Mitsubishi Outlander Plug-in Hybrid Electric Vehicle Forum

The PHEV has to be left in my garage, over the winter, initially for 1 month without use and after that, 2 weeks of usage and then another long period without use - in this case 4 months. Is this going to cause a problem with the batteries or can I just leave the home charger plugged in and on a timer? Any advice on this greatly appreciated.

In general Li-Ion batteries should be stored at about 50% charge. I don't know the self-discharge rate of the Mitsubishi ones, but for four months I would charge them three-quarters full, disable WiFi and disconnect the 12 V battery and hope for the best.
I would certainly not leave the car connected to the charger which would keep the battery at 100% (at least at Mitsubishi's implementation of "full").

Would it be worth putting a trickle charger on the 12V battery? Or would it be better to disconnect it all together? There have been threads where they've lost charge quite quickly

I would disconnect, as the main battery will charge the 12V battery at set times. You could put a trickle charger on the disconnected 12V battery.
The only risk is that the car might lose some settings if a RAM is off-power for a long time, but any dealer should be able to resolve such an event easily.

I'm not sure if the PHEV is the same, but a Zoe charges the 12V battery whenever the car is woken up. So just opening a door every few days will keep the 12V battery charged. So your other option might be to get someone to look at it every week and turn it on. That would keep the 12V battery topped up and they can check on the state of the drive battery and charge it for an hour if necessary.

Thanks all for your tips. The trouble is finding someone to run the car briefly. I think I will disconnect the 12v and put it on a trickle charger. Leaving the Li-ion batteries at 3/4s. My greatest concern is for these but the word seems to be that they can cope. I read somewhere that Lio-ion batteries have a low rate of discharge, 4-5% in the first month and 3% thereafter.

That is correct, in general Li-Ion batteries have a low rate of self-discharge.

Surely the relatively minor inconvenience of having to jump start the car with a flat 12v battery is outweighed by the risk of losing the system settings

Li-Ion self discharge is almost zero

I have old Lithium battery that I stored for years and I can't notice any significant reduce in voltage

Personally ... I would store my PHEV with 10/15km range left .. so with the EV indicator below the 50% charge .. that is around 66% real SOC.

About the 12v battery ... I would keep a trickle charge connected ... there is no issue on having the trickle charge connected even if the car try to charge the 12v battery too

I'm using a cheap "BLACK+DECKER BM3B 6V and 12V Automatic Battery Charger / Maintainer " for my BMW, and I don't even have to disconnect it when I start my BMW for wake up the engine in the long winter break

I took the cheapest charger which start to work (charge) automatically even after a power cut (this can be useful in case of a power black out which my reset more advanced charger unit) .. since actually this charger is connected to a timer that cut the power 2 times a day for take advantage of cheap electricity .. not a big saving, but this plug ins in parallel to the PHEV 10A charger box

If this was me, I would discharge to near empty and then leave it connected to the charger but use the charging timing so it only charges for 10 mins per week. The car will automatically keep the 12v battery topped up at 2pm each day and then the once a week on the charger ensures no overall drain on the main battery.