Month: December 2015

We do our best to make things as intuitive and clear as possible, but sometimes that doesn’t work, or the idea presented simply needs some explanation. The blue aiming bar shown in the driving and braking screens, and soon to be released in the consumption screen too, might be one of those. So, here goes……

The Aiming Bar is always displayed under a Braking Torque bar. Braking Torque is the force of braking. It does not correspond to power, as power is proportional to torque multiplied by speed. In most cars, Braking Torque corresponds to the position of the braking pedal. For the Zoe, this is almost true. Lifting your foot from the accelerator pedal already induces a bit of Braking Torque. Pressing the braking pedal increases that torque.

The Blue Aiming Bar is the maximum braking torque the car can apply using only regeneration. Ideally, you should never brake more than the blue bar indicates: every braking beyond the Blue Aiming Bar is applied through friction braking and the corresponding energy is lost. As I explained earlier, the Blue Aiming Bar is a bit counter-intuitive: at very low speed, the motor cannot regenerate so the bar disappears. At high speed, even a little bit of torque will make the regeneration hit the maximum charging limit of the battery. When the battery is full, there is almost no regeneration at all.

For that reason, avoiding friction braking requires a bit of getting used to. It feels unnatural to “feather-brake” at speed and then apply more and more while speed bleeds off, and it certainly needs stricter anticipation.

Note: this does not apply to the Fluence and the Kangoo, as these cars only use the accelerator to control regeneration. Using the braking pedal applies friction braking only.

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.

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.