You need to do a voltage drop test to see if it helps isolate the problem.

Engine off connect a voltmeter to the BATTERY POSTS not the cable terminal and see what it reads. Shoud be 12.4+ depending on charge. Next with voltmeter connected to BATTERY POSTS, have a helper try to start engine. The voltage should not drop below 10 volts during start. If it does battery is weak.

Next leave meter negative lead on negative BATTERY POST and connect meter positive lead to a good engine ground. Have helper try to start engine. You should read not more than about 0.40 volts. Anything higher and you have resistance on the negative side of the starter. If you get a high reading, do the test again but place the meter positive lead on a good ground on the starter case. Use a good jumper cable if needed. See what you read.

Next, place meter positive lead on the battery POSITIVE POST. Place meter negative lead on the POST where the large battery cable connects to the starter. You may need to use a jumper wire to make the connection. Have helper try to start the engine. You should not read more than say 0.90 volts. If you do get a high reading you have resistance on the positive side of the starter.

If you have a high reading on either side you will have to test, with the meter, to find where the problem resides.

During these tests you are looking for low voltage drop. Under 0.50 is good.

Use caution doing the tests as you are cranking the engine.

 

So if I do this and there is a drop of over 0.5 then what needs to get replaced? Would a direct ground from starter to negative post do any good?

 

If you are getting a large voltage drop, you have to find what is causing the voltage drop. The drop may be on the positive side or the negative side. This test will help ID if you have a voltage drop problem. If you find a drop more testing will be needed to find the problem. First do the tests to see what readings you get.

An example. Say you have internal corrosion in the battery cable to the starter. This corrosion will increase resistance, heat, and slow the current going to the starter. Voltage will drop. In a good cable you want no voltage drop so a low reading 0.50 or below is good. If you read 10.00 volts that means you are dropping 10 volts and the starter is only seeing 2 volts to start. Not good. So then you have to find the voltage drop.

Since all of your equipment is good and relatively new it may not be an equipment problem. This test will let you know if it is an electrical problem. The voltage drop concept is at times difficult to understand. I have to think about it too since I do not work with electricity often enough. This is the first post where I mention this test because the symptoms are good for this test. Anyway up to you to test or not.

 

I worked for O'Reilly's for 4 years. I'll be quick to tell you that their batteries and battery testers suck. It'd be worth swapping the battery out with someone elses to see if that solves the issue.

Drive over here and we'll swap one out with mine.

 

I'll try swapping it with the battery from my bros Toyota and see what happens. I may replace the positive cables off the battery too. They are a bit corroded

 

Well I got a chance to drive the jeep for about 2 hours early this morning and I think it's safe to say that the negative battery terminal and new wires did it! Woo!