batteries, alternator or cables - how to tell?

[GREASE FIRE]

if my truck sits for more than a day or two it needs to be put on a charger to get it started, otherwise it just barely turns over. Once it starts i can drive for a while & shut it down, then it will restart just fine. I stopped by an autozone and they put their battery tester on each battery and said they were both good - i asked if it matters that both batteries are connected to each other when testing them and the guy said no. It seems to me you would have to disconnect them and test one at a time....
the alternator says "reconditioned" and is a 100 amp and looks newish so i don't think that is the problem.
basically i am just trying to figure out if the batteries really need replacing before buying new ones. Any advice? i just don't get the part about why they would work fine but then after a couple days be dead. It almost seems like the lights were left on or something - but i know they were not, nor was the cab light.
thanks,
Paul

 

[Optikalillushun]

sounds like u have something drawing current while sitting.

this is how to check for it:
http://www.flashoffroad.com/electrical/Batteries/BatteryDrain.html

 

[suv7734]

The batteries cannot be tested while they are connected together, they must be tested individually. If one battery is weak it can draw the other one down. What year is your truck? If it has the external voltage regulator (pass side fender) they are known to fail and draw the batteries down.
Have the batteries tested individually to start with. Once the engine is cooled down put your hand on the alternator and if it feels warm there is something allowing the field windings to be 'charged' and it is often the regulator (about a $12.00 piece). To confirm the regulator just unplug the small connector from the top of the alternator and check to see if it cools down in a while.

 

[GREASE FIRE]

1989 is the year - what exactly does the regulator look like so i can find it?
thanks for the tip.

Paul

 

[Mr_Roboto]

You won't know anything until you have the batteries tested properly. All you have to do is remove one connection from one battery and the two batteries are no longer connected.

 

[MIDNIGHT RIDER]

Go ahead and stick a pair of Group 30/31 batteries in it.

It won't be money wasted.

There is a 99% chance that the batteries are causing your problem.

It is pointless to drive to the auto-store to have batteries tested on the vehicle; they are "HOT" and freshly charged from the trip and you will get a false idea of their actual condition.

For a real test, remove the batteries, after a fail-to-start, and immediately head to the tester, with the "cold" batteries.

Don't remove them the night before and set them in the warm garage.



If the problem isn't the batteries, they will soon give it up anyway; as, the constant draw-down will drastically shorten their useful life.

 

[Agnem]

Take the ground lugs off both batteries, and then one at a time put an amp meter between each battery negative post and ground. If you get more than 1 amp, you got a problem.

 

[GREASE FIRE]

i wasn't able to remove the batteries yet because i had to drive my truck into town but while i was there i took it to a place to have them tested - i disconnected the cable connecting them to make sure it was done right even though the guy insisted it did not matter. Anyway, he said one of the batteries tested at 950 cranking amps and the other was around 800. Is that enough of a difference for the weaker battery to run down the stronger one while sitting for a couple days?
I also replaced the external voltage regulator since it was only $12. I was thinking to replace both batteries at first but the person who tested them said they are still pretty good with those readings. He also said the alternator was working fine.
thanks,
paul