I replaced my <1 year old car battery with a new one during troubleshooting, but the actual problem I had was the alternator. So now I have 1 new car battery and 1 used but good battery (and I can’t return the new one). Can I keep my old battery in my car as a backup? How long will it last? Can I just charge it sometimes with jumper cables hooked up to my car while it idles?
Hit Craigslist and sell the new one as exactly what it is: a <1 week old new battery, with exactly the story you just gave us. You should have takers at 2/3 the price of a new one. You'll need the receipt to prove it.
Put the 1-year-old battery back in the car, you will get at least 3 more years out of it. Don't attempt to sell it; a lot can happen to a battery in a year, so ir won't be trusted. its age means its quality will be untrusted, and you'll be lucky to get $20 for it.
For a car battery used in the normal ways, in a non-defective charging and starting system, its #1 enemy is age. The battery sitting on the shelf, well-maintained, will last about as long as the new battery working every day in the car. So both of these two batteries are going to die at about the same time, regardless of which one is actually in the car.
If there is some way to long-term preserve a lead-acid battery, I would imagine it would involve draining all the acid out of it, and store it either dry or wet (can't guess which). These batteries have a dismally short service life (as compared to, say, Edison's NiFe design, or NiCd aircraft batteries, each with 30-40 year life), probably because they are soaking in sulfuric acid ... and also made out of such a flimsy material as lead. Other battery chemistries use alkaline (base) which are less destructive to battery components.
Sell it on the second hand battery market!
I'm pretty sure you can find some online site where you can buy and sell used second hand items.
The reason I'm promoting selling it is that:
- Lead-acid batteries self-discharge
- Lead-acid batteries don't like extended discharge for long amounts of time
- Lead-acid batteries can be maintained on float charge or occasionally recharged, but they have a limited calendar life too; even if you keep the battery continuously on float charge, you'll see someday it has so low capacity it won't start a car
Somebody, right now, right where you live, is looking for a battery to replace a failed one. It's very possible the size that person is looking for is the same as the size of your battery. That person will be the best to take care of one of your batteries.
Flooded lead acid batteries self discharge at the rate of 1-2% per day when not being used (more in hot weather, less in cold). So you should recharge it at least once a month OR connect it to a battery maintainer. Using your car's alternator as a battery charger isn't a good idea, especially if you want to charge the 2nd battery at idle speed. That puts a load on the alternator at load speed which overheats the alt. A cheap battery charger costs a lot less than a new alternator.