battery light came on

[guest]

Battery light came on yesterday. Replaced the battery but light stayed on. Was told (by tech at oreillys) it was probably alternator. went to a mechanic the next day. On the way, the battery light went off. Mechanic says battery and alternator are fine, but I need a starter. I don't know what the starter has to do with the battery light. On the way home, battery light came on again. I'm confused. Don't know what to do now.

[carriedi]

the battery light comes on when the voltage at the alternator is lower than the battery. Check the alternator output when the battery light is on. If the light goes out the alternator is charging. The mechanic may have checked the starting system when the battery was low on charge from the time the alternator wasn't charging. So, he would see that the starter was drawing the battery down too low. But, that's because you're supposed to check the starter with a fully charged battery.

Battery light is telling you the alternator is NOT charging the battery. Not that the battery is bad. If you have a bad battery it could cause the alternator too try too hard to charge the battery and cause the alternator to burn up.

The usual thing to do is to do a starter/charging system check. It tests the alternator, the battery and the starter all together. First is the battery check. If the battery is good then move to the alternator check. If those are both good then you check the starter. Most shops have a tool that will check all of those things together. It will do a conductive or load test of the battery. Then check the charging voltage and wave ripple of the alternator. Then check the battery draw on the starter. Then the tool will usually give a print out of the results. Takes less than 10 minutes. (unless the battery needs to be charged up first)