Went to start RedRam up this morning - starter barely turned over for a sec and then nothing (voltage gauge showed low, at about the 11 o'clock position). Turned key off then on again and turned key to start and it sounded weak but fired up. Voltage gauge was at the normal spot just over half while driving (around the 12:30 o'clock position).
After work turned key (gauge was low) and chimes sounded then turning to on got nothing, turned off then back on and it weakly started. Drove home and noticed that volt gauge was at normal position except when stopping at a light and idling where gauge reading would drop down. Giving it some throttle the gauge rises back to normal reading. Pulled in driveway let it idle when parked but needed to give it some throttle to keep it going. Stopped giving throttle and gauge dropped and it stalled. Grabbed the multimeter and battery reads 12.2v.

Bad Battery, Bad alternator, or?

 

Since it shut down, I'm thinking it might be the alternator. Usually if the battery is weak, the alt will keep the voltage up enough to keep the truck running. Voltage should be around 14 or over with the alternator contributing. I'm not ruling out bad battery, just leaning towards alt based on what you describe. A weak battery places a heavy load on the alt, so degradation goes hand in hand.

 

Took the battery out to charge and it read 11.9v, let it sit a couple hours and it read 12.2v when I hooked it up to charge. The battery is about 3.5-4yrs old, very slightly underrated for the truck as its out of my Summer car, a Mustang. Its rated 590cca and 725 ca whereas the lowest requirement battery for RedRam is 600cca and 750ca as per the Mopar information I saw so not much of a difference and has been working fine for the last 2 months, always starting quickly(Its also a group 59 but not sure the grouping matters as much as the cca).

Will do some tests tomorrow with the charged battery for voltage readings, hopefully can figure out which one it is as I don't want to end up buying both a new battery and alternator.

 

As you've stated, your battery is definitely a problem, and might be the problem. A bad battery can overload the alternator and stall the truck.
11.9v is officially dead for a battery. Since you need a battery, I'd recommend installing a new battery, then testing alternator function as dodgevity recommends.
There are some videos showing how to test an alternator and battery. I recommend testing your system even if everything is working fine after installing new battery. You could have a bad or intermittent ground .

 

You can yank the alternator and take it to the parts store for testing. Probably the best thing to do since the battery is also suspect.

 

I'm pretty sure you have a failing cell in the battery. It should read 12-5 or more when you shut it off after running. If it's below 12 volts after sitting overnight, I'd have it tested. Considering the age and the lower capacity, I'd lean towards the battery instead of the alternator. The alternator is working, it's just putting a charge into a dying battery. I had a BMW like that. I could charge it and get 12-6 at rest. The next morning it would read 9.5 or less. That was a very expensive battery and difficult to get to. Your truck is a breeze by comparison.

 

This is a tell-tail sign that the battery is toast. 5 years is about what I'd expect out of a battery.

 

So put the charged up battery back in the Mustang that's in storage just to fire up the car and run for a bit, worked fine for the few times I started and 10 minutes of running.
Then moved it to RedRam and went to start - starter made an unusual noise and then on second attempt truck started fine and ran. Tested alternator as per Chrisfix's video, alternator checked out fine charging at 14.4 -14.5 volts while running even under full load (Chrisfix says should be 14.2v -14.7v).

Next checked the grounds and all were fine except the braided engine ground from the heads to the ground on the a/c condenser bracket (this must have been used by the mechanic who put a low mileage used engine in the truck in 2018 as a piece of the original braded ground wire was still attached to the firewall in the regular spot but only a few inches long and not connected to anything else). The connection at the bracket read .1 on the bolt when checking for voltage drop (not good according to Chrisfix) and about .08 on the braid near the bolt (ok according to Chrisfix but it was fluctuating a bit), but the braiding is fraying and not a lot left so will redo that one tomorrow (at A/C bracket or 10mm body bolt at heatshield maybe a better position as I've read others use). Truck ran fine mostly for the 10 minutes except for the idle fluctuating a bit down at one point. This truck has always had a fluctuating idle which I have thought was the TPS after it still being there after cleaning throttle body and IAC, but now maybe thinking the frayed engine ground may have something to do with it as well. Battery read 12.4v a few minutes after turning truck off.

Also checked the battery voltage when starting and it never dropped under 10v when cranking (Chrisfix said a bad battery will drop below 10v when cranking).

 

Check all your grounds and you may find some that aren't good. The one that is frayed for sure. There is also one or two grounds from the firewall to the engine. There's also a large ground from the engine to the battery. This is the one that the starter prefers to ground through. I've seen them look great at the battery but at the engine end they were rotten and broken halfway through.

 

Changed the engine to firewall ground, it was fraying even more as I handled it as it broke apart with a small tug. Bought a Dorman 15" braided universal replacement which didn't make it to the stock location but did reach a firewall stud a few inches lower that holds the a/c condenser bracket. Pulled off and sanded another 4-5 grounds around the engine bay plus the negative battery to engine ground which had some corrosion.

The ground for the body to box is not there, only the connectors - is that one braided or just a wire and is it just for the rear lights or?