ZOE Charging Curve (7kW)

ZOE Home Charging (Image: T. Larkum)
ZOE Home Charging (Image: T. Larkum)

The ZOE is unique among all electric cars on the market in its ability to fast charge with AC current, a feature of its patented Chameleon charging system. However, as with all battery charging, the charging rates don’t just depend on the power supplied but also vary with the level of charge so it can be difficult to predict exactly how long a particular charge will take.

In general there are three factors in play:

  1. As the battery fills up the charging rate will slow down. Some people have likened this to filling up a bucket of water so that it is as full as possible with nothing spilt. It gets harder to put charge in as the battery approaches being full, and it has to be put in more carefully.
  2. Related to the first issue, the charging power is typically reduced as the charge level increases. Anecdotal evidence shows that fast charging of a ZOE slows down at rates above 80% and this is presumed to indicate that a 43kW charge point will drop to 22kW (and perhaps again to 7kW) as the charging proceeds (this is something that testing will hopefully show in detail).
  3. When the battery is nearly fully charged the battery management system (BMS) will conduct ‘battery balancing’. Essentially the system will check the voltage of each cell that makes up the battery and adjust each individual cell’s state of charge (SOC) to even them out. This process can take some time, and it appears as though the charging has paused, though it will resume on completion.

To gain a better understanding of these charging processes it is useful to record charging rates for different amounts of power and levels of charge. To start this off I recently did it for my ZOE charging on its home charge point, the simplest scenario.

I began by running down the charge to a low level by driving it at speed along a local dual carriageway. Then I parked up on my drive in front of the garage where I usually charge (the cable is just long enough to reach when passed under the garage door) and plugged in. I recorded the charge level and the time at the start of the charge, and returned to the car every 5 minutes until it was charged, recording the charge level on each occasion (I used the timer on my mobile phone to remind me as each 5 minute period was up). I suspect the neighbours were wondering what was going on!

To cut a long story short, the charging ‘curve’ at 7kW turned out to be remarkably simple – essentially a straight line up to 99% SOC when battery balancing apparently took place (see Figure 1).

Figure 1: ZOE 7kW Charge Curve (Image: T. Larkum)
Figure 1: ZOE 7kW Charge Curve (Image: T. Larkum)

The balancing seemed to take a remarkably long time (about 2.5 hours, after 3.5 hours of charging) which may reflect the fact that the car was outside in cold weather. Similarly a total time of 6 hours is unusually long; usually it takes less than 4 (at least when it’s in the garage).

While it was charging I had the idea to see how accurate its predicted end time was, and I started recorded its ‘time remaining’ figure from about 40 minutes in. Bearing in mind that it was 99% charged after 205 minutes (and 100% after 355) it’s interesting to see that the prediction of 220 minutes it gave for most of the time was about right (see Figure 2). This is if one considers the duration to be the time to 99% ‘plus a bit’, where the ‘bit’ allows for cell balancing but is probably not an easily predicted amount. Conversely, however, for the first quarter or so of charging time its prediction was very poor, seemingly increasing at first (though I don’t have much data for the earliest period) then dropping towards a better estimate.

Figure 2: ZOE Charge Completion Prediction (Image: T. Larkum)
Figure 2: ZOE Charge Completion Prediction (Image: T. Larkum)

Most EV owners will be careful to allow time for the cell balancing phase to extend the life of the battery. However, I have often skipped this stage when short of time (e.g. when fast charging at motorway services) since the battery isn’t mine. I suspect Renault didn’t think of that when they chose to lease rather than sell the battery to drivers. Whether it matters very much long term we may not know for some years.

Anyway, with regard to home charging we can say:

  1. At 7kW the charging is very consistent and essentially linear up to 99%.
  2. At 99% cell balancing occurs, and its duration appears to be unpredictable (and presumably depends on many factors including temperature, and time since last cell balancing).
  3. Unfortunately, ZOE’s display of the predicted time to charge (even to 99%) is largely useless in the early stages.
  4. Fortunately, it is easily calculated by the driver. The rate of charge is about half a percent (actually 0.475%) per minute to 99%.

In summary, as a rule of thumb:

At 7kW a Renault ZOE charges at about 4 miles (6km) every 10 minutes.