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How do I know if my car battery is bad or if it's the alternator?

My last battery went bad to the point where all the water came out of it when I was driving. So I got it replaced with a used battery i got at a battery shop. My turned on quickly with no problem after that for a few weeks. Then one night I left my lights on and had to jump start it. It lasted a few days but now if I leave it off for a few hours I have to jump start it again. It runs fine and everything works fine while I'm driving. But when I turn it off for a few hours it goes dead again? Could this be my alternator sense it's my 2nd battery? Or could the battery just not be good anymore after it went dead the first time ?

13 Answers

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  • 9 months ago

    often its not worh it to buy a second hand battery. Get the battery fully charged and see what happens. The fact there are no warning lihts on when driving tells me the alternator is working OK

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  • Anonymous
    9 months ago

    Usually you blame the battery FIRST BECAUSE IT IS THE EASIEST TO CHANGE. It is the part that AGES.

    I see a few problems here. You bought a USED battery. In otherwords you don't know how old it is, it just works now and it was cheap.(that is the usual story)

    . This is the deal.

    You got to drive at least a good 45 minutes at highway speed at least once a week to keep the battery fully charged ( so 50MPH at least)during the day. So visit your crazy cat lady aunt Mildred...who lives out in the sticks.

    . the new battery will last 5 to 7 years, most making the 7 and THEN IT IS DEAD. End of story. You need to replace the battery with a new one.

    A new 5 year or 72 month battery costs in the neighborhood of $100(with your other battery as core exchange..as they pay you to recycle the old battery about $30 which is $30 off of the price of a new battery. WalMart might not do the battery exchange and credit thing but there are many other places that sell batteries that do do that.

    If you do a lot of short trip driving. Then shutting the car off to WORK for 8 hours and then the short drive home and shut the car off again, or to a store or whatever. Everytime you crank the starter motor it takes a big gulp out of the storage of electricity that the battery holds (think of it like a coffee drink) Every time you take out a big Gulp , that is power gone from the battery and the alternator is peeing in a tiny thin stream to refill it. If you only run the engine for a short time, then the battery is not completely refilled. The next time you use the starter is another BIG GULP of power out of the battery and again it is not being refilled because you live a mile away from work instead of 15 minutes from work....or 15 minutes at highway speed to refill the battery near full.

    . So do a bunch of these short trip stops and starts with the starter motor and your battery has run out of electrical juice (or coffee..depending if you are still thinking of the drink...beer for Homer,"mmm beer")

    This is HOW TO RUN DOWN A PERFECTLY GOOD BATTERY. with a functioning alternator. If I don't go anywhere for the day or 2 then no power is taken or added to the battery. It remains with what it has in it.

    An alternator "trickles in the power"into the battery.

    A starter motor gulps down huge quantities of power TAKEN FROM the battery.

    Having to use headlights or windshield wipers or fan motors slows down the trickle to the battery EVEN SLOWER so it takes more driving to recharge it.

    Alternators have been known to last 40 years. Generators which came before alternators last that long too.

    Batteries have the same story all that time. 7 years is normal for a $100N.A. car battery.

    They do make 10 year batteries (but those are $200) They die on year 10.

    Changing the battery once every 5 or 7 years is not a hardship. Generally no one holds onto a car more than a few years so why stuff in an expensive battery?

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  • zipper
    Lv 6
    9 months ago

    Was the battery fully charged again or did you just jump start it and drive for less than half an hour. It takes a weak battery 45 minutes to an hour of driving with no accessories on to fully recharge. Go to any AutoZone and they will check both for you for free.

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  • Anonymous
    9 months ago

    If your alternator was bad the voltage meter would be jumping on the instrument panel and your lights would be brightening and dimming at night.

    And as far as batteries go just get your schit from Walmart.

    Source(s): nothing
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  • 9 months ago

    The battery is run down and the alternator works, but you aren't driving enough to charge the battery all the way up. Have the battery charged and remember to turn off the lights

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  • Ron
    Lv 7
    9 months ago

    "My last battery went bad to the point where all the water came out of it when I was driving."

    There was nothing wrong with the battery

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  • Dan
    Lv 5
    9 months ago

    You can usually find a new battery on sale cheaper than what the dirt bags sell a used one for. Have the charging system checked and don't let anyone tell you you have to remove the alternator to do it. Chances are good you need a new battery, and when I say "new", I mean from a real store that sells real brand new batteries.

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  • y
    Lv 7
    9 months ago

    If you jump it, and it continues to run, then the alternator is still outputting. If you get it going for a bit, then it dies and needs another jump to get it going again. The the alternator is not maintaining the charge. Now, they can also become fickle at the end, working at time and not at others, that is when it can get a bit tricky. Of course, there are all sorts of lovely testing devices out there now that one can actually use to test.

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  • 9 months ago

    Why are you guessing?

    Go to Advance Auto or Auto Zone and they will test the battery and alternator for free.

    Source(s): Mitsubishi Master Tech
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  • 9 months ago

    A used battery? Really? As Scott recommended, Autozone, Advance Auto or O'reilly will all test your battery & charging system for FREE!

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