What does the State of Charge really mean?
Battery State of Charge (SoC) is similar to the fuel gauge on a car or how full your beer glass is. There are a couple of key ways to think about it, Technical Capacity and Operational Capacity. For charging applications and driving and charging electric vehicles the operational capacity, how much energy you can use between ‘full’ and ’empty’, is almost always the interesting measure. It is important to recognize Operational Capacity is a design choice of the OEM and some of the tradeoffs involved in selecting it. When doing planning and analysis it is also important to check that you are working with the operational capacity.
Technical v. Operational Capacity
Like a beer glass, there are two ways to think about ‘full’ and ’empty’ with a battery, Technical Capacity, and Operational Capacity. You could fill the beer glass right up to the rim, so the beer (not foam) is almost running out; this is equivalent to Technical Capacity. Some pubs use pint glasses and do this, but you tend to spill beer when you move the glass. (here Operational Capacity = Technical Capacity) Likewise, if you are in an extreme performance application like solar cars you might try to use all the technical capacity to win the race, knowing the batteries won’t be usable later. Other pubs use oversized glasses where there is a bit of extra room for foam and so you don’t spill. (Operational Capacity < Technical Capacity) At the bottom of the beer glass, you have a similar difference, you could get a straw and sponge and try to get the very last drops of beer out of the glass (Technical Capacity) but you would get some funny looks at the pub, and it isn’t worth your time when there is a perfectly good beer tap with more beer nearby.
Technical Capacity is how much you could theoretically use if you pushed the battery to its chemical limits.
Operational Capacity is how much you really use.
Why the differences?
Operational capacity is usually a bit smaller than technical capacity for batteries for several reasons (none of which involve spilled beer). The ends of the charge, either very full or very empty are often difficult to work with as the voltages get further from the nominal. Like an empty beer glass where you could try to get more out with a straw, a very empty battery will have high internal resistance, making it difficult to get much power out. In practical terms for driving this could mean you don’t have the acceleration you expect since the battery just can’t deliver the power when it is very low. As the pack gets very full you have to charge slower, taking longer to charge. Similar to a beer glass where you would have to pour more slowly to get it full to brim without spilling. There is nowhere else for energy to go into the battery, so you can’t safely ‘spill’. This can quickly turn into diminishing returns where it could take as long to charge from 80% to 100% as it does from 20% to 80%. The last tiny bit of energy, from 99% to 99.9…% of the technical capacity could take a long time to fill, making inefficient use of charging resources. The battery is also degraded more at the ends of the charge and it will reduce the battery lifespan. Often charging between 20-80% of a battery Technical Capacity will give it a much higher lifespan than charging from 0%-100% of the Technical Capacity. Storing a battery in high temperatures is generally bad for lifespan, but when the battery is also full will make the effect worse. The magnitudes of these effects vary depending on battery chemistry and the specific cell design.
There are reasons to discuss technical capacity. For marketing, bigger numbers are better, so talking about the technical capacity makes a battery sound better. In some cases, the operational capacity isn’t a single limit. It could vary with temperature, or there could be a reserve function where there is a little bit of energy left below zero. (Most cars will still have a little bit left when the fuel gauge needle reaches the bottom of the red, but you better stop at the next gas station.) Some cars also have a ‘reserve’ function. They will only charge to 80%, unless you tell them you are planning a road trip and to please go to 100%, in effect increasing the operational capacity within the same technical capacity when you really need it.
An illustrative example difference between Technical and Operational capacity is shown below. The technical capacity is the full range of the battery, in this case requiring just over 4 hours to charge. The Operational Capacity is then set for this example between 10% to 95% of the Technical Capacity. This costs some (15%) usable energy but will help improve the battery lifetime. It will also shorten the charge time by about 1h, so a 25% reduction in charge time for only a 15% reduction in capacity. Finally,
the 10% lower operational limit will keep the pack voltage in a narrow range between 670V and 735V, instead of going down to 470V at the bottom of the discharge. This could allow smaller cables (for the same power, but with higher voltage) and more efficient power electronics throughout the vehicle since the input voltage range doesn’t need to be as wide.
SoC and Voltage Curves for a Simulated Battery
Most EV batteries will have an operational capacity smaller than their technical capacity to reduce charge time, degradation, and working voltage range. When reviewing data about battery size and charging behavior be sure to check you are working with operational capacity. Operational Capacity is a design choice the battery OEM can make and adjust if needed in the design process, so make sure you have the latest data from them in doing your route and charging analysis. Battery technology is evolving rapidly.
The example shown here is based on a theoretical battery simulation model to illustrate the concepts. It is not representative of any specific pack.
This blog post is intended for general information only and may not be applicable in all cases. ChargeSim BV makes no warranties and accepts no liability for information provided in blog posts.