Bad alternator???

[tfret]

5.7L mercruiser, ran fine in the drive way on the hose two weekends in a row. Put it in the lake today and almost didn't start like the battery was dead. Finally started up and noticed the voltmeter only reading 9 or 10 volts if that. Ran at 2000 rpm for about 5 minutes to charge the battery but the volts never came up. Then intermittently the voltmeter would peg to zero and back up as if there was a momentary short circuit. This happened many times. Finally bad enough to stall engine. I decided the two year old battery must have a cell shorting out. Bought a new marine 800 CCA battery and the exact same thing. In fact this time I had to jump start it off another boat. Both batteries can't be bad. Would a bad alternator create a low voltage with intermittent shorts to zero volts like that, and do they just go bad without any indication? I've never seen anything like this. Please help me out.

 

[GA_Boater]

Take alternator off and take to an auto parts store for testing. Same thing with the battery, test it before assuming it's bad. Saves you money.

 

[tfret]

Actually I did take the battery to the store. First it was too weak to test. So they charged it for about 10 minutes, then retested. The test set said "replace battery". That's all it said but it made sense to me at the time. Good idea about testing the alternator. I didn't think of that.

 

[tfret]

Turns out the ignition fuse holder on the dash had come unscrewed and was loose. I must have rolled it with my knuckle while turning the key or something. All I did was screw it back in and everything works fine now. Wow do I feel stupid. At least now I know how much faith to put in the auto parts store battery tester that printed out "replace battery". I won't bother with them anymore...

 

[Grub54891]

A 10 minute charge is not enough to get a good test. A couple hours at least, then let it rest for a bit, test after that. I have a friend who would buy new batteries for his camper each spring, said they never held up for more than one season. Found out he only charged the for an hour before he went camping, and they were not fully charged constantly. An overnight charge is the best way to go.

 

[GA_Boater]

A 10 minute charge is not enough to get a good test. A couple hours at least, then let it rest for a bit, test after that. I have a friend who would buy new batteries for his camper each spring, said they never held up for more than one season. Found out he only charged the for an hour before he went camping, and they were not fully charged constantly. An overnight charge is the best way to go.

Only a fully charged battery can be tested if it will take a charge. 10 minutes was not a charge, more like a tickle.