XC90 T8 Start Battery Failures...

[Philip Fisher]

Something is evidently very wrong. In 170k miles and 8 years my 12v battery has needed nothing. No top up, no error messages, nothing. It really should just look after itself.

It is either a software problem preventing the 12v battery from charging, or a problem with the hardware of the charging itself. This is beyond a DIYer. What you need is a competent franchised dealer. Who are you using, Marshalls? Is there a better dealer nearby? I think Bells in Bedford has a reasonable reputation.

[carlhunt]

Originally Posted by Philip Fisher

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.

Thank you. This has helped enormously with my thinking on this and the subsequent reading. The car is going back to the franchised dealer but given their response to my initial concerns I'm keen to understand the fault finding process.

At all points the 400v battery has been partially or completely charged.

There is no indication that the wall charger is discharging the hybrid battery at least with the trial I managed over the last 24 hours. I did try a clamp meter which clearly showed the charging but not discharging but it is a fairly coarse device.

I will see if I can get more from Volvo on how the 400V transformer rectifier works and then the intermediate 400 - 12V device. I'd assume that to be some form of DC to DC converter.

The converter between the 400V and the 12v battery does need to be investigated. In the case of the most recent failure after a brand new battery being fitted by the dealer, the car had only been operated for the duration of about an hour before sitting for four days and at that point becoming unusable.

I've not been able to get any charge back in to the battery.

We've only had 7 years and by comparison an embarrassing 50k miles.

Thanks again for the information.

[carlhunt]

So, by way of an update my T8 has spent a bit more than 4 weeks with the Volvo dealer. They have established that there is an intermittent excessive drain on the 12v battery. Much of the time the demand on the 12v battery will sit under an amp but quite frequently it will shoot up to 15 Amps. They have tried a range of methods to try to identify what is driving the increase to 15 Amps but without success yet.

The car has in effect been with them since the 5th Feb. I have borrowed the car from them on two weekends to drive long distances. In less than a day after significant use, the 12v battery will go flat. I carry an additional 12v battery and jump leads to get the electronics on and the car going again. Last weekend I drove 230 miles one day. When I came back to the car 16 hours later the 12V battery was flat again.

The dealer is stuck and now talking about involving Volvo themselves. If they establish the cause I'll post the outcome.

[carlhunt]

Having suggested the IHU, the internal wiring (the car had sunroof issues which the former Volvo Chigwell struggled to repair some years ago) in the end it was attributed to the central electronic module CEM (Part No 36003038) and replacing that does appear to have sorted the issue out. I've had the car back for five days. Must say that I had reached a point where I was looking for a replacement car. I was disappointed that focus/progress only developed after I wrote to more senior people in this dealership. Volvo themselves who I turned to after the first month (01628422522) were not worth the phone calls.

Just to re-iterate the symptoms - rapid 12v battery discharge as a consequence of unexplained drain which continuously varies between 1 and 15 amps. The 12v battery (new) was being discharged in little more than 12hours.

2016 XC90 T8 Momentum (with all the extras at the time apart from HUD).