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Either you have a bad cell in your battery, or your alternator may not be charging. Most cars will have an dash lights which will indicate if battery is low or generator alternator is not charging, you didn't mention what type and year vehicle you have, so have battery checked at your local auto parts store like Advance or AutoZone depending on area you live.
I suggest O'Reilly's checked more than your mechanic did. Just because he says it is chrging does not mean he checked the diodes, which if bad allow the battery to drain whenever the vehicle is not running. Replace alternator.
the alternator gets power on big wire battery voltage the wire runs to starter, fuse box .alternator fuse goes to automatic shut down relay to relay control in ecm/pcm the field wire comes from ecm /pcm to alt.other field goes back to automatic shut down relay back to charge battery . if not charging at alt. try new alt. and retest.
it sounds like you have blown fuse to the Alternator.. I don't know what car you are talking about but they charging systems vary.. But if alternator isn't getting all the power it needs it won't charge. this should be checked.
This may be an indication of a poor battery connection, a slipping (worn or cracked) serpentine drive belt, or a bad battery.
Corrosion around the battery cables prevents the alternator from replenishing full power back to the battery
A bad drive belt tends to slip around the alternator and like in #1, doesn't replenish full power back to the battery
A bad battery with a dead cell won't accept a full charge.
Not mentioned above and less likely is a bad alternator. If you don't have a bad battery connection or a worn drive belt, I'd recommend having the electrical system checked out (most chain auto parts stores will do a free check).
nancycmixon, When you see this "check your charging system" you need to have the alternator checked. This is the main component for the charging system. A normal alternator will put out 13.6 to 14.6 volts. A few tenths of a volt either way won't mean too much but if they are extremely over or under these values means you have a bad alternator. I believe that your vehicle has a voltage meter in the dashboard and it will usually read in the thirteen to fourteen volts range. As far as your battery losing its charge overnight means that you have another problem and that is you have what is called a parasitic drain. There are a certain amount of parasitic drains already in the car with the radio memory and maybe a clock that constantly displays the time and the antitheft system in the car. These will not drain your battery overnight. You have something else draining your battery that is your problem. Have the alternator checked and mention the battery drain. There is test for this but it is too lengthy to post here. Sorry about that. Good luck with this.
I'd check those fuses for voltage, when problem is ongoing. Those fuses are hot all the time, use a test lite. Did you check voltage drop on cables and wiring? Battery has a full charge and charging system is working correctly?
Check the 'Bat' / 'Charge'/ 'Alt' indicator on the dash panel lights when the vehicle ignition is switched on prior to starting the engine.
This indicator should extinguish within approx 1 second of starting engine.
If the indicator is not operating the alternator cannot provide charge current - check:-
1) the wiring connections to the new alternator,
2) the indicator lamp is not blown,
3) the alternator is properly grounded.
If the indicator light works normally, check the output voltage of the alternator is approx 14.5v when the motor is running.
If its about 14.5v, the battery should be charging.
the alternator has a circuit in it that illuminates the battery light on the dash.that circuit is most likely at fault but because the battery went dead once i would think your alternator is just about at the end of it's useful life.
sorry, probably not. the charging specs need to be checked. with accessories on, ck volts and amps. should be 13.8 minimum volts and around 2-8 amps or so. the only other thing i can think of if it is charging is to ck single thin wire behind alternator for good connection. i believe that is the reading for the dash lite.