Replacement Aux battery has now failed again on Outlander PHEV

Anecdotal evidence seems to suggest that this is a common issue with phevs that don't do big mileage.

A neighbour has a phev Toyota, and the battery has had to be replaced., as it was regularly going flat.

The solution from the main dealer.. take it on a long drive twice a week.

Do PHEVs drive the alternator when driving off the traction battery?

I got a 12 v warning on my 2020 2 series phev this week, I presumed it was kids on holidays and the car was idle more and I did not bother charging it, to avoid sitting at 100%.

Having charged and driving around the warning has not popped up.

From a quick google the outlander charges the 12v from the main battery, so that suggests to me you might have been in a similar position.

I found this also,

If the WiFi is enabled, the car apparently charges the 12V battery daily (from the main drive battery) even if it's just parked and unplugged, but if WiFi is disabled, it won't ever charge the 12V battery unless you connect it to a EVSE or put it in READY mode.

BUT, without WiFi, it only loses about 15% SoC per week. So whatever amount of power the WiFi consumes compared to the rest of the vehicle must be significant. It's likely not just the WiFi but an entire computer that's kept running 24/7 when the vehicle is switched off.

Also read that it only every charges the 12 v to 80%, if low use/charging followed by idleness the battery might never get a chance to reach 80%

There is an entire multi page discussion on the Outlander PHEV here:

Might be a better place to get the info as it is specific to the model and there are plenty of owners in there who can offer advice and experience.