Electrical Gremlins

[Skyline]

I've been have a series of electrical issues with my X5. First it was the trio of lights, ABS,DSC SES. Cleared codes, charged battery...no recurrence. That was about two months ago.

Then Trans Failsafe Prog. Cleared codes; no further trouble with that. (that was only a week or so ago.) Car running fine, and shifting properly after codes cleared.

Then yesterday, Engine Failsafe Prog. Went away by itself. Checked the voltage at the terminals in the front, and with car off, it was 12.45v. Battery tester on front terminals says 396 CCA, replace battery. So I replaced battery. Battery tester still says 12.45 volt, 550CCA; replace battery. BTW, when I checked the 1 year old battery that I took out of the car, it checked fine. When I checked the new battery in the car at the battery, attaching the tester to the bolts on the battery cables, (with the cables still attached to battery), it tests fine there. Both batteries are proper 900 CCA Deka batteries.

So I'm clearly getting some voltage loss from the battery to the front of the car. And based on my testing at the battery terminals, it does not seem to be dirty terminals not making proper connection to the battery. Is this normal to have such a voltage/CCA loss from the battery to the front terminals, and am I barking up the wrong tree, or is this an indication of a bad ground or other bad connection or cable somewhere?

I did have a feeling that perhaps some accessory left plugged in to a lighter outlet might have stayed on, (iPhone charger or V1) and discharged the battery a bit. Never had an issue starting the car. Maybe the outlets are staying on instead of eventually powering down. It seems my problems followed periods of disuse of the car for several days. But I have not yet confirmed this.

Also, I should mention, that I also did a charging test with the Snap-on battery tester I was using, and it tested fine, (at that point I had the tester hooked up to the front engine compartment terminals). When I'm talking about the front terminals, I am referring to the + on the driver's side firewall, and the (-) on the LF fender.

Any advice would be appreciated.