Series vs. Parallel: The Architecture of a Battery Pack
How to arrange cells to meet your voltage and capacity needs.
When building a battery pack from individual 18650 or 21700 cells, you are the architect. By choosing how to connect those cells, you determine the final 'personality' of the battery. Do you need high voltage for a fast ebike, or high capacity for a long-range solar bank? The answer lies in the balance between Series (S) and Parallel (P) connections.
Series (S) for Voltage
Connecting cells in series (positive to negative) adds their voltages together while the capacity stays the same as a single cell. For example, connecting ten 3.6V cells in series creates a 36V '10S' string. High voltage is essential for power efficiency; higher voltage allows for lower current (Amps) to achieve the same power (Watts), reducing heat in your wires and motor.
Parallel (P) for Capacity
Connecting cells in parallel (positive to positive, negative to negative) adds their capacities together while the voltage remains the same. If you have three 3000mAh cells in parallel, you get a 9000mAh (9Ah) '3P' group. Parallel groups are also the key to current handling; each cell in the group shares the load, allowing the pack to provide more total Amps without overheating any single cell.
The Hybrid: Series-Parallel Packs
Most batteries are built as 'groups of parallel cells, connected in series.' For example, a 13S4P pack (standard for 48V ebikes) means you have 13 groups in series, and each group consists of 4 cells in parallel. This configuration provides the necessary 48V (nominal) and 4x the capacity and current handling of a single cell.
Why 'P' Groups Must Be Identical
Every parallel group in a series string acts as one single 'super-cell.' If one group has 4 cells and another has 5, the smaller group will charge and discharge faster than the others. This leads to the BMS cutting off early and eventually damaging the cells. Always ensure every parallel group in your pack has the exact same number and type of cells.
FAQ
Should I build the parallel groups or the series string first?
Professional builders almost always assemble the parallel groups first, then connect those groups in series. This makes the pack more structurally sound and easier to manage with a BMS.
Does adding more cells in parallel increase the voltage?
No. Parallel connections only increase capacity (Ah) and current handling (Amps). Only series connections increase the voltage (Volts).
What happens if a single cell in a parallel group fails?
If it fails open, the capacity of that group drops, causing the whole pack to become unbalanced. If it fails short, it will draw energy from the other cells in the parallel group, potentially leading to a fire. This is why using fused nickel strips is a high-end safety feature.