Calculator Inputs
Example Data Table
| Use Case | Cell | Layout | Capacity | Load | Typical Output |
|---|---|---|---|---|---|
| E-bike pack | 3.7 V, 3500 mAh | 13S4P | 14 Ah | 500 W | 48.1 V nominal, about 674 Wh rated |
| Portable station | 3.2 V, 6000 mAh | 4S8P | 48 Ah | 250 W | 12.8 V nominal, about 614 Wh rated |
| Robot battery | 3.7 V, 2500 mAh | 6S3P | 7.5 Ah | 300 W | 22.2 V nominal, about 167 Wh rated |
Formula Used
Cell count = series cells × parallel cells.
Nominal pack voltage = series cells × nominal cell voltage.
Full charge voltage = series cells × full cell voltage.
Pack capacity Ah = parallel cells × cell capacity Ah.
Rated energy Wh = nominal pack voltage × pack capacity Ah.
Usable energy Wh = rated energy × depth of discharge × discharge efficiency × reserve factor.
Cell current limit = parallel cells × max current per cell.
System current limit = minimum of cell limit, BMS rating, and fuse rating.
Pack resistance = series cells × cell resistance ÷ parallel cells.
Voltage sag = load current × pack resistance.
Heat loss = load current² × pack resistance.
Charge time = pack capacity × depth of discharge ÷ charger current ÷ charge efficiency.
How to Use This Calculator
Enter the series count and parallel count first. Add cell voltage, capacity, current rating, resistance, weight, and cost. Enter the expected load power or load current. Add BMS, fuse, efficiency, reserve, and charger details. Press the calculate button. Review voltage, energy, current, sag, heat, runtime, charge time, weight, and cost.
Battery Pack Planning Guide
What This Calculator Does
A lithium ion pack is more than a cell count. Voltage, capacity, current, heat, charging, and safety margins must work together. This calculator joins those parts in one view. It helps size a pack for tools, robots, bikes, storage projects, UPS units, and portable equipment.
Why Pack Layout Matters
Cells in series raise voltage. Cells in parallel raise capacity and current. A 10S4P pack uses ten cells in each string and four strings in parallel. The nominal voltage comes from the series count. The amp hour rating comes from the parallel count. Energy depends on both values.
Runtime And Usable Energy
Nameplate watt hours are not always usable. Real packs need depth of discharge limits. They also lose energy in wiring, electronics, and the load device. Age reserve is useful too. It keeps the design practical after many cycles. For that reason, this tool separates rated energy from usable energy.
Current And Voltage Sag
Load current is a key safety point. A pack may have enough energy but still fail under high current. Each cell has a current rating. Parallel groups multiply that rating. The BMS and fuse can reduce the real limit. Internal resistance also matters. It creates voltage sag and heat during discharge. High sag can cause early cutoff.
Charging And Practical Checks
Charging time depends on pack capacity, charger current, and charge efficiency. A large charger is not always safer. It must match the cell maker rules and the BMS rating. The full charge voltage also rises with each cell in series. Always use a charger built for the exact series count and chemistry.
Design Safety Notes
This calculator is for planning and comparison. It does not replace a qualified battery design review. Lithium packs can start fires when abused. Use matched cells, correct nickel strips, proper insulation, a rated BMS, and a suitable enclosure. Add fuses where needed. Test packs slowly. Stop if cells heat, swell, smell, or drift badly.
Better Inputs
Good inputs improve every estimate. Enter realistic cell data from a current datasheet. Use measured load power when possible. Compare several layouts before building. A small change in parallel count can improve runtime, reduce sag, lower heat, and extend service life greatly.
FAQs
What does 13S4P mean?
It means thirteen cells are connected in series, and four cells are connected in parallel in each group. Series raises voltage. Parallel raises capacity and available current.
How do I choose cell nominal voltage?
Use the value from the cell datasheet. Common lithium ion cells use 3.6 V or 3.7 V nominal. LFP cells often use 3.2 V nominal.
Why is usable energy lower than rated energy?
Usable energy includes depth of discharge, discharge efficiency, and aging reserve. These factors reduce the energy you should plan to use in real service.
Does the BMS increase pack current?
No. The BMS protects the pack. It can limit current. The real safe limit is the lowest value from cell rating, BMS rating, fuse rating, and thermal design.
What voltage should my charger use?
The charger must match the pack series count and chemistry. Multiply full cell voltage by the series count. Always use a charger made for that exact pack type.
Why does voltage sag matter?
Voltage sag reduces output voltage during load. High sag wastes power as heat. It can also trigger early cutoff in the BMS or connected equipment.
Can I mix old and new cells?
Do not mix cells with different age, capacity, resistance, or chemistry. Mismatched cells can drift, overheat, and fail. Use matched cells from a reliable source.
Is this enough for final battery design?
No. This calculator supports planning only. Final packs need datasheet checks, thermal testing, insulation, fusing, BMS validation, enclosure review, and safe build methods.