Electric Model Not Available - which battery?

[pogologo]

I have a C5 Aircross PHEV (2021). I am having the issue where if I don't use the car for 4 or 5 days it says electric mode isn't available (the electric battery is 100%) and I can only run in hybrid. After 2 or 3 decent runs where the engine runs at around 500rpm it will kick back into electric only, which I am presuming is because the 500 revs are recharging the batteries.

Reading around, it appears this is likely to be one of the two auxiliary batteries are at the end of their life. Which one is it though? I believe there is one in the boot and one in the usual position at the front. Do I need to replace both or just one?

Does this need to be done by dealer as it is a PHEV or is it just a case of disconnecting and reconnecting as you would a normal battery?

Thanks

[BlackEdition]

So it seems it's the battery in the back.

Mine had 2 issues related to it.

One of them is the picture attached and the second was full EV mode would not be enabled.

Dealership is changing mine and it's not cheap sadly.

To change the rear battery you need to power down the HV system.....as I don't know how and don't want to cause any issues with the HV warranty I just gave t to them to change.

Cost is around £234