Battery Series-Parallel Calculator | Size 18650 & LiPo Packs in One Step
How many cells, and in what arrangement, gets you the voltage and runtime you need? Enter the series count (S) and parallel count (P) and read total voltage, total capacity, energy in Wh, and max discharge current, with a wiring diagram.
💡 About This Tool
Whether you are building an e-bike pack, a drone battery, a portable power station, or a DIY power bank, the first decision is always the configuration: how many cells in series, how many in parallel. Series (S) adds the cell voltages to raise the pack voltage; parallel (P) adds the capacities to extend runtime. Doing that arithmetic in your head, while also tracking energy and current limits, is error-prone.
Enter one cell's voltage, capacity, and C-rate, then set S and P. The tool reports the whole pack at once. Three 3.7 V cells in series give 11.1 V at the same capacity as a single cell; put two of those strings in parallel and the capacity doubles, and so does the max discharge current. Energy (Wh) is total voltage times total capacity, the same figure printed on power banks and used for airline limits. The diagram draws the cell layout with positive (pink) and negative (cyan) terminals so you can sanity-check the wiring.
🧐 Frequently Asked Questions
Q. What is the difference between series and parallel? Series adds cell voltages (capacity stays the same). Parallel adds cell capacities (voltage stays the same). Series-parallel combines both to raise voltage and capacity together.
Q. What does "3S2P" mean? Three cells in series, two such strings in parallel. That is six cells: the voltage of three cells and the capacity of two. The tool shows this label in the configuration field.
Q. How is max discharge current calculated? Cell capacity (Ah) times C-rate times the parallel count. A 3000 mAh, 10C cell in 2P gives 3 x 10 x 2 = 60 A. A pure series string keeps a single cell's current rating.
Q. Why does energy in Wh matter? Airline carry-on limits and product labels are stated in watt-hours. Wh = total voltage (V) x total capacity (Ah); the tool converts your mAh input to Wh automatically.
Q. Which cell voltage should I enter? Use the nominal voltage: 3.7 V for Li-ion/LiPo, 3.2 V for LiFePO4, 1.2 V for NiMH. Entering the nominal value (not the fully charged 4.2 V) gives results closer to real running averages.
📚 Fun Facts
The "18650" name simply encodes the cell's size: 18 mm diameter, 65.0 mm long. These cylindrical cells powered laptops for two decades, and most laptop packs were wired as 3S2P or 3S3P, neat rows of identical cells. The whole idea of stacking cells in series for voltage traces back to the voltaic pile of 1800, the first device that piled discs to add up voltage. Modern packs pair that arrangement with a Battery Management System (BMS) that balances each series group so no single cell drifts out of range.