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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?

  • Yes. If it's a flooded lead acid battery, just make sure it's strapped down so it doesn't fall over or slide into anything (don't ask me how I know.) You shouldn't need to top it off more than once every month or two if it's a good battery. Hooking jumper cables to it for a bit should work fine, although it will require you to sit and idle your car for probably 20 minutes. Alternately, you could just swap batteries every so often, or get one of those tiny solar float chargers and put in your window to keep it charged. – the_storyteller Jul 19 at 14:27
  • You could also check out split-charge relays and have it properly and safely connected. – Solar Mike Jul 19 at 14:58
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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.

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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.

  • 1
    while you may certainly find someone who will buy a used battery, I wouldn't expect to get a great deal for it. Anyone shopping that market will probably be wary that many people are just trying to sell a destroyed battery, and it can be hard to determine whether it's going to hold a charge or not if you don't know the history. – Christopher Hunter Jul 19 at 18:05
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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.

  • The load on the alternator, excluding the system demands, will be mostly from the battery that did the starting, so overheating won’t be an issue. At least was never an issue for the many split-charge systems I put in place and some were driving winches (both 12v and 24v)... – Solar Mike Jul 19 at 15:56
  • @SolarMike But then if you wait for two weeks (with idle current), and start the car only then, both batteries have a deficient state of charge, and the load on the alternator is higher. For daily use, I'm sure your theory is correct. – juhist Jul 19 at 16:02
  • ...of course, good alternators may have a current limiter to limit the system voltage, but considering that some car manufacturers don't even have a separate voltage sense wire on the alternator, what are the chances the alternator is good? – juhist Jul 19 at 16:03
  • @juhist those alternators that don’t have a voltage sense wire... it’s internal and more than capable and less to go wrong compared to alternators that have external regulators.. The other massive advantage is that there are less wires for “grease monkeys” sorry vehicle technicians to f**k up... – Solar Mike Jul 19 at 16:23
  • You're all reading the post as if he wants the second battery hooked up. That's not what the post says. He wants to know if he can keep it in his car and just use jumper cables to charge it while he idles. If he leaves the spare battery in the back seat/trunk/bed for 30 or 60 days and then hooks it up with battery cables, the spare battery is going to be 30-60% discharged depending on ambient temps. Charging from the alt at idle with jumper cables is doable, but not a good idea. He'd be much better off slapping it on a battery charger. – user9181 Jul 19 at 18:14

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