A laptop battery is a group of lithium cells wired in series to raise the voltage and sometimes in parallel to raise the capacity. Labels such as "3S1P" or "4S2P" describe that arrangement. Knowing it tells you what voltage a healthy pack should show at full charge and when it is empty — the starting point for every battery measurement.
How to use it
- Choose the cell type. Most recent laptops use high-voltage lithium-polymer cells charged to 4.35 or 4.4 V; older packs use 4.2 V cells.
- Enter the number of cells in series (S) and in parallel (P).
- Optionally enter one cell's capacity to get the pack's mAh and Wh.
The formulas
- Voltage scales with the series count: V_pack = V_cell × S.
- Capacity scales with the parallel count: mAh_pack = mAh_cell × P.
- Energy = mAh_pack × V_nominal ÷ 1000.
The "empty" voltage shown is a typical cut-off; the pack's protection circuit and the laptop's firmware decide the exact point.
Reading the S count from a label
A pack's nominal voltage reveals its series count. Labels around 7.2–7.7 V are 2S, 10.8–11.55 V are 3S, and 14.4–15.4 V are 4S. The capacity and energy then follow from mAh to Wh Converter for Laptop Batteries.
Worked example: is this pack full?
A battery labelled 11.55 V, 4900 mAh is a 3S pack of 3.85 V high-voltage cells. Fully charged it should read about 13.05 V; at its cut-off around 9 V. With the pack out of the laptop it measures 12.9 V — close to full, so the cells are holding charge. If it read 0 V, the protection circuit has switched the output off; if it read about 8.7 V, one cell group may have collapsed, since 2 × 4.35 V is 8.7 V. Measuring packs safely is covered in How to Test a Laptop Battery with a Multimeter Safely, and the cell under lithium ion cell.
FAQ
What does 3S2P mean?
Three cells in series (voltage ×3) and two in parallel (capacity ×2) — six cells in total.
Why do new laptops charge to 4.35 or 4.4 V per cell?
High-voltage cells store more energy in the same size. Never charge a 4.2 V cell to 4.35 V.
Why is my pack's voltage lower than nominal at rest?
It is partly discharged. Nominal voltage is a mid-range value, not the full-charge voltage.