The Zoe braking system

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.

11 Comments on “The Zoe braking system

  1. I belive there can be situation that the braking system of the Zoe can fail totaly, both electronically and also the friction brakes fails. There are some reports on the Internet that this has happende with the Zoe. Then one has only the parking brak to stop the car, hopefully.

    • The design seems pretty robust (at least as robust as a conventional vacuum or servo powered braking system), with the added torque of the motor. I am not dismissing your observation “just because”, but would such a car even be certified for the road nowadays? Unless of course it was a double failure (failure of the backup system), but then all bets are off.

      I would be truly interested in a trustworthy account and how his could have technically happened!

  2. Is there a diagram/documentation available showing where the sensors controlling the brake pedal movement are, is there more than one computer controlling the friction brake? If the only one computer that is controlling the braking, including the friction braking, is failing, than I presume the Zoe has a total braking failure? Is there available a detailed description of the braking system where it is describing failure scenarios and then the possibility of total robustness or no-robustness?

  3. The R240 had run 5k km when brakes failed. Brake failure indicator on dash. Speed less than 30. Stopped by using parking brake. Called R service which suggested. Did reset procedure. Start the car but brakes do not function. Do reset procedure second time and brake works. Brake do function the 20 km to Renault service. They could read brake failure information in the car diagnostic system. The car is still at service.

    • If the car didn’t brake at all (meaning it also didn’t when fully pressing the pedal, so un-assisted), that would be a very serious failure mode. If it were my car I would insist on a printout of the failure codes and descriptions as proof, and an answer from Renault Service in your country. At this moment, I can only imagine an air bubble in the hydraulics, but that’s probably my fantasy failing on me!

      DISCLAIMER: I am not a car mechanic, nor involved with Renault. I am a guy interested in technology who happens to drive a Zoe.

  4. The R240 returned after also tech. people from R/France have worked on it, now says it is function and R240 is safe. They said many error code was read from the car, however state only C023. Said no parts were replaced. Said there were air bubbles in the braking system. How does bubbles appear after 1/2 year, and how does the reset procedure (second run) reset braking function? Are the braking computers (software, hardware, sensors) really robust enough in such a bubble situation?

    • Seems my hunch was right then? And that’s about the only positive thing that can be said I guess.

      An air bubble in the hydraulics is a very, very bad thing. No car can handle that well, though front and back should be independent. AFAIK (but you’ve noticed my disclaimer earlier) it can only occur with a bad oil type or a wrongly executed (re)fill. The procedure for the Zoe I have not seen, only referred to, but I know it is complicated and cannot be done without the CLIP diagnostic system hooked up. It’s not like the old days with your rosted 2CV 😉

  5. Internet search revile that similar happenings seems to have been reported. One said also after 5K km use. Is it two independent hydraulic systems? Perhaps there is a leakage where air enters? The shop said they did not know what could cause this situation. Well, have to hope it does not happened again. And, thanks for the link to the information/documentation.
    If you intend to take your Zoe on a visit to Norway, her is some info.:
    And this one:

    • Thank you! That guy in your second link is a mate of mine! I have to defer on how air could have come in. I can only think of two scenario’s: a small leak, draining the reservoir. The car should have warned you way way way before the reservoir was empty though. The second is that the booster servo creating an under-pressure in it’s suction line. You might want to now and then brake with the gear in N, so full hydraulic braking. If the pedal feels “spongy” instead of hard, that is a sign of air. But as said, I don’t feel confident to give any advice. I hope you can still enjoy your Zoe!

  6. Bit of a necroposting here, but I’ve driven a Zoe downhill that both didn’t have its traction battery attached and 12V disconnected. Sure the vacuum was off, but the car did brake, indicating there’s a layer of mechanical fallback on good ol’ unpowered hydraulics if everything falls apart.

    Adds to the version that the car above had hydraulic system malfunctioning in first place, not related to electronics.

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