The first thing I question when a family member or friend asks me to help diagnose a charging problem is to verify that the alternator and battery are, in fact, good. This, of course, after it's confirmed the battery cables are clean, tight and not damaged (cables are known to deteriorate inside the protective jackets, too). It's not enough, in my opinion, to take them to a parts store and have them tested. Too many times in the past I've been told they were good...only to find they weren't once they were replaced. Especially, batteries as they can test okay but have a dead or weak cell that prevents proper charging. So, first double-check both. How old is the battery? If it's more than 5 yrs old and/or sat fully discharged for a length of time, it's suspect. Do you have a battery charger? I can tell when I connect my battery charger whether it's good or not by how it does or doesn't take the charge. A multimeter to check voltage, amps and continuity? You can check alternator output in volts and battery parasitic power draw with it. It's possible the alternator is putting out sufficient voltage, but a blown diode/rectifier can cause battery drain. You can use your multimeter inline on the battery to check for a parasitic power drain...whether from the alternator or elsewhere within your vehicle's electrical system by pulling each fuse individually and watching for a drop in amps.
