Maintenance chargers need to run overnight minimum. They condition in many modes and take many hours. When they finish the voltage is usually 14V plus.Okay so just been out doing some further testing with multiple meters this time. Testing directly across the battery posts gave a constant 12.6v which looks much more healthy although still doesn't entirely explain why my maintenance charger wants to top it up for three hours and more. I'm going to let it run for a longer period when I get chance and see what happens.
There is something interfering with the voltage reading on the line to the vanity light unit for a period of time when the car is switched off and I'm guessing it's the main interior light fitting. Letting it settle for a few minutes after switching off so the dome light goes out, gives the same constant 12.6v reading as the battery posts.
When the car is on, no matter what's running, I get a constant 14.6/14.7v readout suggesting the aux battery is always charging. So why would it ever deep cycle or go flat? Even with utility mode, the battery saver is presumably designed to keep the 12v charged for a certain time. I am guessing if there are duff batteries out there it's because they've been sat around discharged for ages rather than going flat in normal usage.
The other aspect which may impact them of course is heat or lack of it to be more precise. In most ICEs the 12v is under the bonnet with a nice hot engine, In an EV, there isn't much heat to warm a 12v battery when it's -10 overnight. But equally there is no requirement to dump 70 amps into a starter motor so that shouldn't be much of an issue.
Much happier having a voltmeter on the dash though now