State of charge and state of health
State of charge (SoC) of a lead-acid battery, expressed in %, is the ratio of the remaining capacity (RC) to the full charge capacity (FCC) (see Fig. 1). FCC is the usable capacity at the present charge or discharge rate and temperature. FCC is derived from battery full chemical capacity (QMAX) and battery impedance (RBAT) (See Fig. 2). For example, for a new lead-acid battery of 100 Ah design capacity, when it is fully charged, the SoC is 100% since the FCC is 100 Ah and RC is also 100 Ah. If that new battery is discharged so that RC is 70 Ah, then the SoC is 70%. As the battery ages, its full chemical capacity (QMAX) tends to be lower than the design capacity due to chemical degradation. SoH monitors this chemical degradation and is reported in % as the ratio of the FCCH to the design capacity, where FCCH is FCC at 25°C for design charge or discharge rate (Fig. 1). For example, if the battery with design capacity of 100 Ah has a FCCH of only 85 Ah after one year in use, then its SoH is 85%. In short, SoC indicates how much charge is left before a recharge is needed, while SoH indicates when a battery will have to be replaced