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Battery status*

Voltage, state of charge (SoC) and state of health (SoH) of a battery

Voltage

The voltage, expressed in volt (V), is the difference in electrical potential between the terminals of a battery.

Its value is not fixed: it varies according to several factors:

  • the state of charge (SoC),

  • the current drawn (charge or discharge intensity),

  • the temperature,

  • and the ageing state of the battery (SoH).

The nominal voltage depends on the type of cell used:

Cell typeNominal unit voltageModule exampleTotal module voltage
NMC3.7 V13 cells in series (13S)48.1 V
LFP3.2 V16 cells in series (16S)51.2 V

State of charge (SoC – State of Charge)

The SoC indicates the instantaneous charge level of a battery, expressed as a percentage of its total capacity.

A SoC of 100% means the battery is full, while a SoC of 0% indicates it is empty.

The BMS (Battery Management System) calculates the SoC from a coulometric balance, that is, by measuring the amount of energy (current × time) entering or leaving the battery during charge and discharge phases.

The BMS also monitors each cell individually to ensure perfect balance of the pack and to prevent any overcharging or excessive discharging.

State of health (SoH – State of Health)

The SoH reflects the overall ageing of the battery compared to its initial state (100% when new).

It is calculated by comparing, in real time, several parameters to their original values:

  • The temperature (influence on chemical degradation),

  • The current (intensity and frequency of cycles),

  • The number of cycles performed (charge/discharge),

  • Possibly, the internal resistance and the remaining capacity.

A SoH of 80% means that the battery retains 80% of its initial capacity — this is often the limit from which it is considered to be at the end of its first life (for example, for an electric vehicle).



*: The technical information presented in this article is provided for guidance only. It does not replace the official manuals of the manufacturers. Before any installation, handling or use, please consult the product documentation and follow the safety instructions. The site Torque.works cannot be held responsible for inappropriate use or incorrect interpretation of the information provided.