EV Charging Explained – AC, DC, Inverters, 3 pin, Wall box
As electric vehicles are getting more and more popular, keeping them charged will get more and more important – but how does that actually work? Well I’ve got a Renault Zoe to test for a week over on my car channel, At The Wheel, and since I’ve been charging this a lot this week, I thought I’d explain it. This video covers the tech behind EV charging, but if you want to know more about the charging network here in the UK, the connectors and the general process, make sure you are subscribed to At The Wheel as I’ve got a full video on exactly that coming next week. Anyway, it’s cold so lets head inside and talk about charging!
Right, that’s much better. So, EV charging. There are essentially two main types of chargers, AC, and DC. Lets start with the former. AC chargers are the most common, they can range from standard wall plugs like this one, to rapid chargers delivering up to 43kW’s of power. The trouble with AC is that it’s fundamentally different from what the car needs. EV batteries, in fact most all chemical batteries, store power as direct current, or DC. The Nickel Cobalt Manganese pouch cells in the Zoe store 78aH of current at the usual 3.7V nominal (4.2V charged). Those are grouped into series packs so the total voltage I believe is around 400V, then sets of them are put in parallel to provide better power output, and more overall capacity.
But, that’s DC power, not AC. So to charge with an AC charger, be it a 3 pin plug, a home wall box like the one Renault will fit for free when you buy one of their plug in vehicles, or a public charger like the Pod Points at Tesco, you need to convert that AC into DC. That’s done in the car’s charger/inverter, which rectifies the alternating current wave by flipping the negative pulse and smoothing it out into a flat voltage. That is then sent to the DC/DC converter to boost the voltage (at the cost of current) to match the batteries’ charging voltage then finally it can be sent to the battery to charge it. Complicated, right?
The advantage of AC charging is that it’s really pretty simple. You basically hook up mains power straight to the car – or if it’s a fast charger you might need to combine the 3 phase power the charger gets into a single high power phase, then send it to the car, but either way it’s pretty simple. The downside is that it’s not the most efficient. Because of space and weight constraints, the inverter in the car can’t be as effective as having an industrial size inverter built into the charger, where size and weight aren’t big priorities. Because of this, many cars don’t support overly high AC fast charging, with the Zoe topping out at 22kWh on AC.
The solution? Put the inverter in the charger. That’s essentially what DC fast charging does. Now you generally need a different connector for that, either the additional pins that get added to the Type 2 plug to make it a CCS connector, or the CHAdeMO you’ll find on many Japanese models like the Nissan Leaf. But, assuming you have the pins for it, and the charger is able to communicate with the car to ensure safe charging, it can unload much more power direct to the battery. This step also removes the need for a DC/DC converter onboard the vehicle too as the charger will be the one to modulate the supply voltage to meet the batteries’ needs, with only the batteries protection circuitry in the way to stop overcharging.
With DC fast chargers, the common rate you’ll find at most rapid chargers is 50kWhs, although if you can find an ‘ultra rapid charger’ like some on motorway services, and you have a vehicle that can utilise it (which the Zoe can’t), you can get up to 350kWhs of power to juice up on the go. Impressive! Most Tesla Superchargers are 125kWh, although the newer V3 versions are rolling out with up to 250kWh available.
And, if I didn’t make it clear, the more kilowatt hours of power you can put in, the faster your car will charge. In theory, the 52kWh battery in the Zoe can be filled up in a hair over an hour on a 50kWh DC fast charger which is the maximum the car can handle. Although in practice battery chemistry means that even when connected to a DC fast charger, the car might not be able to utilise it all. When charging the Zoe at a 50kWh DC charger, from 30%, the peak charge rate reported was 45kWh, and as the battery filled, the charging slowed to just 25kWh nearing 80%. It gets even slower if you try and get to 100%, as the batteries prefer to not be filled completely. It’s generally recommended to keep your EV below 80% unless you really need the extra range like just before a long journey, otherwise under 80% helps maintain their health and self discharge rate.
So, that’s a look at EV charging and how it works. I highly recommend you head over to At The Wheel and hit subscribe there, and check out my full review of the Renault Zoe which should be live already – and don’t miss the EV charging guide that’ll be following shortly too.
