C4 Spacetourer 2019 - Start Stop not working

[Nick_UK]

With my 2019 C4 Spacetourer - Diesel 2.0, the Start/Stop never works.
VIN is VF7**************[VIN obfuscated, can be read by forum staff]

Things I have tried:
1. Replaced with new Yuasa EFB battery - YBX7096 75 Ah.
2. Charged battery overnight and disconnected/reconnected after 5 minutes to reset the battery control unit.
3. I measured voltage every night, the voltage drops from 13.0 to 12.5-12.7 after 1 night, and then to 12.0 after a 4 nights. However, it does not drop below that after further nights

Should I suspect a parasitic drain? Does this car have a capacitor that could be the culprit? Are there any smart/telematic features that can be disabled to decrease power consumption when parked?

Apparently you can scan the battery ECU with Diagbox to find out the reason why Start/Stop last failed to engage, if someone close to London SE16 can do that I will be very grateful!

Thanks all!

[wheeler]

Its personal choice but i would be totally happy with no S/S. I find it more of a hindrance & would be amazed if it saves you a coffee cup's worth of fuel over a year. To me it is unnecessary extra wear on the starter motor & other engine components.

[wheeler]

It does look like you have an ultracapacitor but its a lot of money to throw at fixing something you dont actually need to then find out it hasn't solved the issue

[Nick_UK]

Thanks Wheeler, there are a lot of traffic lights in my neighborhood and it breaks my heart to idle at all of them. Sadly with this automatic car switching off an on is just not practical, hence my interest in Start Stop. But can leave the S/S in peace if all potential repairs point to expensive things. Perhaps I will look for someone who can scan the relevant ECU for an indication of why S/S does not engage.

BTW, do you know if this model has a reversible alternator, in which case I will need to check the belt tensioners as well as a cause (I do find it cranks about 1.5 seconds before the engine starts, which I find to be on the long side, but may be normal for this 2L diesel). The car has 35,000 miles (and 6 years) so I am surprised that the accessory belt/tensioners could be worn already, but worth ruling that one out as it will impede on other things as well.

Finally, should I be concerned about the low battery voltage? Discharged to 11.95V only 7 days after topping up with a smart charger seems like too much draw, right?
Thanks

[PaulC5]

Do you have any accessories plugged in such as a phone charger or dashcam which could be causing the battery to go flat ?

[Nick_UK]

No accessories plugged in. I just bought one of these: Sealey Vehicle Finder & Battery Monitor Sensor - BT2020
That will give me insight into the charging and discharging activity over time, that way I will find out whether I have a parasitic draw, an alternator problem, or in fact full battery.

On the S/S topic, does anyone know where it says in Diagbox why the S/S is not engaging?

[PaulC5]

Let us know how you get on with the Sealey, could be something to add to my toolbox.

[merlin667]

The issue is, if the battery charge level drops below 75%, it wont go into S&S. To keep it above this value, you need to drive long distances regulary, especially if the battery is older than one or two years (and I'm talking distances > 70km one direction). If your car is equipped with features like the optional alarm system, there will be an additional discharge of the battery.

My opinion: I will not replace almost good batteries every two years just for having proper start stop. Anyhow, if you are driving a lot in stop and go in cities, it might make a significant fuel consumtion difference. In my case i had many occurences, where i had like 5 to 10 minutes with engine stopped.

Personally i like the way the S&S is working in the C4 2.0 BlueHDI, because its restarting really fast (like when i depress the brake pedal and move over tho the gas pedal the engine is allready running).
I was driving some other cars and the engine was just actived when i was pressing the accelerator and after this it needed almost one second to start - this is annoying.

[Nick_UK]

I promised to follow up with insight from the Sealey Vehicle Finder & Battery Monitor Sensor - BT2020, which cost £20 and was easy to fit on the battery terminals (negative can go on the ground point on the C4).

I discovered that the battery does not get charged above 12.25V unless driving above 1600 RPM or unless there is a large draw (headlights or rear de-mister). As soon as I turn on the headlights, the voltage jumps to 14V, regardless of RPM (i.e. this issue is not caused by the alternator belt being loose or the alternator brushes being worn).

The ECU behaviour is the same regardless of battery SoC and engine temperature.

See below.

The effect of this is that if you are driving around the city, each trip will just drain your battery.

I wonder if this is by design from Citroen? If it is by design, then it is a poor design. A diesel can go 20 MPH at 1600 RPM so for city driving, every trip just keeps discharging the battery.

Having said this, my S/S still doesn't work even with a full battery, so I will need to get a read out from the ECU on the cause for that. For reference, there is a section called "Start Stop information" in the ECU (https://scandoc.online/last/0/18/36/2?page=t3&ctrl=). Does anyone if generic code readers can access that data stream (I'm not sure I want to go the full Diagbox route just yet).