Alternator, bad? Definitive test?

With a new battery now what I would do is take an hour long drive and after that go back to my normal routine. Vehicles are to do what we want them to do, not the other way around.

Thanks, VOLVO_V70. Yes, Iā€™ve understood that about vehicles since I first learned how to drive one in 1964. :slight_smile:

FWIW . . . I myself never claimed posting a personal email address was a violation

I merely said it wasnā€™t a good idea, IMO

I like Volvoā€™s comments :thumbsup:

The objective is to get the battery from itā€™s fully charged ā€œnewā€ condition to what the normal charge will be once the ā€œnewnessā€ is gone. Which might be, say, 85% charged after sitting overnight. Not until it reaches that state ā€“ Iā€™d call that the ā€œequilibriumā€ state ā€“ will you be able to verify the alternator is working 100%. Alternators have a batch of diodes, usually 6 or 8 diodes, and if one of those diodes fails as an open circuit it disables a certain percentage of the charging coils; the alternator will still work, but wonā€™t be able to produce the current demanded by the battery and accessories under a high current load condition, so the voltage will drop. You havenā€™t done any tests to prove the alternator is working correctly under these kind of charging conditions. If the alternator is only working at a 75% of what it should, eventually your new battery will run down. It will still be a new battery, but it will be a new and discharged battery. And it wonā€™t crank the engine. So youā€™d be back at square one.

In terms of how much you need to drive until the battery reaches its equilibrium state, hard to say. It depends on the battery and the ambient temperature and whether you are driving with the headlights on or not. Iā€™d guess it would happen in 100 -200 miles. If you monitor the battery voltage right after starting the car in the morning, you should observe it slowing increasing day to day as the battery reaches its equilibrium state. On my Corolla with a 2 year old battery the battery voltage measures 14-14.5 volts right after starting the engine on the first start of the day, which indicates the alternator is up to its job and no diodes are burned out.

I started keeping track of the mileage and number of ā€˜startsā€™ today. After I get a new supply of gas (even doing that is a tad problematic around here), Iā€™ll just drive the darn thing on some longish drives. And Iā€™ll keep track of the voltage in the mornings, too.

Thank you, sir.

ps. Iā€™ve a neighbor and shade tree mechanic who drove the truck prior to the new battery install. He drove it at night, maybe 7 miles and said the lights didnā€™t dim, and so concludes that that mean the alternator is OK. Thereā€™s something in me that says that is not a conclusive indicator of the health of the alternator. (Note that I was charging the old battery virtually every morning, so his drive at night was after Iā€™d driven it only about 6 miles in the morning.)

Harbor Fright has a battery-and-alternator tester for maybe $5. I recommend it. I got it on a whim itā€™d help with some electrical trouble. Low-thinking tool. Crystal clear result. Hard to screw up. Replaced alternator. Problem solved.

Of course the intelligent way is always available for the future.

Yeah, Iā€™ll admit my test equipment is of the low cost variety so just a word of warning, it may not be that accurate. I canā€™t confirm it because all I have is cheap stuff but I believe even though they read out to two decimal places and look really official, I suspect they are off a little. No big deal if you donā€™t get excited about 13.29 versus 13.41, but just something to keep in mind. Iā€™m not building rockets so doesnā€™t matter.

Sorry about the delay. Thanks for the input, sir!