Formula Used
- Vpack = Vcell × S (series increases voltage).
- Ahpack = Ahcell × P (parallel increases capacity).
- Whpack = Vpack × Ahpack (nominal energy).
- Imax,pack = Imax,cell × P × derate (current capability).
- Rpack ≈ Rcell × (S / P) (resistance scales with series/parallel).
- ΔV ≈ I × R and Ploss ≈ I² × R (drop and resistive loss).
How to Use
- Enter your cell voltage and capacity from the datasheet.
- Add per-cell current limit and resistance if known.
- Choose a mode: evaluate an existing S/P or design from targets.
- Enable derating to include margin for heat and aging.
- Press Calculate to see results above the form.
- Export results with the CSV or PDF buttons.
Example Data Table
| Example | Cell Specs | Configuration | Key Outputs |
|---|---|---|---|
| E-bike pack | 3.7 V, 2.5 Ah, 20 A, 35 mΩ | 10S4P, derate 90% | 37.0 V, 10.0 Ah, 370 Wh, Imax 72 A |
| DIY power station | 3.2 V, 6.0 Ah, 10 A, 18 mΩ | 16S2P, derate off | 51.2 V, 12.0 Ah, 614 Wh, Imax 20 A |
| Tool battery | 3.6 V, 3.0 Ah, 30 A, 20 mΩ | 5S2P, derate 85% | 18.0 V, 6.0 Ah, 108 Wh, Imax 51 A |
Examples are nominal and simplified for planning.
Practical Tips
- For lithium packs, BMS limits may be lower than cell limits.
- Use derating when packs are sealed or poorly cooled.
- Long wires and connectors add resistance beyond cells.
- Voltage varies with state-of-charge; nominal is an average.
Nominal Voltage Planning
Series strings set pack voltage by multiplying cell voltage by S. A 3.7 V cell in 10S yields about 37.0 V nominal, while 13S yields about 48.1 V. Most Li-ion cells charge near 4.2 V and cut off near 3.0 V. For LiFePO4, 3.2 V cells in 16S yield about 51.2 V. Use nominal voltage for estimates, then confirm full and empty limits.
Capacity and Energy Sizing
Parallel groups increase capacity linearly, while series does not. If each cell is 2.5 Ah, a 4P group becomes 10.0 Ah, and energy becomes Vpack × Ahpack. The same 10S4P example produces about 370 Wh (37.0 V × 10.0 Ah). Convert to kWh by dividing Wh by 1000. Budget usable energy using depth-of-discharge and efficiency, especially for inverters and motors.
Current Capability and Power
Maximum continuous current typically scales with parallel count. With 20 A cells, a 4P pack can support 80 A before margins. Applying a 90% derate reduces that to 72 A for thermal and aging headroom. Power at nominal voltage is roughly Vpack × Ipack, so 37.0 V at 72 A is about 2.66 kW. Treat pulse ratings cautiously; cooling, enclosure, and cell chemistry change allowable C-rate.
Resistance, Drop, and Loss
Internal resistance affects voltage sag and heat. A useful estimate is Rpack ≈ Rcell × (S/P). With 35 mΩ cells, 10S4P gives about 87.5 mΩ. At 72 A, voltage drop is I×R ≈ 6.3 V, and resistive loss is I²R ≈ 454 W. These values guide worst-case planning. Add busbar, weld, and connector resistance, and expect resistance to rise at low temperature and with aging.
Design Mode Targets and Validation
When designing from targets, the calculator suggests S from targetV ÷ cellV and P from targetAh ÷ cellAh. Rounding up usually prevents undersizing. After selecting S and P, validate mechanical layout, cell balancing, and heat paths. Then test under load to confirm sag, runtime, and protection behavior. Also verify charger compatibility, BMS cutoff thresholds, and final peak voltage for your equipment. Document the configuration as S×P, record total cell count, and keep consistent units (Ah, Wh) so stakeholders can compare designs without confusion across teams easily.
FAQs
1) What does adding more cells in series change?
Series increases pack voltage while capacity stays the same. Higher voltage can reduce current for a given power level, but it raises peak charge voltage and requires BMS and charger settings that match the series count.
2) What does adding more cells in parallel change?
Parallel increases pack capacity and spreads load current across more cells. It can improve runtime and reduce per-cell stress, but it also increases total cell count, wiring complexity, and the importance of good cell matching.
3) How accurate is the Wh and kWh estimate?
Energy is estimated from nominal voltage and rated capacity. Real usable energy is lower due to voltage sag, cutoffs, efficiency losses, temperature, and aging. Treat Wh as a planning baseline, then validate with real discharge tests.
4) Why should I derate the current limit?
Derating adds margin for heat buildup, enclosure constraints, aging, and uneven current sharing. A 10–20% reduction often prevents nuisance trips and extends cycle life, especially when packs operate near maximum load for long periods.
5) How is internal resistance used in this tool?
The calculator scales cell resistance by S/P to estimate pack resistance. It then computes approximate voltage drop (I×R) and resistive heating (I²R). Include connector and busbar resistance separately for closer real-world results.
6) Can design mode guarantee my target voltage and capacity?
Design mode suggests integer S and P values based on targets and your rounding choice. It can meet or exceed targets, but exact matches depend on cell specs, unit conversions, and your selected rounding strategy.