Building a battery pack from cells? Find out what it can pull.
Building a solar storage bank, an e-bike battery, or a power-tool pack? Pick the cell type, set how many in series (S) and in parallel (P): the calculator returns pack voltage, actually usable energy, runtime under your load, and total weight.
Works for Li-Ion 18650 / 21700, LiFePO4 (solar storage), NMC pouch (EV builds), AGM / SLA (UPS systems), and NiMH (rechargeable AA/AAA). Each chemistry has different safe limits: we load sensible defaults, and Advanced mode unlocks fine control.
Example: 4 LiFePO4 280Ah cells in series = 12.8V × 280Ah = 3.6 kWh. Of which 3.4 kWh is usable (95% DoD). That runs a 100W camping fridge for 34 hours.
How to use it
- Pick a cell type from the list (e.g. Li-Ion 18650, LiFePO4, AGM). Defaults, voltage, capacity, safe limits, load automatically.
- S = in series (raises voltage). P = in parallel (raises capacity). Example: 4S2P = 4 cells in series × 2 banks side by side = 8 cells total.
- Read pack voltage (V), capacity (Ah), and total energy (Wh / kWh) from the big summary card.
- Usable energy (Wh), the energy you can actually pull without damaging the pack. AGM gives only 50%, LiFePO4 up to 95%: the gap is huge.
- Enter your load (W), the calculator returns runtime in hours. Enter charger current (A), get charge time.
- Switch to Advanced for: custom DoD, custom cell (V, g, Ah), C-rate, and a warning when load exceeds the pack's safe continuous output.
- The S × P grid visualization shows the physical cell layout. Helps with enclosure planning.
When this is useful
Six typical situations where this calculator gives you a ready answer:
- Solar / off-grid storage. You have a 4 kWp PV array and want a battery for nighttime use. Plug in: 16S LiFePO4 280Ah = 51V × 280Ah = ~14 kWh, of which 13.4 kWh is usable. Pairs with a 48V inverter, done. Decision in 30 seconds.
- E-bike battery. 250W motor, target 50 km range. 13S4P Samsung 30Q (3.0Ah) = 48V × 12Ah = 580Wh. At ~12 Wh/km = 48 km. Short? Bump to 13S5P = 60 km. Hard numbers instead of guessing.
- Power-tool pack rebuild. Replacing a worn-out 18V drill pack. 5S2P Li-Ion 18650 = 18.5V × 6Ah = 111Wh. Stronger than the original at a fraction of the cost.
- Calling out a UPS vendor. You bought a UPS rated "1500VA / 2 hours at 200W". Open it up, find 2× AGM 12V 9Ah inside. Math: 24V × 9Ah × 50% DoD = 108Wh. At 200W = 32 minutes. The vendor lied by 4×. Refund time.
- DIY Powerwall. Building a home battery from 280 salvaged 18650 laptop cells. 80S × 3.5P (averaged) = 300V × 10.5Ah = ~3 kWh. Weight check: 47g × 280 = 13 kg. Fits in a closet.
- Inverter sizing. You have a pack and want the max continuous power. C-rate × capacity × voltage. LiFePO4 1C: 48V × 100Ah × 1 = 4800W. Enough to run AC and a kettle at the same time.
Related: spec a desktop UPS in the UPS runtime calculator, check whether the pack fits a solar setup in the solar + storage calculator, and convert the load into a money figure with the electricity cost calculator.