Thanks. Is there a simple way to test this, please? (I guess involving isolating it etc.).

Can a failing aux battery drain the main one, or is there protection?

Did you Reset the BMS in Sept '19 when you have installed the new battery?

The AUX battery is only used when Start/Stop is used when the engine is stopped. There is a relay to AUX battery and it probably cannot drain the main battery if the relay is OK.

You test the AUX battery directly on its terminals...

Check the charging voltage with engine on, to see the generator health...

I followed the procedure in the handbook, which doesn't mention resetting the BMS. There seem to be various notes on the internet, depending on model. Is there a procedure for a 2013 V70 D4 Geartronic, please?

Just measured voltage, outside temperature ~12C. Car was driven for about 10 mins yesterday, not started since. 12.32V. Started, and was charging at idle of 15V, dropping slowly as expected.

My colleague has a load-tester, that'll be the next step.

The problem is now...You must not reset the BMS with the old battery. You can get away with 1 yr old battery...but yours is 4 1/2 yrs old...

12.32 V sounds too low...but did you measure it while the car is in sleep mode? Wait like 10-15 mins after locking the door... Charging seems OK.

The BMS procedure for Volvo cars 2010+ that worked for me is a few posts above...

When driving, do you use Start/Stop?

And both batteries MUST be AGM type. What is your main battery model and power?

Aux is original, main is Volvo-supplied, part number 31652063. Not near car now so can't check, but it's what the main dealer supplied.

If the BMS needs to be reset, then I'm disappointed that the procedure's not detailed in the handbook. I just followed the instructions.

Start-stop only works if I've had a trickle charger on for several hours, or if the car's been driven several hundred miles in short order. It generally gets lots of shortish trips (not good I know, but I've never had a DPF warning). I let SS operate when available, unless near busy roundabouts etc.

Then the battery is probably suitable. My AUX batt is also the original one. Those really last long time, because they only operate when S/S is in use.

Which handbook was that?

S/S probably doesn't work because of the low battery....

Get yourself some bluetooth adapter ELM OBD2 Icar Vgate for 15-20 EUR (don't go cheaper than that - the one in the picture) and CarScanner App for Android phone (only 2 EUR) (or iphone?) and you will have much easier life with problems like this.

The handbook that came with the car, supplied by Volvo. Actually titled "V70 & XC70 Owners Manual".

Copied from there (fitting a new main battery). There is no mention of resetting anything, so I assumed nothing more is required:

"Lower the battery into the battery box.
2. Move the battery inward and to the side until it reaches the rear edge of the box.
3. Tighten the clamp that holds the battery.
4. Connect the ventilation hose.
> Check that it is correctly connected to both battery and outlet in the body.
5. Connect the red positive cable.
6. Connect the black negative cable.
7. Press in the rear cover. (See Removal.)
8. Fit the rubber moulding. (See Removal.)
9. Align the front cover and secure it with the clips. (See removal.)"

I do have a similar ODB2 reader, which tells me nothing other than charging voltage. I also have a good Multimeter, which I use to measure voltage. More usefully, my colleague has something expensive (don't know the detail, but he can make my wipers & washers operate with it, for example). Again, this lists no faults related to battery or charging.

I've options ranging from ignoring it (car always starts, even in depths of winter), through to paying a main dealer a potential fortune to make the warning go away, replacing whatever they advise.

You're right...just checked mine...

That is irresponsible by volvo, because service instructions require BMS reset...

That same manual has incorrect instructions on how to change front headlight position bulbs. Not mentioning the plastic tab one has to move to pry out the bulb...Manual says to pull the bulb by the wire and it won't budge that way :speechless-smiley-5

So, that's another mistake then...

If you use the Car Scanner app, with Volvo P3 profile, you can read the main and auxiliary battery voltages using that.

Low battery warning is triggered from a battery voltage below 12.4v, which is quite cautious IMO.

Give your battery a long charge with earth to engine or other earth point, and positive to battery terminal. After the charge see what the main battery voltage is. Very healthy is 12.8v, anything over 12.4v is OK - there are better guides on the internet for this.

AGM batteries which are required in these cars go up to 13.00 V