I have a car that doesn't get driven enough. It is modern and the lead-acid/AGM battery drains after a few weeks. There are no power outlets nearby. It is under ground and there is no sun. Disconnecting the battery does weird things to modern cars, so I ain't gonna.

I need a way to keep this battery charged and healthy. Here's my plan:

Starting with a lithium deep-cycle 12V battery (maybe 50-100Ah) and a voltage display, I see two paths:

1) Constant Voltage Boost/Buck

I'd hook up an adjustable boost/buck device to put out 13.2V and just keep it on the car's battery. It would act as a float charge, since the battery will always be pretty much full when this thing is hooked up. At 13.2V the battery should stay full with no ill effects on the life of the battery. They also come with low voltage cutoffs and amperage limiting, so I could have it turn off when the source battery was at 12.8-13.0V (10-20%?).

Pros: Won't kill the source battery. Efficient and cheap

2) Inverter and Tender

I'd hook an inverter up to the source battery and plug a bog-standard Battery Tender Jr into it. Attach that to the car battery and that's it. The con here is it'd probably drain 200mA and I would have to check on it to make sure it didn't kill the source battery.

Pros: Super simple (and I already have the tender, so still pretty cheap)

Every week or month or whatever (depends on how long it takes to drain), I'll carry the source battery up to my apartment (hence the lithium), charge it overnight, and bring it back down to the car the next day and hook it back up.

Is this harebrained?

 

No. You need to keep the car battery topped up, so you need to provide it with energy. A battery of some sort seems sensible.

The first choice is the better choice - the alternative of inverting to line ac then regulating back down is going to waste more power.

 

I would suggest using a VRLA battery, wired in parallel. You could connect it with an Anderson connector.
If you are driving the car, the alternator will charge them both.
If it is parked you can disconnect the second one periodically and take it away to be charged.
You could have two, and keep one on float charge where there is an electricity supply and swap them over.

 

Option 1 will extract nearly all of the energy it can from the backup battery, so it will last far longer than option 2, which will cut out as the battery voltage drops. That and the better efficiency might make it last twice as long as a guess.