Understanding Battery Usable Capacity

Last 12 years I had been working extensively on the Electric Vehicle design and development. Many startups, Tier-1s and OEMs are entering in this fast growing field and I had a chance to work and discuss with many of them technically. Firstly I should say that almost all of them do not understand the battery subject thoroughly and are really not very serious about it. The safety criticality, effects of environment, need of accurate and calibrated BMS, charging profiles, dynamic thresholds, failure prognostics, temperature management, DoD management, accurate SoC / SoH calculation, shock, vibration factors, C rate controls etc. are few of the considerations engineers are missing or ignoring. Among many other parameters, range estimation of the vehicle is very important aspect. And to get that accurately, a very basic aspect is to understand Usable capacity of the battery.

Every battery pack has 2 different capacity values – Installed / Theoretical Capacity and Usable / Actual Capacity. Installed capacity is calculated based on Nominal specifications in the Datasheet. And usable capacity is calculated based on different conditions.

The calculation of Usable capacity is a bit complex and depends on the different operational parameters. All the chosen parameters are additive in nature and need to be tuned to get maximum performance for required result parameters. We also need to understand the trade-off and should design for the ‘required’ system parameters. E.g. if your product life is 5 years and you are going to charge only once in a day, then tuning your parameters to get 3000 charge / discharge cycles is a overdesign. This will lead to compromising in range, size and weight of battery.

Let us see the factors contributing usable capacity

Discharge Rate:

Higher the discharge C rate, lesser is the usable battery capacity

Temperature:

Operating Temperature affects the life of the Cell and the cell usable capacity starts degrading faster. Operating temperature depends on the C rate, cooling system efficiency, ambient temperature, duration of current and some other factors.

Storage Loss:

Along with operating temperature, storage temperature also affects the degradation of cell and ultimately the usable capacity of the cell.

Depth of Discharge:

No of cycles are directly dependent upon the DoD. To get the longer life of battery DoD is limited to upper and lower limit. This directly trims the usable limit of the battery pack.

Cutoff Voltages:

The upper and lower cutoff voltages to adjust DoD with hysteresis also affects the usable capacity of the battery.

BMS Electronics Error:

All electronics components and chips have tolerances and measurement accuracies. These differences and errors contribute to very high amount of error in SoC calculation and may result in reduced usable capacity if they are not calibrated and adjusted properly.

Capacity Fade Compensation:

The vehicle specification specify range of the vehicle in one charge. During warranty period this value should not be compromised. Any EV battery has the usable capacity degradation based on number of cycles. And as the battery starts aging, it capacity starts reducing. If this reduction is not taken into consideration, then the range of the vehicle goes down and can become a warranty issue. To take care of this, the initial DoD of the vehicle is kept less and as battery keep aging the DoD band is expanded by BMS. This gives the same range of the vehicle during the full warranty period. But due to this, the usable power of the gets reduced.

These are few of the important parameters affect the Usable capacity of the battery. There are many more, but effect of them are comparatively lesser than these. The smart BMS should take care of all these parameters to calculate real SoC, SoH values to correctly predict the battery life and range.

At GMS, with our 12+ years of hands on EV experience, we design reliable, accurate and high quality EV solutions for our clients. Get in touch with us for the complete end to end solutions from Electronics, Systems and Certified vehicles.

Thank you,

Jaywant Mahajan, GMS Tech Labs, Pune.

www.getmysolution.com ([email protected])