1984 300d battery loses juice when AC is running
 

i have 1984 300d with 330k miles on it. My battery is 4 years old and it a autocraft silver from Advance Auto.

While using the ac this Summer, my battery seems to losing power. My car was slow to crank over yesterday so I drove about 10 miles to the store without the AC or stereo on.

When I left the store, my car started right up without hesitation. When I drove home, I turned the AC and stereo on. When I pulled into my driveway, I shut my car off. I then tried to immediately start my car but my battery would not turn the engine over. It seemed to be completely dead but my dashboard coil light would come on.

I had the battery tested at Advance Auto and they said the battery failed their battery test. It was only putting out 632 cca and the battery is rated for 850cca.

I am going to buy a new battery today but I was wondering if I also have an alternator problem.

Shouldn't the cars electrical system run completely off the alternator and not use any battery power while it is running?

Any thoughts?

Thanks.

Pete

"Shouldn't the cars electrical system run completely off the alternator and not use any battery power while it is running?"

Not really. The alternator generates energy, but it also has to work WITH the battery under heavy loads because it can't supply enough on its own. Air conditioning is a heavy power draw. Could your alternator also be flaking? Sure. But by the sounds, I'd say most likely it's just a weak battery not being able to live up to its end of the power bargain in your system. It recharged the battery when you were not drawing extra power, so that's a good sign. You'll know pretty quickly once you replace that battery.

It could be a couple of factors. When a battery becomes sulfated, it becomes more and more difficult to charge. Also, any "extra", such as a blower motor for a/c, shares some of the recharging power. If your alternator isn't up to par, it could be barely charging the battery as well.

It would be a good idea to check charging voltages. With a meter across the battery terminals, check both DC and AC voltages. The DC voltage should be ~13V and AC should be <100mV. If DC is low, the voltage regulator is suspect. If AC is high, the rectifier diodes are suspect.