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Check current drain with car off. Start removing fuses until one makes the drain go to zero. then diagnose that circuit. It could be something silly like a glove box light.
Tom, When running voltage should be 14 to 14.8 volts, check all connections,fuses/relays,fusible links if equipped, bad battery, alternator defective, not wired up properly, broken wires loose alternator belt, etc. Diagnosing Alternator Problems EricTheCarGuy
A bad alternator no longer charges the battery, the battery will die once it is drained. This will occur more rapidly when using equipment that runs off of the battery( lights, heater blower, windshield wipers etc.) even electronic ignition will run down a battery. Your battery just went over night the alternator may have stopped working long before. Go to an auto parts store that offers free testing to make sure it is your alternator before making repairs.
Good luck.
Usually that fuse powers the alternator and the fuse box. It would have to be a big short to ground to blow that fuse. Its possible the main wire to the alternator is touching a metal spot, or the alternator is shorting out inside when it is running. What is draining the battery may be something else, or the battery could be discharging after the fuse blows if the alternator is not charging it.
Have you confirmed by going out to the car at night that none of the interior lights or lighted visor lights truck or hood lights are staying on? That is by far the most common cause of a battery drain. Other less common cause is a defective alternator rectifier diode Disconnect the battery overnight as well to confirm you don't have a shorted battery cell, that to will run the battery down, it is called an internal drain. Also make sure the top of the battery is clean of any acid.
Either something is draining the power or your alternator is not recharging your battery. Do you have a DC voltmeter? With the engine running check the voltage accross the battery it should be about 14 volts. If it's down around 12.5 volts with engine running the alternator is not working. If you determine the the alternator is working and the battery goes dead, then there is a draw on the system. It could still be the alternator though, there are diodes in the alternator, a diode acts like a one way gate for electricity. With one bad diode the alternator will still work but, when you shut the car off the alternator drains the battery back down. Something is drawing it down, without testing equipment, you'll have to take it to a shop. Something you could try, after the car has been shut off for while, an hour or more, go feel if the alternator is warm or hot, if so it's drawing the battery down. Disconnect the battery overnight so it doesn't go dead on you.
Check the voltage regulator if there's coming out of it. you may have a good alternator but if nothing goes at the alternator, then, it results to a drained battery. Just replace your regulator, if this is not working.
you need a mech you have a draw whitch meeans something is on at all times or internal short on something or pass bad batt.
make sure things ore off like dome light trunk light cig lighter out of socket cell phone charger etc. good luck
Check voltage at battery before you start, then start & check voltage while running. Obviously your battery reading should start climbing to 13.5 - 14.5 VDC when engine running, otherwise alternator is defective, unless that car has an external voltage regulator on it. Make sure your alternator belt is in good shape & tight, and all connections at battery, & alternator are clean & in good shape. To find a battery drain when vehicle is shut off, use a 12vdc test light connected between positive connection & battery post. If the lights on, you have a drain, & start removing fuses to find the culprit, or remove all & start putting them back in until light comes on bright. Now you've isolated the circuit causing the draw. should give you an idea where to look. Remember your radio will have a constant small draw for memory function.
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