Quote:
Originally Posted by carlhunt
Hi,
I have a curious situation on going with start battery failures in my T8. In December during the cold snap I came to the car to find it dead. Opened the doors, jump started, start battery health looked suspect. Cold weather & 8 year old battery so I assumed it's time was up and purchased a replacement AGM battery.
Two weeks later came out to the car and it was dead again. Charged the battery and all good.
Car went in to the local dealer to have the sunroof fixed two weeks later and at the same time I asked them if they could check the battery/charging system due to what I had seen. They said check on battery fine, all ok. I went to collect the car, paid and they said they would bring the car round to the front. Service manager then appears, embarrassed, as car won't start. Test the battery again, condition poor so this was replaced. I was still concerned but needed to follow the process so took the car home and five days later having sat locked & charging when I went to use it - dead again. Now struggling to charge that new battery.
My belief is that the start battery is charged from the hybrid battery. The only change that occurred about the time that this started was fitment of a new wall charger at home (which has charged the hybrid battery in the T8 and other cars).
So, good people - my questions are
Does the hybrid battery charge the start battery?
Any experience of this hybrid-start battery process failing?
Any ideas of what else could kill off the start battery?
Any possibility that a wall charger (in this instance a Hypervolt Home 3 Pro) could interfere with the start battery (I think I'm clutching at straws with this one)?
Thanks in advance......
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In 8 years and 170k of T8 ownership this is what I understand.
The car has a 12v battery and a 400v battery.
The 12v battery simply runs the 12v systems in the car, such as the electronics, the windows, seats etc... It does not start the engine.
The 400v battery is the traction battery and powers the car electric rear motor (ERAD) and also starts the car engine (ICE) through the starter/generator. The starter/generator is located top righthandside under the bonnet (facing from the front, looking at the windscreen). You can see the big Orange cables running to it (which denote the 400v system).
The 12v battery is charged from the 400v battery via a transformer. I think (but not 100%) they share a common earth in the car body.
So the 12v battery does not start the engine. It is not the "start battery". But it does require to have some charge otherwise the car systems will not power up.
In the instances that this has happened, have you had charge in the 400v battery? A faulty wall charger could drain the 400v battery (this happened to a friend with a hybrid c class merc) and then after time the 12v battery could also go flat, but this is unlikely.
Because the 12v battery is constantly looked after by the 400v battery then generally it causes no issues. I have not heard of any T8 owners having to replace the 12v battery.
My assumption would be either your wall box is draining your 400v battery (and is therefore faulty) or you have a fault in the 400v to 12v transformer/rectifier circuit.