Not using phev for 6 months- how to leave if so it starts when one returns.

I have to leave the car I a garage in Spain every year for 6 months and I have tried all the following and they all result in the phev not starting when I return.

1. Charging the drive battery only with supplied charger

2. Charging the drive battery only with supplied charger AND trickle charging the auxiliary battery by connecting this battery to a trickle charger.

I have asked Mitsubishi dealer what to do and he said just leave the phev Charging the main battery but this does not work.

I have searched youtube and there is nothing.

The first time i had the problem the dealer fitted a new auxiliary battery under warranty

Can anyone advise exactly how one should leave the phev when not being used for 6+ months.

Thanks

To assist readers, you should provide the model and year of your vehicle as part of the question.

Out of interest did the auxiliary battery die when it was left on trickle charge?

I would simply detach the 12V battery and trickle-charge it, reconnect upon returning.

Leaving the 12v battery disconnected will cause the BMU to forget the traction battery levels, and therefore reduce range. The BMU will need to be reset for correct operation to resume.

Leave the battery charger on a low trickle charge whilst connected is the best plan of action

I know it's not the same, but we left ours for 2 months. I left the drive battery at around half and didn't do anything special to the 12V battery. However I did get someone to simply start the car while we were away - they just did it after 6 weeks and just left it running for 5 minutes. The drive battery didn't get significantly lower over the 2 months. I don't know if someone could do the same for you.

partly it depends what model you have. A car with WiFi (that is used) will wake up daily and recharge the 12V battery. From what you write, I guess yours doesn't have (or use) the WiFi.