Thousands of years ago, I diagnosed dying batteries every work day as part of my job. Therefore, I tend to take an extremist approach.
1. What is the charging voltage, and max loaded amperage output of the alternator?
2. Is there excessive AC voltage (Ripple) on the alternator output? Excess ripple indicates diode problems; the battery can drain by having power flow "backwards" through defective diodes.
3. Battery cable clamps free of corrosion, and tight? The battery terminals, and the clamps where they touch the battery terminals, should be bright and shiny--not green/fuzzy, not black, not matte grey.
4. Insert a low-amperage probe--either the 10-amp ammeter built-into most multimeters; or a low-amps inductive clamp--into/onto one battery cable. Verify the amount of current drain (Parasitic loss) when the truck is shut off, doors closed, lights off, etc. When everything has been turned-off for thirty seconds or a minute, there should be almost no amperage draw. 1 amp is too much. 1/2 amp is too much. 1/4 amp or less could be OK. 50 milliamps is definitely ok. Older vehicles had "zero" amps; newer stuff uses a tiny bit of power to keep computer memory alive. If the parasitic drain is excessive, you'll pull fuses one at a time until the drain goes away. The fuse that stops the drain, is the circuit that has a fault. But not every circuit is protected by a fuse. Sometimes finding the drain really sucks.
5. Older batteries accumulate conductive slime on the plastic battery case. The battery will self-discharge through the conductive slime between the two battery posts. WASH THE BATTERY CASE using hot water and a touch of baking soda; or a commercially-available battery cleaning spray.
6.[more later gotta go...
6 continued. Test the battery. A failing battery may not hold a charge, or may not accept a charge. Because I'm old, I prefer the old-style testers as described below. They actually measure what's important. There are new-style testers that "calculate" battery capacity using conductance/impedance/voodoo. The new testers are small, hand-held, look like something out of Star Trek. I don't trust 'em.
7. Verify starter draw. The old in-line high-torque starters would pull up to 225 amps; generally around 180--200. The newer mini-starters draw less current; perhaps 160 amps.
In the end, you'll have tested the alternator, battery, starter; the wire harness that connects them. And also, you'll have verified that the rest of the electrical system isn't pulling power it's not supposed to. It's fairly common to find multiple faults--the battery/starter/charging power team is totally interconnected--a failure of one part often leads to a failure of another part. For example, "shorted" alternator diodes reduce alternator output, and create a battery-draining short circuit. The alternator failure results in a chronically-undercharged battery that sulfates and then has reduced cranking capacity. Then the starter cranks slowly. So "fixing the starter" may mean replacing the battery and alternator.
