How to Calculate State of Charge(SOC)? Battery & Energy Technologies

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Introduction

There is a hiking rise in pollution caused by the adverse effects of carbon-emitting fuel vehicles. Such impacts of pollution cause harm to the environment as well as human health and lifestyle. With this said, there is a booming demand and production of E-vehicles in the market.

The new era of Electric vehicles has begun with the introduction of the following vehicles:

1. Battery Electric Vehicles(BEV)
2. Hybrid Vehicles(HV) &
3. Plug-In Hybrid Electric Vehicles (PHEV)

Just like a fuel gauge in fuel-operated vehicles, battery-operated vehicles, or E-vehicle, you may come across the word “SoC” many times in case of your Battery functionality and capacity.

Now, you might be pondering about the term SoC. Don’t worry, let’s explore this small and a bit complex term SoC. We are here to answer your quest.

What is SoC?

In simple words (The State of Charge) is your fuel gauge for battery-operated vehicles or E-vehicle. It is the real-time or actual capacity of the battery compared to its full capacity. Just like your cellphone battery, it is measured in terms of percentage (%). E.g., 100% SoC means the battery is fully charged to its capacity, and 0% SoC means it’s fully discharged.

The importance of calculating SoC

I will admit that the estimation of the SOC of a battery is not an easy task depending. It definitely varied based on the battery type and the type of application that is using the battery. However, there has been good research work and development in past years to improve the estimation of SOC accuracy. Accuracy in SOC estimation is one of the important tasks for battery management systems, which will help improve system reliability and performance and improve the battery life cycle.

In general SoC of the battery is calculated by the simple formula given below, i.e.ratio of real-time charges present in a battery(Qt) to full battery capacity charges (Qn).

SoCt=Qt/Qn

Though, if you can actually estimate precise SOC calculation, then you will save yourself from unpredictable system blockage. It will prevent you from overcharging your batteries, which, in reality, damages your battery’s internal structure.

However, you cannot estimate the accurate SOC under every condition because it involves complicated chemical and physical processes. But after a lot of research, 3 Simple Methods used for the calculation of SoC are as below.

3 Methods to calculate SoC?

(a) Coulomb counting method

Mathematically integrated over the time formula to calculate estimated SOC by coulomb Counting is as below:

SoCt=SoCt-1+Ic(t)Qnt

Ic(t) ⇛Discharging Current at Time t
SoCt ⇛Estimated SoC at Time t
SoCt-1 ⇛Previously Estimated SoC at t-1 or SoC value Given by Manufacturer
Qn ⇛full battery capacity charges
△t ⇛Difference in time

It is one of the most used methods, However many other factors like temperature, application of battery, life Cycle of battery, fluctuation of battery voltage, etc, need to be considered along with the coulomb counting method to make a more accurate SoC calculation.

(b) Voltage Method

Under a controlled environment, you can determine the real-time pending capacity of a battery, SOC, by using a discharge test method. In the voltage method, battery voltage is converted to equivalent SOC value using the battery’s predefined discharge curve (voltage vs. SOC). Anyhow, the battery’s electrochemical kinetics and temperature affect the battery’s voltage.

You can rely on the voltage method by using a lookup table of the battery’s open-circuit voltage (OCV) vs. temperature. As well as, by a correction term proportional to the battery current and by compensating the voltage reading.

The voltage method is however difficult to implement because it needs a stable voltage range for the batteries. In addition, this discharge test is very time-consuming because it includes a consecutive recharge when there are many applications.

(c) Measuring using hydrometer

To test the state of charge of a battery cell, an instrument called a hydrometer is used. SoC measurement is performed by measuring the density of the electrolyte, which is achieved by measuring the specific gravity of the electrolyte. The density of electrolyte rises higher with the concentration of sulfuric acid.

Conclusion

Frequently Asked Questions (FAQs)