If that's the case, get a good tender and keep it plugged in. You will have problems with any brand because lead-acid self-discharges and the plates sulfate. Either DD, or keep it floated.

The inevitable has happened, I managed to kill my battery, just 4500 miles under its terminals! Subaru has exchanged it under warranty. The replacement is a Varta sealed unit.. I have also invested in a quick terminal release widget as I don't use the car much, planning to disconnect the battery in such cases.

If it sits, no matter what the brand, get a tender on it.

I can very much recommend the Battery Tender Junior. I left my car on this for 6 months while I was deployed. My only preparations for long term sitting were this battery tender and some Sta-bil in the gas tank. The car started up immediately when I returned.

https://www.amazon.com/gp/product/B000CITK8S/

I can't get power to a battery tender. Has anyone used solar panels and if so can they be connected from inside the car?

Would have to find a connection to the unswitched +12V rail. I don't have any experience with panels but am curious.

I have wondered if the cigarette lighter socket works as a route for a trickle charger....

That would work only if the socket were active with the key turned off. Honestly never tried it to see.

I think it is described as a power socket by Subaru.

It dies when the key is in the off position. Not sure about the one in the glove box.

As mentioned by @Tcoat we've had threads about this. I'm on my third battery, all covered under warranty. I admit the third may have been my fault though; but they still covered it so I give Subaru credit.

I think the glove box lighter is the same. turns off when car is off.

Its the same, got my dashcam and radar detector plugged in there. No power. :)

I've had three emails from the dealer I bought my FR-S from.
All offering to sell me a new battery. Hmm.

The original is still working for now.
Don't think I'd buy another from them if they are counting on that to get customers in.

Fused Battery Tender pigtails to go on the terminals are only $6 at the autoparts store.