# What is the SOC ( State of Charge )? | Battery Monday

### Intro

When we check on the status of a Li-ion battery, we will often look at the battery temperature, the SOC, SOH, SOS, SOF and SOE. The SOC, in particular, is important because of its role with the BMS. The BMS monitors the battery based on the SOC and prevents battery overcharge and overdischarge, and extends the service life of the battery. As shown in the picture here, this is the SOC power indicator seen on a smart battery with a BMS.

### Calculation formula

The SOC is the measurement of the amount of charge remaining on a battery. It represents the ratio of the current capacity of the battery to its nominal capacity after a period of use or long-term shelving, which also indicates the maximum amount of power that can be stored in the battery. The SOC is calculated as a percentage: when it’s at 0, it means the battery is completely discharged. When the SOC is at 100, it means the battery is completely full.

### Absolute charge state

There is also what’s called an absolute charge state, which is a reference value calculated from the fixed capacity value when the cell is manufactured. The absolute SOC of a new fully charged battery is 100% while an aged battery, even if fully charged, cannot reach 100% in different charge and discharge situations.

### Influencing factors

Most SOC estimations require precise data measurements, whether it is the chemistry of the cell (type of electrolyte), its operating conditions, or the variables of the cell (voltage, current), etc., thus being suitable only in the laboratory and not in real applications. The SOC of a battery is a combination of voltage, charge/discharge current and internal resistance parameters to estimate its size. These parameters are also affected by a variety of uncertainties such as battery aging, temperature changes and usage status.

### Conclusion

Accurate SOC estimations reflect important information, such as battery performance, remaining battery life, etc., which ultimately leads to efficient management and utilization of battery power and energy. In addition, SOC estimation can be used to regulate battery life degradation, explosions or fires due to battery over-discharge and overcharge, as well as aging and permanent damage to the battery cell structure. Therefore, the SOC indicator on a BMS is vital for user convenience, as well as the efficiency, safety and life of the battery.

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