Battery Pack Calculator
Estimate a practical battery configuration (series/parallel), total cells, usable energy, and runtime.
How to Use This Battery Pack Calculator
Designing a battery pack means balancing voltage, capacity, current delivery, runtime, and safety. This calculator helps you quickly size a pack and identify a realistic cell arrangement before moving into detailed engineering.
The main output is the pack configuration in S (series) and P (parallel). For example, 13S7P means 13 cells in series and 7 cells in parallel, for a total of 91 cells.
Core Concepts You Need to Know
1) Series cells (S) set voltage
When cells are connected in series, voltage adds while amp-hours stay the same. If each cell is 3.6V nominal and you place 13 in series, your pack is about 46.8V nominal.
2) Parallel cells (P) set capacity and current capability
When cells are connected in parallel, capacity and current capability add while voltage stays the same. If each cell is 3Ah and you place 7 in parallel, that group is 21Ah.
3) Watt-hours (Wh) describe energy
Energy is what determines runtime. A simple estimate is:
- Energy (Wh) = Voltage (V) × Capacity (Ah)
Because real systems have losses and you usually avoid full 0–100% cycling, usable energy is less than nominal energy.
4) Depth of discharge (DoD) and efficiency matter
Two packs with the same nominal Wh can deliver different real runtime depending on:
- How deep you cycle the pack (DoD policy)
- Controller, wiring, and conversion losses
- Temperature and cell aging
What the Calculator Is Doing Behind the Scenes
- Series count: rounds up target voltage ÷ cell nominal voltage.
- Parallel count (capacity-based): adjusts requested Ah for DoD and efficiency, then divides by cell Ah.
- Parallel count (current-based): divides required pack current by cell continuous current.
- Final parallel count: uses the larger of the two, so both capacity and current demands are met.
This approach prevents the common mistake of sizing only for energy while ignoring discharge stress.
Example Scenario
Suppose you want a 48V-class system, 20Ah usable capacity, and 40A continuous output. You choose a Li-ion cell rated at 3.6V nominal, 3Ah, and 10A continuous.
- Series count ≈ 48 ÷ 3.6 → 13S
- Capacity may suggest about 10P (depending on DoD and efficiency)
- Current requirement gives 40 ÷ 10 → 4P minimum
In this case, capacity is the binding constraint, so you would pick the larger P value. The calculator handles that automatically and reports total cells, nominal/usable Wh, and runtime estimate for your average load power.
Battery Chemistry Tips
Li-ion (NMC/NCA)
- Higher energy density
- Common in e-bikes and portable systems
- Usually around 3.6–3.7V nominal per cell
LiFePO₄
- Excellent cycle life and thermal stability
- Lower nominal cell voltage (~3.2V)
- Often preferred for stationary storage and safety-focused builds
LTO
- Very long cycle life and strong low-temperature performance
- Lower nominal voltage (~2.3V), so higher series counts are needed
- Great for high-reliability applications
Safety and Engineering Checklist
- Use a BMS rated above your continuous current demand (with margin).
- Design for peak current, not only average current.
- Add fusing and short-circuit protection.
- Plan thermal paths and ventilation.
- Match cells by model, age, and condition.
- Never skip insulation, spacing, and mechanical retention.
Common Mistakes to Avoid
- Sizing by voltage only and ignoring capacity.
- Sizing by energy only and ignoring discharge current.
- Assuming full nameplate capacity is always usable.
- Ignoring efficiency losses in real operating conditions.
- Under-rating the BMS and busbars.
Final Note
This calculator is ideal for early planning and comparison. Before purchasing cells or assembling hardware, validate your design with detailed electrical and thermal analysis, vendor datasheets, and relevant safety standards. A good pack is not just powerful—it is stable, serviceable, and safe over its full lifecycle.