Hi all - wondering if I can get some advice on a battery issue.

I've got a 97 F-350 with 2 batteries (unmatched - a 750cca motorcraft and a 850cca gold from advance auto parts). I've had some trouble maintaining a charge and/or getting a full charge. I've also had the battery voltage spike up to as high as 16 volts when operating at highway speeds (which I suspect is a different issue from the charging).

Generally I assume a good battery will only drop a small voltage when a load is put on it - maybe less than a volt. With the 750cca battery as the primary (hooked directly to the solenoid) and the 850cca in the secondary (drivers side) the primary battery drops no voltage when I crank the starter - it stays rock steady at 12.75 volts. The secondary battery drops from about 12.5 volts to just under 10 volts when I crank the starter. A voltage drop of 2.5 volts seemed pretty severe to me on the secondary and I'm thinking that one might be bad.

When I switched their positions so the 850cca was the primary and the 750cca was the secondary, they both dropped about 1.5 to 2 volts.

Is this typical? I took the 850cca battery back to advance auto parts to get a warranty replacement (its only 8 months old), but when they put their tester on it they said it was working properly - and they refused to warranty the battery. It reported 12.57 volts and 890 cold cranking amps. That surprised me, but then again their tester probably has a small load - nothing like the load that my diesel starter and GPS relay combine to put on the batteries.

So what do you guys think - are there any other tests that I should run? Also, any thoughts on the voltage spiking up to 16v when driving at highway speeds?

Thanks in advance.

 

Alternator is probably on its way out.
Mine would randomly do that for the last 6 months till the electronics on it went out. The part that controls charging.

Having miss-matched batteries is not really a good thing, it causes weird strain on the alternator. There is probably a way to hook them up for best efficiency though when using that style of system.

If your having voltage issues like that on drop across the terminals, I'd suggest either new cables or going over all your cables and connections and cleaning them.
Makes a world of difference.

 

Measuring from the PCM or some down-line voltage meter I'd agree, but those numbers were taken right on the battery terminals. Essentially I put my pocket voltmeter leads right on the respective battery terminals to take the measurements (while the wife was starting the vehicle). Cables shouldn't matter in that case, no?

 

Well, if the cable cannot properly draw from the battery, but after you moved batteries both were taking a draw.