Basics

How a lithium-ion cell works

Every cell has two electrodes: the anode (usually graphite) and the cathode (a lithium-containing metal oxide). Between them is the separator, which prevents a short circuit; the electrolyte allows the lithium ions to pass through.^[1]

• Charging: The ions move from the cathode to the anode and are stored in the graphite.
• Discharging: They move back, while electrons flow through the external circuit at the same time – the device operates.

Lithium-ion cells have no memory effect and low self-discharge.^[1]

Series and parallel: what “10S4P” means

S (series) adds up the voltage, P (parallel) adds up the capacity. The energy content is the product of voltage × capacity.^[1]

A 10S4P battery therefore has 10 cell groups in series, each with 4 cells connected in parallel – 40 cells in total. Calculate your own configuration

Why all cells are always replaced

Cells age differently – even identical cells under the same conditions. In a laboratory test with 48 identical cells, the variation in ageing was more than 25 %.^[2]

The BMS stops discharging as soon as the weakest group reaches its minimum voltage.^[3]

A mix of new and old cells therefore does not work well together. This is why all cells are replaced during cell replacement.

Capacity and current rating

Capacity (Ah) describes how much charge a cell can store. The permissible continuous current (A) is a different quantity and depends on the cell type and the temperature conditions stated in the data sheet.^[4]

More Ah does not automatically mean more motor power. New cells must match the current requirement, the existing electronics and the charger.

The battery management system (BMS)

Every lithium battery needs protective electronics. They keep each cell within the safe range – and are often the reason why a battery “suddenly” switches off. The BMS^[3][1]

• monitors the voltage of each cell group,
• measures temperature and current,
• estimates the state of charge (SoC) and state of health (SoH),
• equalises the cell groups (balancing)
• and switches off when limit values are reached.