SOC : State of Charge
It determines the the remaining capacity of battery pack.
SOC of the battery is defined as the ratio of its current capacity {Q(t)} to the nominal capacity {Qn} .
Where, {Qn} Nominal capacity is the maximum amount of charge that can be stored in the battery.
This rating is given by the manufacturer.
SOC(t) = Q(t)/Q(n)
In other words, SOC is the ratio of the remaining charge of the battery and the total charge.
During charge & discharge, Batteries internal parameters like resistance,temperature,etc vary with SOC.
SOC is expressed in percentage.
SOC= 100% --> Battery fully charged
SOC= 0% --> Battery fully discharged
Classification of SOC Estimation method.
1. Direct Method
- Open Circuit Voltage (OCV) method
- Terminal Voltage method
- impedance method
- impedance spectroscopy method
2. Indirect Methods
- Coulomb counting method
- Modified coulomb counting method
3. Adaptive System
- BP(Back Propagation) neural network
- RBF(Radial basis function) neural network
- Support vector machine(SVM)
- Fuzzy neural Network
- Kalam Filter
4. Hybrid System
SOH : State Of Health
SOH of a battery is defined as the ratio of its maximal instantaneous releasable capacity,(Qmax(t)) capacity of new battery (Qnew).
SOH(t) = Qmax(t)/ Qnew
- SOH is a figure of the present condition of a battery cell compared to its ideal conditions.
- The SOH is represented in percentage form.A SOH equal to 100% means it is a fresh/new battery.
- The SOH could be derived by capacity and the internal resistance, and it could also be derived from one battery parameters like AC impedance, self- discharge rate, and power density.
- Take the capacity as an example, SOH could be defined as the ratio of the current capacity and the rated capacity given by manufacture.Generally,if the battery capacity is 80% less than initial value,which means the SOH is less than 80% then the BMS would warn the user to change the batteries.
- The SOH decrement of a battery cell is mostly caused by the battery aging and degradation, namely, durability problems.that means with the using or storing of the battery cells, the battery capacity would decrease and the internal resistance would increase. Thus the SOH of the battery cells worsen.
- For the battery in the PHEV (Plug-in hybrid Electric Vehicle) which requires both enough energy and sufficient power, both the capacity and internal resistance should be considered for SOH estimation.
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