If we want a battery cell to last a lot of cycles, extend the life in a power application or to ensure the available power is consistent then we need to set a usable SoC window that is smaller than 100%. That is we will limit the top end charge to perhaps 95% SoC and the bottom end discharge to 5% SoC.
High SoC and hence high cell voltage stresses the cell and significantly reduces the lifetime. Going over the maximum cell voltage risks safety of the cell and pack. Although this top end SoC is controlled by cell voltage, any error in SoC estimation needs to be taken into account when setting the SoC limits.
At low SoC the Open Circuit Voltage (OCV) is decreasing and the internal resistance of the cell increasing. Hence with a discharge load the cell voltage will drop even further and more rapidly approach the minimum cell voltage. This minimum cell voltage will be set by the cell manufacturer to avoid damage and extend the cell lifetime.
The downside is the usable capacity has just been reduced by 10%.
For higher power packs the usable SoC window will be even smaller. A high power hybrid battery pack may have a usable SoC window of just 30%. This is required to allow the pack to operate with a more consistent power capability and to extend the lifetime with very high micro-cycling.
Benchmarking
The problem in benchmarking this is that it is easy to find the usable energy and the total energy of a battery pack. However, it is more difficult without instrumenting the battery pack to ascertain the upper and lower working capacity limits.
Therefore, as an approximation we use the usable to total energy ratio as an alternative. This percentage is calculated as the Usable or Net kWh / Total or Gross kWh.
Examples of usable energy windows:
- 95.4% Mercedes EQA 250 66.5kWh usable (69.7kWh total)
- 92.5% Nissan Leaf 37kWh usable (40kWh total)
- 92% Ford F-150 Lightning 131kWh usable (142kWh total)
- 90% Nissan Leaf 56kWh usable (62kWh total)
- 90% Porsche Taycan EV 83.7kWh usable (93.4kWh total)
- 90% Renault Zoe ZE 40 41kWh usable (45.3kWh total)
- 89% Lexus RZ 450e 63.4kWh usable (71.4kWh total)
- 87.5% Formula E 2014-18 28kWh usable (32kWh total)
- 82.8% Lexus UX 300e 45kWh usable energy (54.35 kWh total)
- 82% BMW 330e PHEV 9.8kWh usable (12kWh total)
- 40% Ford C-max Hybrid 0.56kWh usable (1.4kWh total)
Where this window sits within the total window is more difficult to ascertain in benchmarking.
From the battery pack database we can plot the usable to gross energy ratio. For PHEV’s this is typically around 80%.
For BEV’s this is typically around 95%.
State of Charge
State of Charge, abbreviated as SoC and defined as the amount of charge in the cell as a percentage compared to the nominal capacity of the cell in Ah.
Increasing Usable SoC Window
If you are in a battery design role there is always pressure on increasing usable SoC window. Nobody wants to pay for and carry around unused battery capacity. However, there are some really good reasons to restrict that window.