Charging up on the advantages of electric cars

Electric vehicle (EV) charging instead of refuelling is perhaps the biggest difference in ownership experience between EVs and internal combustion engine (ICE) vehicles – so it is no surprise that the hows and whens of charging are not yet well understood.

On the flip side: knowing what charging methods and choices are available is an important step on the path to driving electric – and the key to taking advantage of the change.

First up, AC charging at home or locally is the norm when it comes to charging, and real-world usage data shows it is used for over 90% of EV charging.

For local commuting in the longer-range EVs now available, it is not uncommon to charge only once every week or two, and to time that event to take make the best use of solar or off-peak electricity.

On the other hand, AC charging is not fast enough to give sensible travel times on long runs beyond the range of one charge – this is where the ‘other’ charging system (DC) comes into play. (See table 1).

And this is also where much confusion seems to arise. DC charging an EV requires a different plug, and DC chargers themselves may be termed ‘fast-charge’, ‘rapid charge’, ‘ultra-fast charge’ and for Tesla, Supercharger 2.0 or 3.0. (See table 2).

Table 2: Common charging rates and names for DC chargers

So why the difference? Well, batteries must be charged with DC current – but our grid supply is AC. Consequently, charging a battery means converting grid-supplied AC to DC for the battery using what is called an ‘inverter’.

Low power inverters are quite cheap and small – so inverters to suit the lower power availability in homes and the like are easily built into the car. Bigger inverters to provide the high charge rates needed for DC fast-charging are very large, heavy and expensive – so they need to be fixed items with big connections to three phase grid power.

Given the limits of household electricity supply (and AC to DC inverter size and cost), EV manufacturers mostly stick to an upper limit of 32 Amps (7.2kW) for AC charging, although some go as high as 11kW three phase (or even 22kW three phase in the case of the Renault Zoe).

So there you have it: AC charging is, therefore, perfectly suited for a full overnight charge at the cheapest tariff at home, or for smaller top-ups in the down-time between trips.

Having the AC to DC inverter built into the car also makes it extremely handy to be able to charge at any available power point if the need arises. AC chargers are mostly installed in homes, offices, etc, where cars are parked.

For long-distance travel, the inverter is moved outside the car and connects to a different part of the inlet plug, bypassing the on-board inverter to supply DC direct to the battery.

Therefore DC chargers are different in appearance and plug type, and mostly to be found on the major intercity routes in roadhouses and shopping strips.