Power battery management system-state of charge (SOC)-concept

The state of charge of the battery, State of Charge, referred to as SOC, as the name implies, refers to the available state of the remaining charge in the battery, which is generally expressed as a percentage. The definition of the most classic SoC can be expressed by the following formula:

(1) SOC=Q_remain/Q_rated ×100%

Among them is the nominal (rated) charge capacity of the battery, and is the remaining charge remaining in the battery. If it is regarded as a constant value, that is, the remaining charge margin is always equal to the nominal capacity minus the discharged charge, the SoC can be expressed by the following equivalent formula:

(2) SOC=Q_remain/(Q_discharged+Q_remain )×100%

Among them, Q_discharged represents the charge that has been discharged in the battery after the most recent full charge.

The actual amount of charge that the battery can release is not exactly equal to the nominal value. In addition, with the aging of the battery, the actual maximum charge that the battery can release is constantly decreasing.

Therefore, in actual work, we give two suggestions:

1. The denominator of formula (1) can be the actual maximum charge capacity Q_ture, provided that the actual maximum charge capacity can be accurately obtained. This can be obtained by frequently evaluating the battery and continuously calibrating the actual maximum capacity Q_ture (or Estimate the actual maximum available capacity online in real time).

2. In the case that the actual maximum charge capacity Q_ture cannot be accurately obtained, the nominal capacity Q_rated should be used, because Q_rated does not change with time and other factors, and it is easy to convert Q_remain through the value of SoC. Of course, using Q_rated as the denominator may cause the SOC evaluation value to be greater than 100% in some cases, which is also allowed.

The remaining charge Q_remain is affected by many factors and cannot be completely released.

Generally speaking, the molecule Q_remain in the classical definition of formula (1) is affected by many factors and cannot be completely released. If the evaluation of the remaining power is to estimate the remaining mileage that an electric vehicle can drive, it is not appropriate to use the generalized concept of the remaining power. From the results of the discharge rate characteristic test, although the remaining power of a certain power battery at a certain moment is certain, they may not be completely released with the power required by the electric vehicle. This can be compared to in daily life, when we use a mobile phone, when the mobile phone is in the "low battery" alarm state, it will automatically shut down as soon as a call is made, but if it is restarted, it can continue to stand by for several hours. This shows that it is not that there is no remaining charge in the battery, but that the remaining charge cannot be released with a larger current according to the user's needs.

The maximum charge that a fully charged battery can release is affected by factors such as discharge rate, ambient temperature, etc. It is not a constant value, nor can it be completely equal to the nominal capacity Q_rated.

The discharged charge Q_discharged is not necessarily measurable

The use of formula (2) implies a prerequisite, that is, the last charge of the battery is 100% full. In the actual use of the power battery, the following situations often occur: the battery is briefly charged after a period of use, but the charger is disconnected before it is fully charged and ready to continue to use. At this time, the measurement of Q_discharged loses its meaning, and the value of SoC cannot be evaluated by formula (2). This situation is more common in hybrid electric vehicles.