Battery Pack Configuration Calculator

A physical pack can only be built from whole cells -- you can't wire in three-quarters of a cell -- so if your target voltage or capacity falls between two multiples of what one cell (or one series string) provides, the pack has to use the next whole number of cells, which pushes the actual delivered voltage or capacity slightly above the target rather than landing exactly on it. That's normal and expected in real pack design, not a rounding error to correct for.

Every cell in a series string carries the identical current, since they're all part of one continuous electrical path, but individual cells can still drift apart in state of charge over repeated cycles due to small manufacturing differences. Without active balancing (typically provided by a battery management system), the weakest cell in the string can be driven to full charge or full discharge well before the others reach their limits, risking damage to that cell even while the pack as a whole appears to be within its normal voltage range.

Cells wired in parallel are all held at the same voltage, but if their capacities or internal resistances differ, a higher-capacity or lower-resistance cell can push current into a weaker neighbor even while the pack is just sitting at rest -- a circulating current the pack's own outer circuit never sees. That circulating current generates localized heat inside the pack, which for lithium cells is a real contributor to the conditions that precede thermal runaway, so parallel groups are normally built from cells matched in capacity and resistance before assembly.

No -- cost per kWh is purely a function of total cell cost divided by total energy stored, both of which are driven by cell price and cell electrical capacity/voltage. Cell weight matters for the pack's total mass (important for vehicle or portable applications) but has no relationship to how much energy the pack stores per dollar spent, so it leaves cost-per-kWh completely unaffected.