Battery pack builder

Pack — S cells in series multiply the voltage; P in parallel multiply the capacity and current: nominal voltage = S × cell nominal, capacity = P × cell capacity, energy = nominal V × Ah, and maximum continuous current = P × the cell’s rating. 4S2P of 3.7 V / 3000 mAh cells is 14.8 V, 6 Ah, 88.8 Wh. Weight adds about 15% to the cells for strip, holders, BMS, and wrap. The diagram shows the usual layout: each column is a parallel group, joined in series across.

BMS — the same series count and chemistry as the pack, a continuous rating with about 25% headroom over the load, and per-cell over- and under-voltage protection. Passive balancing currents are small, so start with matched cells at the same voltage.

Fuse — about 125% of the maximum continuous load, DC-rated above the pack’s full voltage, with a breaking capacity well above the pack’s short-circuit current (lithium cells can deliver hundreds to thousands of amps). If the fuse rating comes out above what the cells can carry, the layout is wrong.

Nickel strip — resistance = ρ × length ÷ area, with ρ = 6.99×10⁻⁸ Ω·m for pure nickel and about 1.4×10⁻⁷ Ω·m for nickel-plated steel (which runs about twice as hot). The tool gives each link’s voltage drop and heat. There is no single ampacity figure — it depends on the temperature rise you accept — so builders often keep pure-nickel links to roughly 5–10 A/mm² and add layers or a copper bus for high-current packs. Use pure nickel, spot-welded; never solder to cells.