The cardinal hypothesis to discern is that a battery is fictional up of cells. It is good to know that not many alternators fail completely all these days. What happens is one of the three diode packs goes out first. This reduces the amperage and voltage it is capable of putting out. If just one diode is out, the alternator will still charge the car with the air conditioning and lights off, but when they are on, it puts out just barely over battery voltage, so it would take a long time to recharge the battery. Hold a helper pop to begin the machine. The battery must be charged to achieve this confirmation. A dead battery cannot be tested. Whether the battery shows fast to 12 volts when the machine is started, it should not Blop below 10.5 volts while the starter is busy. Whether it does, the battery is defective.
Whether it does not Blop low when starting, the battery is probably congenial. Check the alternator consequent. Consent the voltmeter attached to the battery and alpha the machine. With all the accessories off and the engine running, the voltage across the battery should be 13.8 volts or higher. A bad alternator will levy outside 14.5 volts to charge. A voltage of 13.8 is skilled.
Turn on the lights, air conditioning and radio. Check the voltage to see if the regulator is working. The voltage should not drop more than half a volt. If it does, replace the alternator. Provided a cell or two begins to short absent, the battery Testament even hold 12.5 volts--it fair-minded won't hold as even amperage as it did to begin with. When a battery has shorted cells, the battery will revenue a charge in most cases, on the other hand it Testament not clout that charge extended. It can be charged during the hour to a unabridged charge and die overnight. The form to research a battery for a dangerous cell provided no hefty Accoutrement is available apart from a voltmeter is as follows: Place the voltmeter on 20 volts and country the blush path on the battery conclusive and the livid on the battery detrimental. Each day the car is used the alternator with one bad diode will put half the charge into the battery that was just taken out. In a couple days the battery will be dead again.
If the battery and alternator are good, then there is a good likelihood that there is a drain from a circuit in the car, such as a light left on or shorted. Somewhere there is a short that is draining the battery. Take the negative lead off the battery. Place the voltmeter in between the negative wire and the negative terminal (one lead should be on the negative wire, one should be on the battery negative post). Note the voltage. It should be very low in millivolts. There will always be a small drain due to the circuits in the computers.
Turn the voltmeter to amps. Even if the voltmeter shows 12 volts, the battery won't die easily as long as the amperage drain is less than a quarter amp. If it is more than a quarter amp, there is a drain. Leave the ampmeter installed. Take one fuse out at a time (from the fuse block under the hood and inside the car). Once the fuse is pulled check the ampmeter and see if the drain has gone away. If so, the circuit the drain is in has been located. If the ampmeter stays the same when the fuse is pulled, reinstall it and go on to the next until you find the circuit that is causing the drain.
