6.2 The pack

Outcome 2 of 8 in domain 6 · Power, panels, packs, motors, converters

Series and parallel, balancing, BMS, spot welding, casing, rebuilding a tool pack.

My targets

I declare:

Read a designation like 10S3P, calculate the pack’s voltage and capacity, and find that structure in an opened tool pack.
Explain why series cells drift apart and what balancing does, passive or active.
Explain what a BMS protects (overvoltage, undervoltage, overcurrent, temperature) and read its datasheet: continuous current, cutoff current, thresholds.
Match cells (capacity, internal resistance, voltage) before assembling them, and explain why.
Spot-weld nickel strips, choose the thickness for the current, and check the weld.
Rebuild a tool pack: disassemble, record the wiring and the thermistors, replace the cells, reassemble, test under load.
Explain a pack’s mechanical and thermal protections: cell holders, insulators, cell fuses, thermistors, and why a tool pack has so many.
Diagnosis: a pack that cuts out mid-cut, a weak cell that reaches the low threshold first; find it without opening, through the balancing voltages.

Read

There is no Kuphaldt chapter for this: his textbook predates lithium. The useful sources are elsewhere: the datasheet of an 18650 cell (Samsung 25R, Molicel P28A), the datasheet of a commercial BMS (JBD, Daly), and Cadex’s Battery University, BU-302: Series and Parallel Battery Configurations, readable and honest. For spot welding, the forums (Endless Sphere) beat the books.