Video blogger Alloam, in his latest “Living with my Renault Zoe” episode mentioned something very interesting. A Renault battery engineer basically told him the extra capacity is added most of all because the LiPo’s have a fairly rapidly decreasing capacity (SOH) in their early life. With the extra headroom, they have been able to accomplish two things (at the cost of that headroom no less!):

  • The customer doesn’t experience that rapid capacity decrease in early life. To me that is an argument that is slightly in the “to avoid complaints about customer service, we will not be providing customer service any more” category, but I do see their point. You want to avoid customers complaints about that, especially in early life. But……
  • By not using that headroom, the overall SOH curve is higher than when using it all. That is no surprise really. It is no secret that topping a LiPo to the rim does stress it quite a bit. It was interesting to see the effect in a graph, even while it was unit-less.

Also the cumulative SOH of all Zoe’s graph is quite revealing. Dig in at 08:30

Edit: I had not read the link to the original presentation by Masoto Uriguchi.

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!

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.

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.

We get a lot of questions about the braking system. Here is how it really works.

Zoe braking system

  • Coasting without braking is not a braking function and is performed entirely by the EVC (the motor management computer);
  • As the driver starts pushing the brake pedal, the requested torque is computed by the UBP (braking computer) based on main cylinder pressure and pedal position. This requested torque is passed to the ESC (the ABS computer that controls the oil valves to the friction brakes);
  • The EVC permanently sends messages to the UBP stating the maximum torque of the motor. This is determined by gear, SOC and temperature;
  • The UPB requests the EVC to apply braking torque. In principle this is the same as the driver requested torque, up until the maximum the motor can deliver;
  • The EVC sends the truly applied torque to the ESC. The ESC computes the difference and applies friction braking for any difference.

Note that if you put the car in N, no motor braking is possible and the ESC will command all braking through the friction brakes.

In normal operation this means braking is almost entirely regenerative, with the following exceptions:

  • very fast braking: the hydraulic system is faster than the electrical system (see below for some more details);
  • very powerful braking: the hydraulic brakes are more capable than the electrical system;
  • related to the above: if fast, powerful braking is applied, it’s hydraulics all the way as the car might want to apply any form of EPS which requires individual control of all 4 wheels;
  • when the electrical system cannot apply the requested torque (max charging power reached, you can see this in the driving and braking screen);
  • at very low speeds, when the motor simply cannot brake.

Here is a revealing graph: yellow (hard to see, hidden under purple and red) line  is the driver requested torque. The blue line is the regenerative torque and you can see the hydraulic system (red line) compensating for any difference. Note that the final cut over is at a very low energy state (roughly last half second before full stop, 10% of the time, 1% of the energy).


*) Other than this graph shows, when serious braking is applied very fast (3rd bullet above), and therefore, the hydraulics kick in immediately and substantially, they are not released anymore and replaced by motor torque. So, for economical, max regen braking, it is better to not only push the braking pedal not too deep, but do do it gently too.

ps: look here for a description of the computers in the Zoe.

pps: there is a ton of information in this document on scribd.

ppps: Here is some more info on braking.

Today I have spoken with a Renault specialist and to make a long story short: my definition of friction braking is WRONG. In normal operation, the brake system is entirely “fly by wire” and does not perform friction braking as CanZE reports. I will fix this, but I doubt it will make this weekend-release. Long story short, Zoe uses far more motor braking than anticipated. To be continued.