I have a 2014 cx9 grand touring. 3 weeks ago I noticed my drive belt was squealing when I turned the ac on and I bought a new tensioner and belts and my mechanic changed it all. Well after I got it back the battery light would come on after about 12 minutes of driving and would go off when I would get up to about 2500rpm and my engine sounded super clunky when I would back up with my AC on. Took it back and they said the tensioner was faulty and they replaced the tensioner and said my alternator was fine but my battery needed to be replaced. So I replaced the battery and once again my battery light came back on. So I took it back and when we tested it again it showed the alternator was failing. So they put a new alternator on and everything was reading great. They test drove it for a while to make sure the light didn't come back on and it didn't. I get it in and 3 miles down the road... my battery light came back on. When they tested it again everything came back fine. What are we missing?

 

Find a new mechanic who knows his job. This guy is just guessing and throwing parts at the problem.

I have a battery charger and a voltmeter. Here's how I test...fully charge the battery, let it sit for a couple of hours for the surface charge to dissipate. Check the battery voltage. It is probably high 12+ volt region. (A fully charged "12 volt" battery is 12.6 volts.) Start the engine and run at a high idle rpm. Check the battery's voltage again. If the voltage is higher than measured with the engine off, the alternator is good and the battery is the problem. If the voltage is lower, then the alternator is the problem.