorts the cells from largest to smallest into the group with the lowest total, then looks for swaps that reduce the spread.
What to look for
- A lithium battery pack needs a BMS rated for the right number of S and current. It monitors the voltage of each group and protects against overcharging, over-discharging, and short circuits.
- Before assembling, balance the cell voltages in each parallel group: a voltage difference causes a large equalization current when the cells are connected.
- Use capacity and current values from the datasheet or measure them yourself: cheap cells often have wildly inflated figures on their labels.
Previous version of the tool (rePackr)
Frequently asked questions
What does 4S3P mean?
4 groups in series, with 3 cells in parallel in each group. Using 18650 cells rated at 3 Ah gives 14.4 V and 9 Ah, or about 130 Wh.
How do I match used cells for a pack?
Measure the capacity of each cell and sort them into parallel groups so their totals are as close as possible: the weakest group limits the whole pack. It’s best to reject cells with high internal resistance.
Does a lithium battery pack need a BMS?
Yes. A BMS monitors each group and protects against overcharging, over-discharging, and short circuits. Choose one based on the number of series groups (S), with some headroom for current.
