Size your DIY lithium-ion battery pack. Enter your cell configuration and get instant energy, voltage, and state-of-charge calculations.
Uses standard 18650 Li-Ion cell specs: 3.7V nominal, 4.2V full, 2.8V cutoff.
Multiply cells in series (S) by 3.7V to get nominal pack voltage. Multiply cells in parallel (P) by your cell capacity in mAh to get pack capacity. Total energy in watt-hours = (S × 3.7) × (P × mAh ÷ 1000). For example, a 7s4p pack with 3000 mAh cells gives 25.9V × 12 Ah = 311 Wh. The calculator above runs all this automatically and shows voltage, capacity, and SOC levels.
A 1 kWh DIY powerwall typically uses ~120 cells (e.g., 14s9p with 3000 mAh cells = 1.13 kWh). A 5 kWh wall needs around 600 cells. Most DIY builders start with a 7s or 14s configuration to match 24V or 48V battery chargers and inverters. Use the calculator above to see common configurations from 7s to 16s.
A 13s pack has nominal voltage of 48.1V (13 × 3.7V), with full charge at 54.6V (13 × 4.2V) and minimum at 36.4V (13 × 2.8V). 13s is popular for e-bikes and matches the typical 36V “48V” naming convention used in the EV battery world. The calculator displays state-of-charge voltages for any series count.
Yes. A BMS (Battery Management System) is mandatory for any lithium pack. It protects against overcharge (>4.2V/cell), over-discharge (<2.5V/cell), short-circuit, overcurrent, and cell imbalance. Match the BMS S-count to your pack (e.g., 14s pack needs a 14s BMS) and rate it for your maximum expected current with 25% headroom.
No — soldering overheats the cell internals and can damage the separator, increasing the risk of fire. Always spot-weld nickel strip to the cell terminals using a battery spot welder. Quality spot welders for DIY use cost $80–$200 and are essential for safe pack assembly.
Salvaged cells from old laptop batteries can work but require capacity testing and internal-resistance matching before assembly. Mismatched cells in parallel cause uneven loading and reduce pack life. Most DIY builders test every cell with a hobby charger or dedicated tester (e.g., Opus BT-C3100) and group within 50 mAh tolerance. New cells from reputable suppliers eliminate this work.