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.
| Config | Li-ion Nominal | LiFePO4 Nominal | Common Use |
|---|---|---|---|
| 1S | 3.7V | 3.2V | LED torch, IoT sensor |
| 2S | 7.4V | 6.4V | RC car, small robot |
| 3S | 11.1V | 9.6V | Drone, 12V systems |
| 4S | 14.8V | 12.8V | Lead-acid replacement |
| 7S | 25.9V | 22.4V | E-scooter (24V) |
| 10S | 37.0V | 32.0V | E-bike (36V) |
| 13S | 48.1V | 41.6V | E-bike (48V) |
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.
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Pre-built 3S1P pack with BMS – skip the assembly for simple projects
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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:
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2S 20A BMS for small packs up to 20A draw
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Price: ₹88
3S 40A BMS with balance charging
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Price: ₹39
1S 12A BMS for single-cell projects
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Online Calculator Approach
Use this simple formula approach:
- Determine required voltage → choose series count (S)
- Determine required capacity (Ah) → choose parallel count (P) = Required Ah / Cell Ah
- Total cells = S x P
- Total energy (Wh) = Nominal voltage x Capacity (Ah)
- Max continuous current = Cell CDR x P
Frequently Asked Questions
How many 18650 cells do I need for a 48V 20Ah e-bike battery?
For a 48V pack using Li-ion: 13S (for 48.1V nominal). For 20Ah with 3000mAh cells: 20/3 = 7P (rounded up). Total cells: 13 x 7 = 91 cells.
Can I add more cells to an existing battery pack later?
Adding parallel cells to an existing pack is risky. New cells have different internal resistance and capacity than aged cells. This causes uneven current distribution and accelerated degradation. It is better to build a new pack.
What happens if one cell fails in a pack?
In a series pack, one failed cell can bring down the entire pack. In a parallel group, one shorted cell can overload the others. This is why BMS protection and cell matching are critical.
Ready to Power Your Next Project?
Shop 18650 cells, BMS boards, battery holders, and nickel strips to build your custom battery pack at Zbotic.in.
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