ZOE Charging Curve (Ecotricity 43kW)

Charging at Newport Pagnell 43kW Charge Point (Image: T. Larkum)
Charging at Newport Pagnell 43kW Charge Point (Image: T. Larkum)

Despite the regular problems I have had with public charging points (see The Pain of Public Charging 5 and previous episodes) I have had little trouble with my home charging point. The exception is that occasionally it doesn’t charge overnight when using the ZOE’s built-in R-Link timer app. The majority of these occasions have been on a Saturday evening and it has been speculated in the forum that the charging requires a connection to the Renault online services to start, but Renault does some server maintenance on a Saturday night that prevents it.

Normally this lack of charging causes us little inconvenience as we don’t tend to use the car much on a Sunday (though one time the resulting extra daytime charging required made us late for an appointment). Last week, however, the ZOE failed to charge on Tuesday night/Wednesday morning leaving me with insufficient charge to get to work and back. I didn’t panic, as this area (around Milton Keynes and Northampton) is quite well provided for with Ecotricity fast chargers. Instead I decided to return home after work along the M1, rather than my usual cross-country route, so I could use the Newport Pagnell fast charger (plus there are also 22kW charge points there as back-ups).

Unfortunately, it turned out to be not quite as straightforward as the range remaining on leaving work was barely enough for the journey to the services. Combined with a wrong turn, that then took me through a traffic jam from a broken down lorry, I was limping along the motorway (with the range remaining display alarming) by the time I got to the services. I had been driving at about 30-40mph on the motorway to eke out some range and that had clearly annoyed lorry drivers, as they hadn’t failed to let me know.

Anyway, this is by way of being a long preamble to say that getting to a fast charger with little charge remaining gave me an ideal opportunity for me to repeat my charging curve testing on a 43kW charge point. I sat in the car while it charged and recorded the state of charge (SOC), and its predicted end of charge time, every minute. The results, equivalent to those from last time, are shown in Figures 1 and 2 respectively.

Figure 1: ZOE Ecotricity 43kW Charge Curve (Image: T. Larkum)
Figure 1: ZOE Ecotricity 43kW Charge Curve (Image: T. Larkum)

One thing to note is that it never achieved 100%, only 99%. I waited for an hour after it reached 99% before giving up and ‘pulling the plug’. It seems to me that it’s not a meaningful fast charge if you have to wait more than twice as long for it to finalise the charge as to do the bulk of it! This confirms my previous view that if you’re in a rush you should just charge to 99% and then move on. Of course, I may have got very close to 100% and I’ll never know if it was just about to display that when I unplugged. Certainly I noted that when I restarted the car it claimed to be 100% charged.

The prediction for the charge duration was less erratic than before, but the general point that it is not worth much until the charging is well underway still applies. As before, it tends to be pessimistic.

Figure 2: Charge Completion Prediction – Actual was 99% after 35mins (Image: T. Larkum)
Figure 2: Charge Completion Prediction – Actual was 99% after 35mins (Image: T. Larkum)

The SOC curve is more of a straight line than I had expected. I previously had the impression that it slowed down significantly towards the end, above about 85%, but that isn’t borne out by the chart – perhaps that was just my impatience waiting for charges to complete. Instead there appears to be a slight slowdown above about 70%, though the change is subtle. There is then clearly another slowdown above 90%, but even then not so much that it would be worth stopping prematurely (before 99%) unless you were very short of time.

If, for now, we consider ‘fully charged’ to be 99% SOC then we can see that that was achieved after 35 minutes. The charging started at 8%, so the charge point provided 91% of a full charge in that time. The nominal capacity of the battery is 22kWh, so we can deduce that the charge point was supplying at an average of 34kW (0.91 * (60/35) * 22).

More interesting, however, is to look at each of the three apparent main sections of the curve. For these the power provided is as follows:

  • First section, from time zero to 20 minutes (8% to 72% SOC): 42kW.
  • Second section, from 20 to 27 minutes (72% to 89% SOC): 32kW.
  • Third section, from 27 to 35 minutes (89% to 99% SOC): 16kW

Of course, since the SOC readout is only given to a whole number these values are necessarily approximate. Similarly the assessment of the sections of curve is subjective – in practice the charge system is probably adjusting the power smoothly down over the range from 70% to 99%.

Nonetheless the ‘headline’ figure, the one we can be most confident about, of charging at 42kW from a charge point with a nominal rating of 43kW is very reassuring. This can be compared with the results posted by Alloam that show 18kW from an APT charge point with a nominal rating of 43kW.

Here we can say that the much-trumpeted Chameleon charging system in this ZOE at this charge point on this occasion fully lived up to the hype of fast charging.

So can we deduce some useful rules of thumb? I think so, by extrapolating back from the start of the charge and assuming it would follow a straight line back to empty (0% SOC), we can calculate it would have taken just an additional 3 minutes to charge if the battery had been empty. Therefore we know how long a complete charge would take.


ZOE Ecotricity 43kW Fast Charger Rules of Thumb

1. Typical rate of charge is 3% per minute (so 2-3 miles per minute).

2. An 80% charge takes half an hour.

3. A 99% charge takes 40 minutes.