12 Volt battery issues

[Wotters]

Thanks for that very comprehensive reply. Excellent.

Surely though it would have made a lot of sense to charge the 12 volt at the same time as charging the main batteries. Oh well maybe in a future version.

[RichR]

This seems to be the case for most if not all EVs, so there's possibly a safety related reason for it. Generally it's not an issue though, as with petrol/diesel cars.

[Xtopher]

Been reading into this a bit myself. It does generally seem to be down to safety reasons that the 12v battery is only charged when the car is ready to drive (i.e. VWG expect it only to charge when being driven). My understanding is because of the way the high voltage systems work, the 12v battery requires all the HV systems to be energised to start charging. This is done by some beefy switches called contactors which make the electrical connection from the battery to the rest of the HV system (motor, inverter etc). They are engaged when the car has woken up and decides it is happy for the HV system to be energised (after doing its safety checks). Its a design decision taken by VWG to ensure the HV system is only powered up when the car is ready to be driven, so can't be left in a potentially dangerous energised state when the car is otherwise idle, like if a technician is working on it in the garage, the motor wont be energised if the car isn't 'ready' so can't get a shock off it. Now as I understand it they could have created another connection to the HV battery just for the 12v system which could be operated independently of the other HV systems, and allow it to charge independently but I guess this would add complexity and cost, which they decided they don't need as most of the time the 12v would be charged just fine when its drive and it would be driven enough to keep it topped up.

Hopefully most of the issues being seen around this are just cars needing to have their 12v batteries charged during the PDI process at the dealer and that will top it up enough to get the car back on track.

[RichR]

It's the same on my Mitsubishi Outlander - in fact, when you plug in the charge lead you can hear the contactors connecting (noticeable 'donk-a-donk' noise from under the car). People rarely get flat 12V batteries if the car is used in the average manner. Just the same as with petrol/diesel cars - one 30 minute trip a week is usually enough. Bear in mind the 12V battery doesn't have to rotate a heavy internal combustion engine like on a petrol/diesel car too (though it doesn't on my PHEV either, that's done by the high voltage battery).

[Xtopher]

Bear in mind the 12V battery doesn't have to rotate a heavy internal combustion engine like on a petrol/diesel car too (though it doesn't on my PHEV either, that's done by the high voltage battery).

Yes at least you can just go sit in the car on the drive and it will charge it

[Toolman77]

I wanted to run an electric cooler in the car when it's parked so i did some measuring with a voltmeter.

When the car is turned off the voltage on the 12 volt battery is ca 12V
When the cooler is connected to either the 12V in the trunk or the 230V in the back there is a volitage drop. So both theese outlets run off the 12V battery. It's therefore not a good idea to connect the cooler since it draws 5A and would deplete the battery quite fast.

With the charging cable is plugged in the voltage on the 12V battery is 15V so it seems that it is charged everytime the traction battery is charged.

When the tractionbattery is fully charged I will check again if the 12V battery is still charging