A pack is described as xSyP — x cells in series, y strings in parallel. Series sets voltage, parallel sets capacity, and the product sets energy.
To build a nominal 48 V, 100 Ah pack from 3.2 V LiFePO4 cells: 48/3.2 = 15 cells in series. If the cells are 25 Ah, four strings in parallel give 100 Ah. The result is a 15S4P pack of 60 cells storing 48 × 100 = 4.8 kWh.
Nominal voltage is the least useful number in the pack. What the connected equipment must tolerate is the full range: that 15S LiFePO4 pack reaches 15 × 3.65 = 54.75 V fully charged and falls to around 15 × 2.5 = 37.5 V at the low cut-off. An inverter or charger rated only for "48 V" without checking that window will see voltages well outside it.
In series, the weakest cell governs the whole string — it hits the upper limit first on charge and the lower limit first on discharge, so the pack stops using the capacity the other cells still have. Balancing circuitry in the battery management system equalises them, and without it a pack progressively loses usable capacity even though no cell has failed.
In parallel, cells share current according to internal resistance. A lower-resistance cell takes more than its share, ages faster, and the imbalance compounds. Cells for parallel strings should be matched for capacity and internal resistance, and ideally come from the same production batch.
Fifteen cells in series and four such strings in parallel. Series multiplies voltage, parallel multiplies capacity, so a 15S4P pack of 3.2 V 25 Ah cells gives 48 V at 100 Ah.
Multiply nominal voltage by capacity in ampere-hours. A 48 V 100 Ah pack holds 4.8 kWh.
The full charge-to-cutoff window, not the nominal figure. A 15S LiFePO4 pack runs from about 37.5 V at cutoff to 54.75 V fully charged, despite being called a 48 V pack.
Because the weakest cell reaches its limits first and stops the whole string, leaving the other cells' capacity unused. Balancing equalises them so the pack delivers the capacity it actually has.
They share current according to internal resistance, so a stronger cell is worked harder and ages faster, which compounds the imbalance. Matching capacity and resistance, ideally from one batch, avoids this.
Monitor individual cell voltages rather than only pack voltage, block charging below roughly 0 °C, limit charge and discharge current to rated values, and balance the cells.