Battery or Alternator?

So, I have a 1991 Mercury Capri. Recently - on Halloween, no less, spooooooky - I drove to work, and like a dumbass, left my lights on. When I came back out, my battery was ofcourse dead. I got a jump from a helpful stranger, but after a few moments, the car died. Another jump, another car death - it appeared that either my battery was simply not holding a charge properly, or the alternator was dead. Most would say that it would have to be the alternator, since most battery problems like that aren’t that sudden - this happened all at once, and I hadn’t had nary a problem before this.

Here’s the interesting part - since the first step in diagnosing alternator problems is to make sure it isn’t something else, I went and had my battery tested. It was ofcourse dead, so they had to charge it at the AutoZone where I took it. When the guy put the battery on their charger, the charger accepted it, and it tested good to be charged. However, once it had gotten to full charge, it tested it again at the end of the charge cycle, and it tested bad. The voltage was between 13 and 14 volts, which should have been just fine, but it apparently failed on amperage. However, their machines (and the machines at a few other auto places I visited) won’t show the specific amperage the battery is putting out, just a pass or fail. I have no way of determining if the amperage is just barely falling short, or if it is really failing.

So, my issue then, is that I think that the battery is good-e-nuff to chug on for a while, and thus I need to go ahead and replace my alternator, but I have no way of being sure. The symptoms match a dead alternator, but there is enough evidence of a bad battery that I’m not comfortable going ahead and acting on the supposedly dead alternator. Does anyone have a way I can rule the battery out as the culprit here? I only have the funds to replace one or the other, not both.

It is possible that running the car with the dead battery killed the alternator. If the freshly charged but iffy battery can start the car, a simple voltage check will tell you if the alternator is working. Also, is the battery light on in the car once the engine is running?

At a minimum you would need a voltmeter to check the battery. A fully charged, good battery should be about 12.6 to 12.7 volts. The voltage may drop into the 11s or upper 10s while cranking the engine over. If it drops to 10 volts even or less the battery is either in need of charging or is bad.

As to the alternator you might turn the key to the RUN position (engine not running) and touch the alternator pulley with a screwdriver tip. You should feel a magnetic attraction in the screwdriver.

I’m going on the assumption here that skills and tooling is limited so the above is strictly of the DIYer backyard method. It may be crude but it does work.

Also make sure the dashboard battery light is operative. If not, the alternator will not charge. Hope that helps.

Check the battery voltage while the engine is running. It should be around 14VDC or so. If the alternator can’t provide that kind of voltage then it may need replacement. I also check the voltage with the engine running with all the lights on and the radio just to be sure the alternator is up to snuff.