Quote:
Originally Posted by MLL67RSSS
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If you have a volt meter and can measure your battery voltage with car off and no loads it should read 12 volts, with engine running if the alternator is functioning properly it should read about 14 to 14.5 volts. Alternator is three phase with a diode rectifier for each, if one diode is bad so one phase gone it will read somewhere between 12 and 14 volts. That can gradually cause the battery to loose charge but generally not a one day fine/next day not thing. Also if you do have a shorted cell you want to get a new battery right away before the excessive load on the alternator overheats it causing it to fail. I suspect you have a bad battery, if you are unsure you could remove it and take it to an auto parts place and have it load tested.
P.S. BTW if you don't know, ALWAYS remove the negative cable from the battery first. Ever seen the end of a wrench melted off in an instant when someone put it on the positive and the other end hits grounded metal somewhere? With the negative cable off battery is isolated so no current can flow if the wrench touches something when removing the positive cable.
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Quick way to narrow it down
Grab your volt meter as suggested above, read the voltage between each terminal of the battery. If the battery is still "good" it will read 12 or so volts (11.7 or so - 12.3 or so)
Now while still taking a voltage measurement, have someone start the car. If you see the voltage drop below 10 the battery is probably going. If it drops below 9 then your battery is likely shot.
Now, let the battery charge for a while and retest. If its fully charged, and you still get voltage droop your battery is probably gone, see below. If at a full charge it works fine, then it could be your alternator.
If its a refillable battery, then really hot temps can have the battery fluid evaporate over time, to refill it CAREFULLY (ie wear gloves, there is sulfuric acid in there) open up the caps, and check the levels, if its low (ie not filled to the top) then fill it up with distilled water ONLY. Recap it, and let it charge overnight, and test it in the morning.
if not refillable, and you still have the voltage drop...time for a new battery