Calculating the State of Charge of a Lithium Ion Battery System using a Battery Management System

The State of Charge (SOC) is like the fuel gauge in your car, telling you how much energy is left in your battery system. Calculating the State of Charge in a lithium-ion battery system requires an understanding of how the battery pack capacity and energy change. This understanding is critical before diving deeper into Coulomb counting.

Topics Covered
State of Charge
State of Charge (Capacity) (SOCcapacity or SOCc)
State of Charge (Energy) (SOCenergy or SOCe)
Fuel Gauge
Coulomb Counting
Depth of Discharge
Open Cell Voltage Lookup

The state of charge is defined as the capacity remaining divided by the total capacity of the battery pack. We look at an example of how the state of charge is calculated. Using a discharge curve, he explains that this curve is not linear. Given this, the true state of charge is often different from the energy remaining. Looking at State of Charge (capacity) and the State of Charge (energy) we can understand the differences between these values and how to calculate the most relevant information.

Coulomb counting is discussed next. Coulomb counting integrates amp hours and time to calculate the capacity removed from the pack. This provides a basis for SOC. The current sensor has drift and measurement error, so it is also important to have an open cell voltage lookup.

[apurvawatekar]

Thank you for your videos. So simply explained. Your videos helped me to prepare for my interviews.

[johnnyflanker]

Excellent explanation ! Glad I found your you. Will continue listening to all your videos prior to asking a question. Thanks !!

[FINwithFAME]

Your explanation beats what my professor has covered.

[oliverheeg]

In you example 50% SOCe corresponds to 58% SOCc, doesn't it?...I think you mixed it up as you plotted over Ah discharged and not over SOCc initially.

[joechi]

Thanks for the introduction. I really learn something.

[ChuckJ]

GREAT ONE!!!! The calculas everyone needs!

[vishnoor]

Wonderfully done. Explained in very simple manner. So I guess one needs the charts for a battery pack to implement the socc soce and dod.

[tonymunn]

At the 5:00 min. point, doesn't that point actually represent a greater than 50% SOCcapacity or is that 42% capacity used at that point? I think I understand. It seems that the 42% is more Depth of Discharge????

[cangudem]

Thanks for the explanation. You save my life

[writerconnect]

Excellent job. Clearly explained . Very eloquent !!!

[dhadumia]

Thank you very much for such a great video!

[energyeve]

Clear and concise. Thanks for sharing!

[peppi]

Informative video of what i exactly didnt understand about how you know what nn% charged means of a battery. I would ve appreciated if you defined the first SOC you wrote right away as SOC_c and in two different graphs you once mention Ah-discharged and DOD. Could have made them the same aswell.

[stevecoscia]

Very informative video. Thank you for this. I just bought a 100Ah lithium battery and this explanation helps in understanding the app's SOC metric.

[mohamedzain]

Thanks a lot, good explanation, keep going.

[Doctorbasss]

WATT-HOUR is the real term that define remaining range/energy. Capacity is voltage dependent and each Ah at the end of charge worth less than the first Ah. So the REAL SOC is Watt-hour defined and not Capacity (Ah)

[erfquake]

These videos are brilliant, Mr. Stafl, thank you, very easy to follow along. One gets into the noble field of lithium battery technology for environmental concerns, tries to brain-up on lithium battery characteristics and electrical fundamentals, and invariably gets stuck between principles and real-world application. Me for example. A coulomb meter out of the box needs initial settings ...specific to that battery config that it's monitoring... like capacity, full voltage "empty" voltage, and drift. A lithium battery pack's stated Ah may be, say, 40Ah, but is it the kind of parameter that ought to be tested for? And if it's "72v, " obviously the range is going to be a few volts above (for full) and a bit above. (for depleted) But these are rules-of-thumb, and sort of nebulous to the amateur. To get specifics, should I test the voltage a.) immediately after charging, or after a day or two, and b.) idle or under load? In the interests of mitigating range anxiety, I'd like to make sure my settings are as specific/accurate as possible. You've got several videos. Is there one that may address these real-world scenarios best? Thank you.

[DennisMathias]

Great presentation! I knew there were nonlinearities in monitoring SOC and wondered how they did that. So now I know.

[edsonferraz]

OK, excellent information. Do you have a SoC code using C++?

[smde]

I would really like to see a LiFePO4 BMS among your product offerings .