## Calculating Charge Times

The basic calculation is simple but there are important factors to account for with the input numbers:

Time to Charge  (hours) =  Amount of charge required (kWh) / Charging power (kW)

### Amount of charge required

The amount of charge required is the number of kWh you need to put  into the battery which in most cases  will be less than the full capacity of  the battery even assuming it will be charged to 100%.

The first factor to consider is the “usable capacity” of the battery.

For operational reasons and in order to protect the condition of the battery, some vehicles will not allow the battery to drop below a certain level of charge – the vehicle’s gauge is a measure of the usable not the actual capacity.

For example, a 24 kWh battery might have a usable capacity of 21kWh which means even when the vehicle is charging from “0” to 100% it is actually charging from 3 to 24kWh (a total of 21kWh) so it is the usable capacity that is needed for the charge time calculation.

The second factor is that in normal circumstances charging will be carried out long before the indicator reads zero so it will only be a partial charge (e.g. from half to full). The number
of kWh remaining can be subtracted from the usable capacity of the battery to calculate the amount of charge required i.e. if the same 21(usable)kWh battery has 50% remaining, the
amount of charge required to reach 100% is 10.5kWh.

### Charging rate behaviour

The charge times for high power chargers are often quoted to 80% because this is where the performance advantage is gained and what they are designed to do in public use scenarios.  This is because the battery’s ability to charge at a high rate diminishes as it
gets nearer to full capacity.

Home charging is relatively unaffected because of the typical power involved
but the higher the power of the chargepoint the more apparent this phenomenon is. For instance, a 100kW chargepoint may charge the battery to 40% very quickly but beyond this the charge rate will progressively reduce. By the time the battery reaches 70% capacity, the charge rate may be the same as a 50kW chargepoint, and the very last bit of charge will be carried out at the same rate as a domestic socket.

This article has been extracted from the "BEAMA Guide to Electric Vehicle Infrastructure"