XC90 T8 Start Battery Failures...
 

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......

I can’t answer your questions but I’ve posted about a few issues recently.

I’ve had the turtle/tortoise symbol appear claiming an issue with the hybrid drive train and the battery having insufficient charge. It had plenty of charge so that was nonsense and the irony is the car will only run on battery in this situation.

Today, a week later I’m being shown 12v battery warning issues, and not remedied by trickle charging.

So whilst I can’t answer your questions I’m wondering if the two are related and a hybrid battery issue is knocking on to a 12v issue or vice versa.

Given the complexity of the system and the fact my car is 3 years old at the end of March I’m getting itchy feet.

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.

Thanks for this Philip.

I’ve experienced a low 12v battery recently. I mostly do short journeys on electric and keep the 400v battery charged up after use.

Is your understanding that the 400v battery should prevent my 12v battery from ever being low?

Thanks

That is my understanding yes. The only way it can get charged is from the 400v battery. The car has no 12v alternator.

Interesting. I had a botched software update recently, Volvo told me my 12v battery was low at the time and it transpires this seems to be the cause of this update issue.

Given they would have known my car was a T8, I wonder why, if your hypothesis is true, that Volvo didn’t think it was strange, or that there was some problem, if my 12v battery was as low as it seemingly was.

Furthermore see a reply I made to a trickle charging thread I started. Trickle charging from the front didn’t charge the battery in the boot….. the plot thickens.

What do you mean by trickle charging from the front? You mean plugging in the type 2 socket to charge the traction battery? Or using the 12v terminals under the bonnet?

In normal use the 400v system charges the 12v battery through a transformer.

To manually charge the 12v battery you can charge through the terminals or connect to the battery. Both should charge the 12v system.

I attached a CTEK 12v trickle charger to the positive and negative terminals under the bonnet. This charged the front battery to full overnight. When I attached the CTEK to the battery in the boot 24 hours after connecting to the front battery, the rear battery was showing all but flat.

I can’t see any battery would have drained in 24 hours, therefore believe charging the front battery via the terminals, as I was told was correct procedure in another thread, did nothing to charge the rear battery for me.

I think you are mistaken. You have a T8 right? If so there is no such thing as a "front battery". You have a 12v battery in the boot which runs a 12v system in the car. This system has some charging terminals in the engine compartment. The battery itself is in the boot.

You also have a 400v battery for the electric drive motor. This 400v battery is either charged via a Type 2 electric car charger or charged via the engine running.

The 400v battery keeps the 12v battery charged.

So if you are driving along with the lights, heated seats, fan, wipers etc.. All running, then these items will be depleting the 12v battery (which is in the boot). The 12v battery will then be being topped up by the 400v battery and once that is near to discharged the engine will start to top it up.

Thanks. Sorry, when referring to front battery I meant the terminal points found under the bonnet.

Irrespective, I have some weird issue going on with the 12v system, whereby my CTEK charger was showing full battery charge this morning at 8am when connected to the front terminals.

By 18:00 it was showing all but flat when I connnected the CTEK directly to the battery in the boot. I’ve driven approx 30 miles today using either AWD or PE mode.

I’m struggling to think I would have drained my 12v battery from full to all but flat in the space of 10 hours? And a low 12v battery isn’t a new problem to be (reference the botched software update). The 12v system service message is new. I’ll try and monitor it, but the behaviour is weird. More so that I regularly keep the traction battery topped up.