Renault Megane 3 1.5 dci - battery charging error problem

[clau8829]

Hello.
I have a strange problem with my car. Initially my alternator died and I runout of electricity in the middle of the road. I got it back to service and repaired and now is on car again. This time battery charges but I get error from time to time : battery charging errror. This error rarely appears when engine is cold and battery is depleted but it appears very very often when battery is charged.

Someone diagnosed my car and it found errors stored related to start-stop module DTC926741, DTC926755 not working.

- I tried to replace battery but I have the same error. On car I have 8years old AGM battery which starts fine even after 7-8 days in cold weather.
- Someone advised me to double check ground from engine to alternator and to battery but they all seem fine.
- I managed to record a graph few days ago with charging process. On the graph error appears after alternator power is under 250w and battery voltage drops under 12.2 Volts. Error stays until battery voltage goes up to 14 volts.

Video here : https://youtu.be/wf1reyCcXrc

Any suggestion is appreciated. Thank you.

[Stelaras26]

sounds like you should recheck the altenator :P

[clau8829]

Indeed it looks like this but don't know ...there was so much work to remove it and now it looks that it charges battery well. I wanted second opinion because it requires almost a day of work to remove, fix alternator and put it back on. I removed ecu wire which connects to alternator and I have permanent error on dash but also on diagnose which points to alternator meaning that ecu communicates with alternator. With alternator wire connected it says nothing. I was thinking about voltage regulator to have some kind of problems or something but I was expecting to have some kind of error pointing to alternator or at least to be a broken alternator. This kind of error is tricky.

I'm afraid that I will remove alternator again, test it and it says is good. This would be really bad because would mean that there is another component broken and not alternator. I forgot to mention that I was previously consulted an electrician and fuses are ok including the 5Amp one from the back of the battery case.

[Stelaras26]

if voltage drops below 13 when engine is running ,the only component faulty should be the altenator...So,first,make sure that it is rebuilt or repaired...

[clau8829]

I remember when old alternator was working fine I also had under 13V with engine running. This happens because ECU sends command to stop or reduce charging power to the battery and for few minutes everything draws power only from battery. I think is normal behaviour for a micro hibrid engine with brake energy recovery system. This doesn't mean that still alternator is not to blame. Actually I'm hoping is the alternator because don't know what else can be the cause of this problem.

I will try to log what happens when battery is discharged because that is the time when no error appears on dash. After ecu thinks battery is charged and tries to drop voltage it gives error.

[clau8829]

I changed alternator with a known good one and nothing is changed : I have same error. This means the problem is maybe in the UPC. I also got error "battery disconnection unit".