Battery State of Charge and Fuel Gauge Systems

- details of the technologies used in determining battery state of charge and battery fuel gauges for battery management systems.

Battery state of charge is a particularly important aspect of battery management systems.

Knowing the battery state of charge enables a system to determine when the next charge is required for the battery. It also enables the system to shut down in an orderly fashion should the battery run out of charge.

Battery state of charge monitoring

There are a number of different aspects of a battery that can be monitored by a battery management system. Each of the parameters gives an indication of different aspects of the state of the battery.

  • Voltage:   The voltage is one of the easiest parameters to measure to give an indication of the battery state of charge. This battery measurement can give an indication of a number of aspects of the battery state. In particular, there are two states it gives a good indication about:

    • Low charge:   As the level of charge in the battery falls and the battery is getting close to empty, the battery voltage starts to fall. This is a good indication of the battery of cell being close to empty.
    • Overcharge:   In many cells as the cell or battery has been fully charged, the voltage rises. If this rise is detected, it is a good indication that the cell or battery is fully charged and charging should cease.
  • Current:   While current is not always as easy to measure it indicates how much charge has entered or left the battery or cell, and again this gives a good indication about the battery state of charge. This is important in determining the level of charge within the battery.
  • Temperature:   The operating characteristics of a battery may change with temperature. Also during the charging process, many types of cell dissipate heat as charging is complete. This gives a temperature rise and this is one method of determining the completion of the charging process.
  • Air flow:   In some battery installations air flow may be used for cooling the battery system. In these circumstances the air flow may be measured.

By Ian Poole

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