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Battery discharges after 30 min of cruising

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AustinJim

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Hi all,
I have a 78 Spitfire which, after 11 years in a barn, I've just gotten back on the road. Now that I can drive it (instead of running it in the garage) I'm beginning to address the things that road tests show. One of the problems is that after about 30 minutes of neighborhood driving the battery becomes so discharged that it can't spin the started hard enough to start the engine. I can recharge the battery over a fairly short period and restart the car with ease. The alternator has been replaced and I've had it checked at the parts store. They say no problem. I put an ammeter on the brown lead and found that it starts out reading 5 amps or so but goes to around 0 in a couple of minutes. This is all at around 1000 rpm. When I turn the lights on the ammeter shows negative by an amp or 2. I suspect the alternator but since it's been checked on the part store tester I'm wondering what everyone thinks I should do next.
 
Jim;

With the car off put you amp meter in series with the battery. If it is a faulty alternator the amp meter will read somewheres up to 10 A.
 
It sounds like the alternator is not charging. It may test good on the bench, but once in the car it might not charge due to several reasons. First thing to check is the drive belt tension for the alternator. There should be about 3/8"-1/2" deflection in the belt at the longest run.

Does the ignition light on the dash go on when you turn on the key before you start the car?? If it doesn't, the bulb may be burned out and then there will be no current to activate the alternator.

You can also do some voltage tests. Hook up a voltmeter to the battery...

With the engine running and no load, voltage should be
approx. 13.8 to 14.5
With a load...lights, wipers, fan, etc. voltage shouldn't drop below 13.2. You may have to rev it to 2-3000 rpm when you load it up to get a better reading.
 
Alternator is the first suspect, good checks listed above.
But, how old is the battery?
If it's more than about 5 or 6 years old you should change it anyway, it may not be accepting the charge if the alternator is actually giving one.
 
Well, after more investigation using the advice all you great folks gave me in this post I don't think I have a batter or alternator problem at all. Everything checks out ok. I think I may have 2 problems that I first thought were related. The starter seems to not crank well when hot and the engine won't accelerate when hot. I'm going to post a new message about the engine. Thanks for all the help!
 
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