Battery Pack Builder
Design custom Li-Ion packs (S/P configuration).
Watch: How to design a Li-Ion battery pack from scratch
The S/P notation is easy to read on paper and easy to get wrong on a workbench. A video showing real 18650 cells being spot-welded into series groups, then wired in parallel, makes it obvious why cell matching, nickel strip sizing, and BMS placement matter as much as the arithmetic.
Suggested video topics to film or source for this page:
- Spot-welding a 13S4P pack from matched 18650 cells, group by group
- Wiring a BMS balance lead harness and explaining what each tap does
- Measuring individual cell voltages before and after balancing
- Comparing a 4P block's discharge current against a single cell's rating on a load tester
Designing a custom battery pack
Off-the-shelf batteries come in fixed voltages and capacities, and most projects don't match one exactly. When that happens, the answer is to build a custom pack from individual cells, commonly 18650 or 21700 Li-Ion cells, arranged to hit a specific voltage and capacity target.
Pack layouts are described with "S" and "P" notation, as in 10S4P. S stands for series, which sets the voltage. P stands for parallel, which sets the capacity. This is the same design process used for e-bikes, RC drones, power walls, and electric skateboards, just scaled up or down depending on how much energy the application needs.
Configuration formulas
Cells are arranged in blocks to reach the target voltage and capacity:
Series Count (S) = Target Voltage / Cell Voltage
Parallel Count (P) = Target Capacity / Cell Capacity
Worked example: 48V e-bike battery
Using standard 3.6V, 2500mAh cells to build a 48V, 10Ah pack:
| Step | Calculation | Result |
|---|---|---|
| Series steps | 48V / 3.6V = 13.3, rounded to 13S | 46.8V nominal |
| Parallel groups | 10Ah / 2.5Ah | 4P |
| Total cells | 13 × 4 | 52 cells |
Applications
| Use case | How S/P configuration is used |
|---|---|
| Personal electric vehicles | Custom packs for Onewheels, scooters, and e-bikes, tuned for range or torque |
| Power walls | Storing solar energy in large parallel blocks of recycled high-capacity cells, sometimes 14S80P or bigger |
| Robotics | High-discharge packs (high C-rating) for combat robots and industrial AGVs |
Frequently asked questions
Do I need a BMS?
Yes, without exception. A Battery Management System is mandatory for lithium packs. It monitors each series group to prevent overcharging, which is a fire risk, and overdischarging, which permanently damages the cells. It also balances the cells so the pack stays healthy over its life.
Can I mix old and new cells?
No. A pack is only as strong as its weakest cell. Mixing capacities means the weaker cell drains first, which triggers the BMS to shut the whole pack down early. Use matched cells from the same batch, ideally with similar internal resistance and cycle history.
What is current capacity, or C-rating?
Parallel connections add up current capability. If one cell handles 10A continuous discharge, a 4P block handles 40A. This matters most in high-power applications like electric vehicles and combat robots, where the pack needs to deliver a large burst of current without sagging.
Why round the series count instead of hitting the target voltage exactly?
Cell voltage comes in fixed steps (3.6V or 3.7V nominal per cell for most Li-Ion chemistries), so an exact voltage match is rarely possible. Designers round to the nearest series count and work with the resulting nominal voltage instead, since most downstream electronics (motor controllers, chargers) tolerate a range rather than a single fixed value.
How do I calculate total pack energy?
Multiply the pack's nominal voltage by its total capacity in amp-hours: Energy (Wh) = Voltage (V) × Capacity (Ah). For the 13S4P example above, that's roughly 46.8V × 10Ah, close to the 481.0 Wh shown in the calculator.