Batteries Series and Parallel

Batteries in series vs parallel: voltage and capacity

Wire batteries in series and the voltages add up, but capacity (Ah) stays put. Example: take two AA NiMH cells (1.2 V, 2000 mAh) and in series you get 2.4 V @ 2000 mAh. Wiring them in parallel flips that around. Now the capacities add and the voltage holds, so those same two cells give 1.2 V @ 4000 mAh. Either way the total energy (Wh) comes out the same, since Wh = V · Ah. Critical safety note: with lithium-ion (Li-ion / LiPo) packs, a BMS (Battery Management System) is mandatory. It balances the cells, blocks over-charge and over-discharge, and guards against short circuits. Never put Li-ion cells in series without one.

Applications: power banks, e-bikes, power tools

Power banks usually parallel a bunch of 18650 Li-ion cells to pile up mAh at 3.7 V, then boost-convert that to 5 V for USB. E-bike packs mix both layouts: a "13S4P" pack runs 13 cells in series (48 V nominal) with 4 in parallel at each stage. Cordless power tools such as the Makita 18 V put 5 cells in series (5 × 3.6 V) and add parallel groups when they need more current.

FAQ

Can I mix old and new cells? Don’t. The weakest cell ends up setting the pace for the whole pack, and in series it can even get reverse-charged, which is dangerous. Stick to cells that match in brand, chemistry, age, and capacity.

How long will my pack last? A rough estimate is Runtime ≈ Ah / load current. So a 2 Ah pack feeding a 0.5 A load runs about 4 hours, before you account for efficiency losses.

Series or parallel for more range? Go parallel: same voltage, more Ah. Series buys you speed and power through higher voltage, but at the same power draw it won’t make the pack last any longer.