22 maart 2016

In my first blog I discussed what DC fast charging is and how it differs from AC charging. In this part I will go into more details on car batteries and the variables that influence the charge speed when using a fast charger.

Car battery packs
A car battery consists of many ‘cells’. A single cell is quite similar to a rechargeable battery you use at home only bigger. A Tesla Model S with a 85 kWh battery pack contains 7, 104 individual cells. A BMW i3 with a 21.6 kWh battery has just 96 cells, but its cells are larger than the cells used by Tesla. Together with all wiring and packaging the cells form the battery pack as depicted below.

BMW i3 battery pack

Today's battery packs are designed with fast charging capability. For example the powertrain of the BMW i3 is rated at 125 kW peak power and 75 kW continuous power while fast charging is done at 50 kW.

Battery life
The battery pack of a car is never used 100%. The usable capacity of the 21.6 kWh i3 battery pack is around 19 kWh. The reserve of 2.6 kWh is used to 'cushion' the impact of charging and discharging. The battery pack automatically cycles between around 5% and 95% of the battery pack. All of this is handled by the Battery Management System (BMS) and completely hidden from the driver.

There are many factors influencing battery life including heat, battery age, duration of keeping a battery fully charged and number of charge - discharge cycles. Research shows that exclusive use of fast chargers hardly affects battery life when tested with the Nissan Leaf MY2012. And other research indicates that fast charging might actually be better for battery life. As a general rule, a battery will last longer when its size increases because fewer charge - discharge cycles are needed for the same mileage.

Charge speed
During fast charging there is continuous communication between the BMS and the fast charger. The BMS instructs the fast charger to set the charging speed. This speed is usually expressed in kilowatts (kW). Charging a car for 1 hour at 50 kW puts 50 kWh into the battery pack. On average an electric car uses 1 kWh to drive 5 km. Tesla also expresses the charge speed in km/hour. So 50 kW equals about 250 km/hour (‘250 km of range charged in 1 hour’).