Torque Shop: How long does it take to charge an EV?

The battery capacity divided by the charger's rated power will give the theoretical charging time in hours. PHOTO: ST FILE

From the charging station's power capacity, how do I tell how long it would take to charge my electric vehicle (EV)?

Charging time for an EV's battery is a function of the charging station's rated power, the energy capacity of its battery and the capacity of the vehicle's on-board charger.

Wall chargers designed for home installation are available in 3.7kW and 7.4kW power outputs (the latter with three-phase supply). Charging up a 37kWh battery with these will take 10 or five hours respectively. At a Shell Recharge 43kW charger, it would take just under 52 minutes.

Basically, the battery capacity divided by the charger's rated power will give the theoretical charging time in hours. In real life, this can vary according to the number of appliances running in the house at the same time.

Charging time is also determined by a vehicle's on-board charger. This charger controls the maximum input current allowable. If the on-board charger is limited at 11kW, charging time for the same vehicle mentioned above at a Shell station will take three hours and 20 minutes. But an EV with a 40kW on-board charger will take 56 minutes to "fill up".

The above examples apply to AC power sources. If you use a DC fast charger - like the 50kW ones at Shell kiosks - charging time will be proportionately shorter. But there is literature suggesting that DC fast-charging will degrade batteries faster.

Charging time aside, there are a number of practical factors to consider if you drive an electric car.

Firstly, the actual usable energy ranges between 90 and 95 per cent of the manufacturer's specified battery capacity. For example, the BMW iX3 has an 80kWh battery, but the power control system limits the usage to 74kWh.

Drivers also do not drain their batteries to zero, nor is it necessary to charge up to 100 per cent. The common practice is to plug in when the battery is down to about 20 per cent and then charge it up to 80 per cent.

Using the iX3 example, that would mean a nett energy requirement of 60 per cent or 44.4kWh, which requires six or 12 hours with a household wall charger.

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