Your first two sentences suggest the battery was run down, most likely from a system draw. A different battery will solve that until it runs down too.
Here is a guide to help you see what to problem is:
In your third sentence, unless the new battery was also discharged, you likely have a loose or dirty battery cable connection. You can verify that by turning on the head lights and watching their brightness. Typically you will see they are fairly bright, then when you try to crank the engine, they get real dim or go out. You can quickly figure out which cable connection is causing the problem with a voltmeter, but if you do not have one, you may also see one of the cable clamps spark or smoke when a helper tries to crank the engine.
Once the cables are repaired, have your mechanic test the charging system. If you do have a voltmeter, measure the battery voltage with the engine running. It must be between 13.75 and 14.75 volts. If it is low, the battery will never fully charge. If the voltage is okay, the second half of the test requires a professional load tester to measure full-load output current and "ripple" voltage. If output current is one third of the alternator's maximum rated current, and ripple voltage is high, there is a failed diode in the alternator. 30 amps from the common 90 amp alternator is not enough to run the entire electrical system under all conditions. The battery will have to make up the difference until it slowly runs down.
Please let us know what happens.
Monday, August 24th, 2015 AT 4:19 PM