Went out of town last weekend, only to come home to a truck with bad batteries. One battery (motorcraft 750 driver side) is about 20 months old, other one was bought about 8 months ago (republic 850). I charged up the batteries and jumped it off. Drove it to work the next morning and when I went to leave for lunch it didn't have enough to start the truck. I got a jump and went to oriellys had the batteries and alt checked out. The lady that had the handheld tester said the alt was bad. So I drove home and parked it. Bought a new alternator friday and swapped it out friday night. Charged the batteries for a bit, jumped it off and started the truck. It ran for a little bit with high idle on, and it started uping the rpm, im assuming the truck new it didn't have enough voltage/amps to contiune running so it shut down when it got to 2000 rpm. I though I could have gotten a bad alt so I took it down and had it tested along with the oldest battery. The alt passed the test and the battery was no good. Then I had them check the original alt, and it passed. So I barrowed a battery for the driver side for the time being. So right now I have the alt still out of the truck and have charged the batteries. Not sure where to go from here. I have been checking fuses and wires, im not finding anything that looks odd or blown. Any help would greatly be appriciated.

 

Both batteries, even though only 1 tested bad, should have been replaced. The bad battery will always take the new one down with it and you end up with a new, bad battery. ALWAYS replace both with identical, not mismatched batteries.

 

When you originally had the batteries and system checked did you separate the batteries from each other by disconnecting one cable? If not then you didn't get a true test on the batteries.

 

I did not have them isolated. I pretty much assumed that the batteries were ok, just low. I took the motorcraft out of the truck and the checked at orielly and they tried to charge and said it was bad. So what you are saying is check the other one, the republic battery and if its good charge it up and go and get one just like it for the driver side. My question is why are the batterys loosing a charge when its just sitting? What about the fusable link, should that be a concern? Are there any fuses between the alt and battery? Or are their any fuses that I may have missed?

 

This will answer your question. In a dual battery system when you start replacing batteries one at a time you're asking for trouble.

 

so i need two batteries, so it I go and get two new batteries and they discharge, anything else I can check or have checked?

 

Yes, there is a fusible link between the alternator and the battery. If you get two new matched batteries and they discharge then you, obviously, have another electrical problem for which I'll let one of our resident electrical experts walk you through on diagnostics. I can give you a LOT of the basic's but discharge problems are VERY hard to diagnose.

 

I know on a single battery car you take the positive wire off the battery hook a test light between the two the battery terminal and the positive battery cable if the light lights up you got a bad wire somewhere thats has chaff or something and is grounding out drawing power and draining your batterys

 

you should remove or disconnect both baterries and completely charge them then let sit over nite and do or have both load tested to determine condition of both batterris if one or both fail that is your problem i also agrea that both baterries must be replaced simultaneously if one is bad