My daughter's 93 Protege, which is just a few miles short of being a paperweight, stopped charging the battery. It only put out 11.5 to 12v, and would go down to 9v when lights, defroster, etc. were run. Nattery was 2 yrs old (but cheap). Bought a new alternator, had the same problem. They Autozone guy said my battery was bad, got a new one (somewhat warrantied). Same problem. Next they told me the alternator was bad. Got another one-same problem. I'll ask here before I install the next alternator (btw, the first two replacements bench tested ok after I returned them). The battery light lights when the voltage drops, making me think that the wire between it and the alternator is ok, and the other two wires have constant voltage. I wire stuff for a living and have all the meters I need, but cars are confusing to me. Any ideas? Thanks!

 

My 1997 mazda protege had a corroded wire on maf sensor which caused battery light to come on and alternator to stop charging found this after buying a new alternator and paying to have the ecu checked so always check everything

 

A ground problem can create voltage drops, especially if the alternator or battery ground cable is not properly connected or is corroded.

 

The first thing to have done before exploring electrical problems tis to have your battery load tested. make sure that the battery has minimum 12.50 volts for the test to be accurate.
Then clean the battery terminals connectors and ensure that the cable at the starter and ground lead is secure with good contacts.

After that if the problem is still present you can move on to the charging system components testing.

Age of the battery as in when you purchased it does not mean how old the battery actually is.