LFP pack and state-of-charge calculator

Build a pack from cells in series and parallel, then see how a small voltage error becomes a state-of-charge error on a flat LFP curve.

Read this first. Defaults reproduce the page example: 3.2 V per LFP cell (CSTEP 2020; Victron sheet), 51.2 V, and a 60 Ah cell capacity that we inferred from a Sunsynk sheet (3.07 kWh at 51.2 V). The 10 mV error is invented. The 0.77 mV per % gradient is our arithmetic from one paper's iron-phosphate cell at 23 °C between 10% and 70% depth of discharge. Real cells and ageing differ. Use a dot for decimals; "10,000" is read as ten thousand.

1. Cells in series and pack energy

Target nominal pack voltage (V) Nominal cell voltage (V) Cell capacity (Ah) Strings in parallel (whole number)

Result	Value
Cells in series for the target (voltage ÷ cell voltage)	-
Whole-cell options either side of the target	-
Nearest option: pack voltage	-
Nearest option: pack capacity	-
Nearest option: nominal energy (volts × amp-hours)	-

2. Voltage error to state-of-charge error

Voltage error on one cell (mV) Curve gradient (mV per 1% of charge)

Result	Value
State-of-charge error from voltage alone (mV ÷ gradient)	-
Same error if the curve were ten times steeper (illustrative)	-

Method: series cells = pack volts ÷ cell volts, rounded to whole cells; energy = pack volts × amp-hours × strings. State-of-charge error = voltage error ÷ gradient, in the range where that gradient holds; it changes along the curve. This is a sizing aid, not a battery design tool: the maker sets cell count, limits and protection.