HV Battery discharging when parked

[Bertduursma]

My 2016 i3 94Ah discharges when standing still.
Discharge is about 8kWh per 24 hours so quite significant.
When I have charged my car to 100% SOC and leave the charger connected, the battery drops down to 99% in about an 1.5 hours. The charger then starts to to top up the HV battery. This process repeats endlessly.
Could this be a bad 12V battery?

 

[mojo]

If the 12V battery is still the original, I would change it anyway as a precaution. You should be able to find a date code on it, to confirm that it is the original.

There is one process that could partially explain what you are seeing: cell balancing. This is an automatic step that only happens when SOC is above ~80%, and it can only happen when charging has finished. It involves selectively discharging some cells so that the SOC of each cell is more or less equal. Unavoidably, it partly discharges the HV battery, so once it is complete, you should expect to see charging resume, to top the battery up again.

However... I think you could expect to see a 1% drop in SOC occasionally - but not repeatedly. It may suggest a problem with the HV battery. It would be interesting to see what your car's "max kappa" figure is...

 

[EvanstonI3]

When the 12v battery fails, it self-discharges over time at rate that increases. The HV battery comes on to try to maintain the 12v battery above 12.0 volts. I've documented this on my 2017 with a Bluetooth 12v battery monitor. It started out slowly but increased over 2 weeks until I just had to replace it as the HV DC-DC converter was coming on every hour to "boost" the 12v battery for exactly One Hour. If I had left it, the charging process would have eventually been continuous.

Also- if there is a fault somewhere that is causing a drain on the 12v battery when shut off, it will suck power from the 12v battery while the car is off and you will get the same result. Anecdotally I have read of one or two owners who had a faulty headlamp module that was shorting out causing this excessive draining.