I have a 1973 k-20, I'm running the factory alternator and dual batteries. The problem I'm having is every so often my batteries go dead. I'm wondering if I need a more powerful alternator. Any ideas will be helpful.

 

Have you added more power hungry accessories than when it first came from the factory? If yes, then you may need a bigger alternator.

Or, maybe you have developed a key-off drain on the circuit. Meaning, with your keys off and engine not running, some power is being pulled from the battery.

You can check that with a voltmeter, using the amp setting.

Disconnect negative lead of battery, touch it to negative lead of meter.

Touch other meter lead to negative battery post.

If you get a reading, that means that something is consuming power when key is off. From there, you can make other decisions.

 

Hi 73K20,

Welcome to the Chevy forums.

Are the batteries hooked up in tandem without any interruption in the circuit?

If so, your max power will always be that of the weakest battery.

More than likely, one battery is bad, and it's pulling the bad battery down.

If you charge both batteries and they test out as good, try seeing if the diode plate on the alternator is blown. This will discharge both batteries when the vehicle isnt running.

Needless to say, a system like this absolutely must have pristine battery cables and engine-to-firewall-ground-strap...

Try running the vehicle. Disconnect battery #2. If battery #1 goes dead, start the vehicle with #2, run it long enough to charge both batteries, and then disconnect battery #1. If #2 also goes dead, you know the alternator diode plate is shot.

Usually there is a stamping on the alternator clearly showing the amperage rating on the alternator. While it's perfectly safe to change to the higher amp alternator, be sure you get the proper size belt. The belt is usually just a little larger for the larger alternator, but the smaller belt for the smaller alternator will not stretch to fit.