Battery Pack Calculator: Series, Parallel, and Mixed

Building a custom battery pack requires understanding how series and parallel connections affect voltage, capacity, and current capability. This guide walks you through the calculations for designing battery packs for e-bikes, solar storage, power banks, and robotics projects, with practical examples and component recommendations.

Series vs Parallel Basics

There are only two ways to connect battery cells:

• Series (S): Positive of one cell to negative of the next. Voltage adds up, capacity stays the same. 2S = 7.4V, 3S = 11.1V, etc.
• Parallel (P): All positives connected together, all negatives connected together. Capacity adds up, voltage stays the same. 2P = double the mAh.

A pack configuration is written as [S]S[P]P. For example, 3S2P means 3 cells in series, with each series position doubled in parallel. Total cells = S x P.

Calculating Pack Voltage

Pack voltage depends only on the number of series cells and the cell chemistry:

• Li-ion/LiPo: Nominal 3.7V per cell, fully charged 4.2V, empty 3.0V
• LiFePO4: Nominal 3.2V per cell, fully charged 3.65V, empty 2.5V

Example: A 13S Li-ion pack: Nominal = 13 x 3.7V = 48.1V. Fully charged = 13 x 4.2V = 54.6V. Empty = 13 x 3.0V = 39.0V.

Calculating Pack Capacity

Pack Capacity (Ah) = Single Cell Capacity x Number of Parallel Cells

Pack Energy (Wh) = Pack Voltage (nominal) x Pack Capacity (Ah)

Example: A 10S4P pack using Samsung 30Q cells (3000mAh each): Capacity = 3.0Ah x 4 = 12Ah. Energy = 37.0V x 12Ah = 444Wh. Total cells = 10 x 4 = 40 cells.

Mixed Configurations Explained

Mixed configurations combine series and parallel. Build parallel groups first, then connect groups in series. Each parallel group must use identical, capacity-matched cells. The BMS balances between series groups during charging.

Example – 3S2P Power Bank: Use 6x 18650 cells. Make three pairs of 2 cells in parallel (2P), then connect these three pairs in series (3S). Result: 11.1V nominal, 4400-6000mAh depending on cell capacity.

Common Pack Configurations for Indian Projects

• Power bank (5V USB): 1S2P or 1S3P with boost converter to 5V. Simple, cheap, safe.
• Drone (11.1V): 3S1P LiPo (buy pre-made) or 3S1P Li-ion for longer flight time at lower power.
• Raspberry Pi UPS: 1S2P with 5V boost converter, or 2S with buck converter.
• E-bike 36V 10Ah: 10S4P using 2500mAh cells (40 cells total).
• Solar inverter 48V 20Ah: 15S7P LiFePO4 or 13S7P Li-ion (91 cells total).

BMS Selection for Your Pack

Every multi-cell battery pack needs a Battery Management System (BMS). The BMS must match your pack’s series count and maximum current draw:

Online Calculator Approach

Use this simple formula approach:

1. Determine required voltage → choose series count (S)
2. Determine required capacity (Ah) → choose parallel count (P) = Required Ah / Cell Ah
3. Total cells = S x P
4. Total energy (Wh) = Nominal voltage x Capacity (Ah)
5. Max continuous current = Cell CDR x P