It’s happened several times before. Good alternator, good battery, dead battery. Over the last two or three days, I noticed my good starter getting slower and slower. Since vehicles these days, inexplicably, have only a voltage meter, I couldn’t tell from my instrumentation whether I had extra drain, a dead battery, or dirty terminals. You do get a slight clue though, if you watch the voltage closely immediately after start-up. If everything is working normally, you’ll see the voltage start out a bit lower, then creep up to normal running voltage in a few seconds. My meter was rock solid. With a current meter, such as was the norm in the 1960s, you’ll see the charge or discharge current. Much more useful in my opinion. Best would be to have both volts and amps.
The pickup barely started this morning, and at work it didn’t start at all. Like a jerk, only then did I clean the battery terminals. Still no go, so I got a boost from Dan. The terminals get corroded and that can sometimes allow current to flow one direction, but not the other (that’s a diode, see). In this case the batt would discharge just fine, but it couldn’t get any charge current (hence the full voltage immediately after starting– the voltage wasn’t being pulled down by any charge current). Eventually you’re a walkin’, yo.
I went and bought a new battery anyway, because I’ll have a use for it either way. By the time I got back from across town, the old battery was fully charged and snappy as ever, so now I have a new one I can use either in my son’s van or my old beat-up T-Bird. Fun fun fun. Plus I got a decent charger for the garage, just in case. If the battery had had only a few less electrons to give up this morning, I’d have been knocking on doors for a boost. No more. I have used my ham radio 12V supply to charge a car battery, but it doesn’ like that. The huge current load tends to blow some fairly important components.
Yeah so; if your starter slows down, just a little, clean your terminals before it gets worse. You may just have solved the whole problem, right there. You do keep a terminal brush and some tools in your vehicle, right?