Battery? Or alternator?

ford
focus

#1

Kids alternator light came on. Drove it to work and later home. I went over to look at it. No light quite a bit of corrosion on battery terminal. Cleaned that off… 2 weeks later battery is too weak to start car. They jumped it with booster pack and runs fine. No alt light. Next day it did same thing. Bad battery? Any link to alt light a few weeks ago?


#2

Did you have the battery and the charging system tested? How old is the battery?


#3

Make sure the battery connections are clean and tight. If you have a volt meter, check to make sure the battery measures around 12.5 volts with the engine off before the first start of the day, then measures 14-15 volts after first starting the engine. That’s a good way to get an idea if it is the alternator or battery that is the problem. As posted above, the best method is for a shop to do a charging/battery test with their test machine.

fyi, Alternators can be bad but not completely fail. They have 6 or 8 diodes in them, and sometimes a single diode can fail. That causes them to work as long as they aren’t being asked to produce a lot of current. The symptom is the battery becomes too discharged to crank the engine, but only after several weeks of driving.


#4

Just how old is that battery . . . ?!

If it’s older than 5 years, replace it before doing anything else

Then you can check out the charging system

The battery is the foundation of the car’s electrical system

If it’s not in good shape, the alternator won’t be able to do its job properly, either


#5

So, car started today. Date is 04/12. Put new battery in. Tested old battery at store and it failed with load tester.