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Alternator question

1K views 3 replies 3 participants last post by  the bubba 
#1 ·
My battery was dying after sitting for two days (not starting) but my alternator checked OK at 13.5V after jump starting. I went ahead and replaced the battery and now my alternator reads 14.5V, what's happened. Usually when a battery is weak, the alternator reads higher (than 13.5) in order to charge it. Why is it reading higher now after a battery change?
 
#3 ·
Most modern (and all Jeep) alternators require (!) +12v from the battery, that is what powers the alternator. The days of being able to run a vehicle without a battery are over, at least for most vehicles. So when your battery is weak, the alternator will be weak. If the battery is weak enough, the alternator won't work. If the battery is dead, the alternator will be dead.

So that you had a weak voltage out of the alternator with a weak battery makes perfect sense. Generators will produce power without a battery, and SOME alternators which are "self excited" like GM's single-wire alternators don't require batteries either. The the vast majority of the rest of the alternators all require a good strong +12v from the battery to work/produce power/keep the battery charged.

That's one reason you can't charge a dead battery with the alternator after removing the jump start battery. And that's the reason most vehicles will die with a dead battery within a minute or two after the jump start battery is removed. The alternator isn't getting the power it needs from the battery to produce power to power the ignition system.
 
#4 ·
Thanks for the responses, you learn something every day (especially me). I'd always thought the alternator would/could charge the battery back up somewhat after driving it a while. Mine would last the rest of the day after jumping it but would be dead after a couple of days of not driving (wouldn't start the jeep but still had lights etc). I finally figured the battery was toast and got another one. Glad to hear the stuff about the alternator.
 
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