There are many misconceptions about fast charging. One being that “the battery should be as cold as possible when fast charging”. I mean, when hooking up the battery to a fast charger, all these fans start to run right? So it must be true. Like so many assumptions, unfortunately it isn’t. Zoe’s Batteries are very, very happy when they are over 25C and actually, when they are colder than that, the BMS will rightfully cap the maximum charging power. 43kW fast charging a pack that has been freezing overnight to 0C would almost certainly damage it beyond recovery.

Having said that, overheating the cells, that will still happily charge at a temperature of over 40C **), is a very, very bad idea. Renault implemented a pretty clever solution for that, installing an extra evaporator of the climate system in the air inlet of  the battery compartment, which is why you not only hear the battery fans kicking in, but also the climate control when fast charging.

Which of course leaves the question, why would the car do that when fast charging and the cells are way below that happy temperature of 25C and higher? Well, consider that about 10% of the energy is lost to heat *) when fast charging the battery, that is over 4kW of heat being generated, which is substantial. The car is simply using a pre-emptive strategy, blowing cooled air over the batteries. If that annoys you (those fans can really “take off”), simply put the car in ECO mode before powering it down. The climate system will not kick in now kick in at a much higher temperature. And of course, the batteries will heat up faster. Which might or might not be a good idea really. Some chargers do not appreciate the interruption of the charging process and ECO mode might avoid it.

*) some say full cycle energy loss in LiPo is 3%. While that can be true under ideal circumstances, 43kW (2C strategy) is not that.

**) Masoto Uriguchi, battery engineer at Renault states the batteries are happy up to 60C, but should not be taken above that.

“Granny charging” is used for ultra slow, normal plug charging. So it’s not about charging up grandma, but charging AT grandma’s place, if she lives just a tad over half the range away and no decent public chargers on the way, read: emergency charging.

And there is another reason why I keep it in the trunk: I have had occasions where a flaky grounded charger put my Zoe in the “red nose” mode. I’ll talk about the reset procedure in another post, giving me back my precious cruise control, but one part is: a successful charging session, however short.

Here are two video’s of my home-build. It cost me roughly 150 Euro’s. A “real”, clunky one could easily set you back 400 Euro’s and Renault retails (retailed?) theirs here for more than 700 Euro’s.




For the technicians: no, this is NOT a fake system that simply puts the proper pilot signal on the CP pin. It is a decent OpenEVSE system, doing all the checks and balances.

LED schematic (thanks to user “seti”)



I took the liberty of posting a picture of a rebuild that reader “seti” made, see the comment thread below.

Granny Cable1

And another one by Andy Fraser.

Thank you all for rebuilding!

OK, maybe I am a non-friction-braking junkie.

Today, I had a discussion with a friend who owns a Tesla model S. The single motor type, but with the complete performance pack. As we started to talk about braking, we figured the S’s stategy is quite different than on the Zoe and is actually closer to the Fluence and Kangoo. In simple terms, on the S, touching the braking pedal does friction braking, period. Regeneration is applied through not, or barely touching the accelerator. He calls this “one foot driving”.

He also told me the “average” tesla driver doesn’t do any aiming-braking. With that I mean unpowered coasting, letting the motor basically run free. It seems to be popular with Fluence hyperdrivers to avoid the regen-use cycle.  I have to assume this is because hyperdriving is less of an issue with a 80kW battery.

The regenerative braking strategy itself is different too. The Zoe seems to aim at fixed torque, mimicking a traditional car. It is transparent to the driver if that torque is generated through regeneration or friction braking. The S seems to aim at a fixed regeneration power level (up to 60kW, which is lower than the Zoe per kg). As I explained in the previous post, that means increasing torque as the speed bleeds off. When the car reaches roughly 50km/h it seems to switch to constant torque, probably as otherwise the braking would get too brisk and uncomfortable. It is an interesting approach (irrespective to whether it is controlled through a braking pedal or not) as it is the behaviour I am trying to mimic through following the blue bar in the driving screen.

I got an interesting question about the blue “Aim bar” in the new driving screen. Confusion arose how, when increasing speed, the maximum braking torque aim bar actually shrinks, while common sense would dictate it should stay the same or even grow a bit.

Well, common sense is not always right! Assume we are running at a speed where the motor itself can apply it’s maximum braking torque. The power this potential braking would generate is the torque multiplied by the angular velocity. So, as the speed of the vehicle goes up, by definition, the power regenerated with this maximum torque goes up too. Pretty quickly we will hit the limit of the battery: 43kW, and that is under ideal circumstances. At any speed above that, you’d have to actually decrease torque not to go over this maximum, and this is exactly what the power management does and what is displayed through the blue bar..

Consequently, if you “follow” the bar coming down from i.e. 120 km/h, you will find you’ll easily hit that maximum with only a little bit of braking pressure. While speed bleeds off you’ll find yourself braking harder and harder following the blue bar, chasing the maximum power. That goes on until you reach the maximum torque the motor itself can apply. For a short moment, this is a fixed value. As the car decelerates further, now at a constant rate, at constant pedal pressure and with decreasing power generation, it reaches the point where the motor is simply turning too slowly to apply its maximum generating torque and the ability to brake through the motor collapses. I you don’t do anything, the friction brakes will kick in. This is the “traffic light effect”.

Hope this helps.

Most prominent change is the driving screen now showing braking torque, with the bar extending to the left, and an aiming point for maximal motor based braking. This is more of a feed-forward instead of the old feed back system with the red Friction Braking (avoid) bar now for you to peruse.

And of course there were a couple of strange bugs and spelling errors that we fixed.

Today’s release contains, among others, a few serious bugfixes. In the previous versions switching between the different screens filled up the field request list, which slowed down considerably the entire application. This has now been solved.

Another feature that has been added is that Main, Technical and Experimental fragments can now be accessed by simply sliding the entire page to the left of the right. You may also notice some graphical improvements.

Last big change I want to point at is the new Tires activity where those of you who have TPMS installed can read out the actual pressure of the 4 wheels.

When time is money (both re. your own time as well as how the operator calculates the rates), the following guidelines will help you, especially in winter. The’re all fairly obvious:

1. Try to avoid fast-charging starting at a high SOC to avoid entering the area where the car squeezes the charging power. This squeezing can start as low as 35% SOC when it is cold. Drive as far as possible to keep the charging power high for as long as possible.

2. Try to charge with the highest possible battery compartment temperatures. As driving increases the temperature substantially, try to fast-charge at the end of a drive, not i.e. the following morning. Fast charging itself also increases the temperature.

3. Quit fast charging as soon as you can. If there is a slow-charger at your destination, just fast charge until you can reach it.  This ensures fast-charging at the highest possible power and trades “real” waiting time (twisting thumbs) against “virtual” waiting time (car is charging for a longer time, but you’re not waiting for it doing nothing).

A rule of thumb is that squeezing from 43 kW starts at 30% SOC plus twice the battery compartment temperature for a Q210, and from 22 kW at 65% SOC plus the battery temperature for an R240. Note that this is for the 22 kWh battery. The 41 kWh battery behaves substantially different, but we don’t have enough data yet.