Configure the pack
Series (S) adds voltage. Parallel (P) adds capacity.
Configuration: 3S2P, 6 cells total.
Resulting pack
What the pack does, and what your electronics must tolerate.
The formulas
Pack capacity = Cell capacity × P
Pack energy = Pack voltage × Pack capacity ÷ 1000
Cell count = S × P
Series adds voltage, parallel adds capacity
Wire cells in series and their voltages stack while capacity stays put: three 3.7V 3000mAh cells give 11.1V at 3000mAh. Wire them in parallel and the opposite happens: 3.7V at 9000mAh. The total energy is identical - 33.3Wh either way - because you have not changed how much is stored, only the shape it comes out in.
That choice is not arbitrary. Higher voltage means less current for the same power, which means thinner wires and lower losses. It is why e-bikes and tools run 36V or 48V packs rather than enormous parallel banks.
The voltages that actually bite
Nominal voltage is an average, and it is not the number that breaks things. A 3S Li-ion pack is nominally 11.1V, but it leaves the charger at 12.6V and is empty around 9.0V. If your electronics cannot take 12.6V, the pack will destroy them on day one regardless of what the label says. Size your electronics against the charged voltage and your low-voltage cut-off against the empty voltage - never against nominal.
Matching cells is not optional
Cells in a pack must be the same chemistry, capacity, brand, and ideally the same age and batch. In series, the cells share current but not capacity, so the weakest cell empties first. Keep pulling and the remaining cells drive current backwards through it - reverse charging - which destroys it, and with lithium can start a fire. This is the same reason we say never to mix cells in a device, only it matters far more here.
Common questions
What does series and parallel do to a battery pack?
Series adds voltage and keeps capacity the same. Parallel adds capacity and keeps voltage the same. A 3S2P pack of 3.7V 3000mAh cells is 11.1V and 6000mAh, using six cells. Total energy in watt-hours is the same either way - only the voltage and current change.
What does 3S2P mean?
It describes the arrangement. 3S means three cells in series, doubling to 11.1V from 3.7V cells. 2P means two of those series strings in parallel, doubling the capacity. So 3S2P is six cells: 11.1V at twice the single-cell capacity.
Can I use different cells in the same pack?
No. Cells in a pack must be the same chemistry, same capacity, same brand and ideally the same age and batch. Mismatched cells drift apart in charge level, and in series the weakest cell gets over-discharged and reverse-charged by the others. With lithium that is a fire risk, not just a performance problem.
Do I need a BMS?
For any lithium pack, yes. A battery management system balances the cells, and cuts off on over-charge, over-discharge, over-current and over-temperature. Series lithium cells drift out of balance in normal use, and an unbalanced lithium pack is a genuine fire hazard. Building a lithium pack without a BMS is not a corner worth cutting.
Next: convert your pack to watt-hours for travel, or estimate how long it will run.