Despite the fact that my smart charger suggests the battery in a 2002 4 cylinder Mitsubishi Eclipse is ok, I don’t think it is. How do I truly test the battery to find out if it is taking a charge and holding it.
To test if the battery is holding its charge, you are going to have to take a measurement over a few hours.
a) 2A for a day should just drain a 48Ah battery. Side lights (parking lights) might do. So your own calculations, adding up the power of all the bulbs concerned. The voltage should go down from approx 12.5V to say 10V and then tail off fairly quickly.
b) Try starting the car a few times, but don't let it run. That way, the battery did not get charged in between. Try a few the next day. Look at the voltage across the battery when the engine is cranking, because there is a the greatest load then. See how the battery drains. If it ain't start in the morning, it isn't holding much charge, is it!
How do I know for sure the alternator is working? I have a scope (not automotive). Perhaps I could look across the output of the battery cables unhooked from the battery to see if there is a ripple (indicating a bad diode). I could look at voltage at the same time. I would love to measure the current, but am uncertain as to how to do that. I probably need to know the max current my multimeter can tolerate without frying it and couple it in series between the battery cables and the battery.
The alternator output ripple current should be even from peak to peek. A longer gap is an open circuit diode. I can't remember what a shorted diode like like. An interesting thing about alternators is the voltage on the collector of the output of the control block. It's connected to one side of the alternator exciter windings. It's used to control the alternator output current, and hence the voltage. It tends to oscillate more than I expected. A low voltage means more output current. Turn on all the lights and heaters to prove this.
A battery under charge should measure 13.8V and the current depends on how much charge the battery has left. A battery at 12V is not charging!
Measure the current by measuring the village drop along the wire that goes to the battery. Do not try to put your lovely multimeter in series with 10A charging current or >100A cranking current. Start the multimeter at a 10V range. Calculating the power into the headlights might help calibrate the arrangement.
To test the battery, I suppose I need to place the battery under load and watch the rate of discharge. Maybe the most simple way is to take the battery to autozone and have them test it? Do I trust their setup to test the battery correctly?
See above for loading the battery. Professionals often have a "battery tester" which is a high power heating element that will stimulate creaking indefinitely. It has a built in volt meter, and a poorly charged battery goes to zero pretty quickly.
Maybe the battery my friend has is a rebuilt battery. How an I tell if it is? How do I tell if one I purchase, as a replacement for the one she has, Is rebuilt?
I suppose you should go to a reputable dealer to be reasonably sure. A rebuilt battery will hold slightly less charge, and supply less cranking power: measure the bills while cranking. Perhaps you have storage scope? 🙂
Enough. Any help will be appreciated.
Long reply? You have only yourself to blame 😉