What does 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 recognise 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 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 near by.

  • 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 a 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 no where 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 it 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 behaviour 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. Wise Charging BV makes no warranties and accepts no liability for information provided in blog posts.

Fast Charging v. Slow Charging in Depots

The case for faster battery electric bus charging is obvious en-route. If the driver is waiting, or only has a set break period, getting as much energy into the vehicle battery as possible as fast as possible maximises the possible range.

On a depot the choices are a bit more complicated. Faster charging can mean using fewer, more powerful chargers, but more movements around the depot as vehicles are moved between chargers and storage for example. There are several factors to consider for faster or slower charging on depots which are discussed here. Our expertise and ChargeSim analysis tools can help you find the right balance between these factors.

Benefits of Fast Charging on Depot

  • Faster Turn Around Time for Vehicles. If the vehicles are only charged overnight, this usually isn’t an issue. The chargers need to be powerful enough to charge in the available time. If future schedule changes may need more dispatches per day, or shorter turn around times, more powerful chargers can allow more flexibility.
  • Reduced Run Time for Ancillary Systems. While charging, buses and other vehicles will usually need to run fans, cooling pumps for the batteries and other systems. This consumes energy and adds wear to the systems, eventually adding maintenance costs.
  • Potential Electricity Cost Savings. For many locations, off-peak power and energy tariffs can be much lower than peak tariffs. It can make sense to charge faster, so you can do more charing in cheaper off-peak periods. If your utility has demand response, or variable tariffs, faster charging can let you take more advantage of low cost time windows.

Benefits of Slow Charging

  • Less Chargers, Transformers, Grid Connection Required. Lower peak powers mean smaller chargers and the transformers and grid connections to support them. This translates into lower capital and operating costs.
  • More Efficient. Slower charging is done at more efficient lower currents. The resistive losses in wiring are related to the square of the current. (I2R) Charging twice as fast will require twice the current, and have quadruple the resistive losses.
  • Better for Battery Life. Faster charging can reduce the battery life. The impact of a given charge power will vary with the specific battery chemistry, though typically a charge power which can charge the battery in <1.5 hours will reduce battery lifetime.

With Switching and Dynamic Charging you can connect more than one dispenser to a charger, and electronically control which vehicle gets the available power. This can allow installation of fewer, faster chargers, without needing to move vehicles. This can offer many of the benefits of fast charging with the smaller grid connection and lower costs of smaller chargers. Switching and Dynamic charging need to be planned carefully to work with your layout for the best performance.

Selecting the size and speed of chargers to install requires balancing several considerations. Wise charging can help you find the right balance for your depot to ensure the right balance of cost and capabilities. Our ChargeSim tool can help analyse and validate a range of scenarios.

This blog post is intended for general information only and may not be applicable in all cases. Wise Charging BV makes no warranties and accepts no liability for information provided in blog posts.