22 March 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.In part 4 I discuss the relationship between station capacity and economies of scale.
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.
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.
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’).
P = V * I
Power (expressed in Watts) is the product of voltage (Volts) and current (Amps). When charging at 50 kW this is typically done at 400 V and 125 A (400 * 125 = 50.000 W = 50 kW). Note that this means that the charge speed is influenced by both the voltage and the current.
You can compare charging electricity with running water from a tap. Think of voltage (V) as the water pressure and current (A) as the size of the tap. If you increase the pressure more water will flow, and the same is true when increasing the size of the tap.
The voltage is a characteristic of a battery. Most car battery packs today operate at around 400 V when fully charged. But when a battery pack is not fully charged, the voltage will be lower e.g. 325 V. Voltage will gradually increase while charging, so this has a positive effect on the effective charge speed (see the blue line in the graph below showing a fast charge session of a 30 kWh Nissan Leaf).
The current can be increased or decreased by the fast charger based on data received from the BMS (see yellow line in the graph below). Most fast chargers can provide a maximum current of 125 A, but Tesla superchargers and the upcoming 150 kW CCS chargers can provide more than 300 A.
0-90% charge of a 30 kWh Nissan Leaf
What influences charge speed?
Now let’s take a look at the factors that have an effect on the charge speed other than voltage. There are four main aspects:
Battery pack capacity. In general, a larger battery pack can be charged quicker. So a Tesla Model S with a large 90 kWh battery can be charged quicker than a BMW i3 with a 21 kWh battery. This is also the main reason why most plug-in hybrid electric vehicles (PHEVs) cannot fast charge: their battery packs are simply too small. Most PHEV manufacturers do not include the additional hardware (e.g. extra inlet and wiring) in the car.
State of Charge (SoC)>. When the battery is almost fully charged the charge speed drops to prevent the battery cells from overheating. Typically at 80-90% SoC the speed drops and charging will slow down further closer to 100% SoC. That is the reason why fast charging is most effective between 0% and 80-90% SoC.
Battery temperature. Battery cells operate most effectively between 20 - 25 degrees Celcius (68 - 77 degrees Fahrenheit). When battery temperature is too low or too high, the BMS reduces the requested current to protect the health of the battery cells. If the battery pack is equipped with a heating or cooling system the BMS will activate this system in order to control the cell temperature. Note that battery temperature is not only influenced by the outside temperature, but also by driving and charging as this will generally increase battery temperature.
Power level of the fast charger. There can be several reasons why a fast charger cannot provide full power. For example the grid connection might not be sufficient for the charger to operate at full power. Or the fast charger needs to share the available grid connection with other chargers on the same location. In that case the chargers will communicate with each other to ensure that the total power used does not exceed the available power of the grid connection. At a Tesla supercharger site two stalls (chargers) typically share their capacity. So if one car charges at full power, there is limited power left for the other car. At Fastned all chargers can work at full power, because we ensure the grid connections are sufficient to deliver peak capacity.
Electric vehicles share a lot of similarities when it comes to the factors that influence charge speed, but the exact impact of each factor differs. For example the 30 kWh Nissan Leaf is able to charge to 90% in the same time as the previous 24 kWh version did. And the BMW i3 can actively heat and cool its battery so temperate will have less impact.
My next blog will be about charging the next generation of EV’s at 150+ kW.
Roland van der Put is the CTO of Fastned and heads the Network Operations Center. He graduated as computer science engineer at the Delft University of Technology and has a background in computers. His interest in electric cars was triggered after installing his own solar panels and driving a plug-in hybrid.