i would load test the battery's individually, and check the voltage with it running, the alt. should charge somewhere around 14v but my guess is you have a bad battery that is draining the good one or maybe they are both weak. i dont think a lot of powerstroke guys are fond of the optima batts.

 

I too have Yellow Tops and they work Great, I can't imagine why there would be those who didn't "like" them...

But to the OP, First what do you mean "the starting battery"? Your batteries are both starting batteries, they're in parallel and work together 100% of the time.

You need to be sure all your battery connections are clean & tight too.

I agree, it's time to have them evaluated with a Load Tester. Charge them both up to full (you know Optimas DO NOT like to be charged at a high rate) and have them individually tested.

On a side note, you know there's this Optima guy around on YouTube who says to leave an old one upside-down for a day or two to "rejuvenate" it (might be worth a try before you chuck one :wink[3]:

 

I was once told that the passenger side was the starting battery, but now that you mention it...that doesnt make sense since they are connected. Without one, the truck wont start. It could be that I just simply have a bad battery....

 

both batteries are connected in parrallel (pos to pos, neg to neg). the cable wire guage to the starter should be heavier than the one that connects the batteries. check the lug on the passenger side positive battery terminal. double the batteries in parrallel means double the amperage. if the positive cable is still the original i would check this. corrosion on the inside the cables can cause this. remember the amperage (electrons) is carried over the outside of each wire strand surface not through the core.

 

Also, I would imagine that the reason why it starts up when I've got the block heater plugged in is because the glow plugs won't draw as much, not completely killing the battery.... correct?

 

I think the GPs will draw the same either way (they're either On/Off) but, being warm just makes starting easier with less cranking so, yea, less current needed from the batteries....

 

had similar trouble with this one a little over a month ago with my early 99. i found out after 2 new batteries that the battery cables can get corrosion at the lug where the cable passes through the positive lug on the passenger side battery and will not deliver the full amount of amperage from the batteries. it will show full voltage, and new batteries in my case did not solve the problem. the glow plugs wont pull enough amperage (am i close at about 180 amps, 20+ amps x 8 ) to show the large voltage drop at the guage until the key is twisted to start. while replacing the cable i also removed the starter to check the brushes and found two strips in the commutator melted pretty bad. so a new starter also. the 7.3 has never spun over this fast since i have owned it!