Originally Posted by sparky34kv
Sailor, no disrespect but you are wrong on this one. The alternator can charge a normal battery, but they are not made to charge a discharged battery. A car should, and will run off just the alternator, that is how cars are designed. The batteries sole purpose is to turn the starter on start up, after that the car runs off the alternator, with some draw going to bring the battery up to full charge. A car will draw from the battery sometimes when sitting in traffic or idle with the ac running. I could be wrong, if you can name an instance when it has killed a good alternator, I'd like to know. All I know is it is a simple and reliable way to see if the alternator is working.
Sparks, no "dissin' " here either, just some extra info..
Your description of where power comes from and when is perfect, but there are some special case reasons why "logical" test corollaries aren't a good idea, and the "dead" battery instance also depends on the details...
As to "dead" batteries, correct as to charging a totally "dead" battery - when there's no power at all from the battery the alternator won't produce any output so no charging. Engine prob. won't run either for lack of electricity, but in a few cases it may run, using all the power avail. so there's still no charging output - bunch of odd special case possibilities here, general rule to make testing valid is to use a known good battery when checking the charging system on any vehicle to get valid results & avoid the odd cases that may give confusing results. So, severely discharged batt. needs some outside source charging before the car's system can do the rest of the job. Low batteries (often called "dead") that need a jump to start the car will recover from normal alternator charging, but a short drive isn't adequate...
Pulling the batt. terminal to check for adequate charging dates back to the days of generators & cars with no electronics - a more "robust" system with regard to voltage spikes...
While doing this doesn't always cause damage, the potential is there & risking severe damage to alternators & electrical components from voltage spike when disconnecting the battery just isn't worth it.
My best "proof" can be found if you look up battery switches, either racing safety disconnects, or marine versions for multiple batteries. Cheaper versions have no alternator protection circuit, and carry warnings about possible damage if battery is disconnected while engine is running.