Enter Battery Pack Details
Example Data Table
| Configuration | Cell Rating | Nominal Voltage | Capacity | Raw Energy | Common Use |
|---|---|---|---|---|---|
| 3S2P | 3000 mAh, 10 A | 10.8 V | 6 Ah | 64.8 Wh | Small lights and portable tools |
| 4S3P | 3000 mAh, 10 A | 14.4 V | 9 Ah | 129.6 Wh | Robotics and compact storage packs |
| 7S4P | 3200 mAh, 8 A | 25.2 V | 12.8 Ah | 322.56 Wh | Mobility projects and backup systems |
| 13S5P | 3500 mAh, 10 A | 46.8 V | 17.5 Ah | 819 Wh | Higher voltage electrical builds |
Formula Used
Total cells: Series count × Parallel count
Nominal pack voltage: Series count × Cell nominal voltage
Full charge voltage: Series count × Cell full voltage
Cutoff voltage: Series count × Cell cutoff voltage
Pack capacity: Parallel count × Cell capacity Ah
Raw energy: Nominal pack voltage × Pack capacity Ah
Usable energy: Raw energy × Depth of discharge × Efficiency
Safe current: Parallel count × Cell current × Derating factor
Pack resistance: Series count × Cell resistance ÷ Parallel count
Voltage sag: Load current × Pack resistance
Runtime: Usable watt hours ÷ Load watts
How To Use This Calculator
Enter the rated voltage values for one 18650 cell. Use the cell datasheet when possible.
Add cell capacity, discharge current, charge current, and internal resistance.
Choose how many cells are connected in series and parallel.
Enter your expected load current or load power. Use one value if only one is known.
Set practical values for depth of discharge, efficiency, and current derating.
Press the calculate button. The result appears above the form and below the header.
Use the CSV or PDF button to save the result for project records.
Understanding 18650 Pack Design
An 18650 pack is built by joining cells in series and parallel. Series groups raise voltage. Parallel groups raise capacity and current support. A correct layout protects cells from stress and gives the device stable power. This calculator helps compare those choices before parts are purchased.
Why Series Count Matters
Series count is the first design choice. One lithium ion cell is often rated near 3.6 or 3.7 volts nominal. Four cells in series create a pack near 14.8 volts nominal. The full charge voltage is also multiplied by the same count. That value must match the charger, controller, and battery management system.
Why Parallel Count Matters
Parallel count sets the available amp hours. Three 3000 mAh cells in parallel provide about 9 Ah. It also raises current capability because each cell shares load current. The calculator includes a derating factor, because datasheet current ratings should not be used at the edge in real projects.
Runtime And Energy
Watt hours describe stored energy better than amp hours alone. A high voltage pack with the same amp hours stores more energy than a low voltage pack. Runtime depends on usable watt hours, load power, allowed depth of discharge, and efficiency. High loads can reduce effective runtime because voltage sag and heat rise.
Safety Notes
Use matched cells from the same model, age, and condition. Avoid mixing unknown reclaimed cells. Add a suitable battery management system for balancing, over charge, over discharge, and short circuit protection. Cell holders, nickel strips, fuses, insulation rings, and proper spacing also matter. Spot welding is usually safer than direct soldering to cell ends.
Practical Use
Enter the cell ratings first. Then choose series and parallel counts. Add expected load current or load power. Review voltage range, capacity, pack current, internal resistance, sag, and runtime. Treat the output as planning guidance. Final designs should follow datasheets, enclosure limits, thermal testing, and local electrical rules.
Common Mistakes
Many weak packs fail because the design ignores heat. Current ratings assume good cooling and new cells. Long wires, poor welds, and small connectors waste power. Always leave space for airflow. Test the pack at moderate load before trusting it in expensive equipment or daily transport devices.
FAQs
What does 4S3P mean?
It means four cells are connected in series, and each series group has three cells in parallel. Series raises voltage. Parallel raises capacity and current ability.
Can I mix different 18650 cells?
Mixing different cells is not recommended. Use matched cells with similar capacity, resistance, age, chemistry, and discharge rating. Mixed cells can become unbalanced and unsafe.
Why is derating included?
Derating adds planning margin. Cells may heat up, age, or perform below datasheet ratings. A lower design current can improve reliability and reduce thermal stress.
What BMS should I select?
Choose a BMS with the same series count as the pack. Its current rating should meet or exceed the safe current required by the load.
Why is watt hour value important?
Watt hours combine voltage and capacity. They show stored energy more clearly than amp hours alone, especially when comparing packs with different voltages.
Does voltage sag matter?
Yes. Voltage sag can trigger low voltage cutoffs and reduce usable performance. High current, weak cells, and high resistance connections increase sag.
Should I solder directly to 18650 cells?
Direct soldering can overheat cells. Spot welding nickel strip is usually preferred. Use proper insulation, holders, and safe assembly methods.
Is this calculator enough for final battery design?
No. It is a planning tool. Final designs should include datasheet checks, thermal testing, protection circuits, fuse selection, enclosure review, and safe charging rules.