No Power and No Start after Second Battery Install

A few days ago I had a friend help me install a secondary battery in my '96 XJ. The setup is a solenoid/isolator and a relay that's run to a switch in the cab so I can combine the batteries and charge both while driving when I want to.
The solenoid is also wired to the ignition so that the batteries cannot accidentally drain each other while the vehicle is off if I forget to turn the new battery combining switch in the cab off. My friend does this stuff for a living and I trust his install (not saying there couldn't be a mistake) but everything was working fine and I put over 300 miles on this new setup while watching the volt meter in the dash like a hawk and it never was drawn down while everything started and ran perfectly. I went to start it in the driveway yesterday after having driven it all morning with no issues and the starter cranked once and then EVERYTHING shut off. No dash lights, no starter, no clicks no anything, DEAD. I disconnected the second battery from the starter battery and tried jumping it with another vehicle and that did absolutely nothing, didn't even get the dash lights to turn on. Both batteries are new, the cables and terminals are all new except for the ones that go to the starter but we cleaned everything up on top during the install. I checked all fuses. I tried putting it in neutral and starting to see if the NSS had something to do with it, nothing, plus I just replaced the NSS last winter. The suddenness with which this happened makes me think that the amperage from the starter overloaded some part of my new battery system and fried something. But what?

So..Everything sat all night and this morning I turned the key and the dash lights came on and I heard the fuel pump prime, so I tried to start and I heard a *click* and the dash lights turned off and I'm back to nothing again. I got underneath and did notice that my oil filter has been dripping onto my starter which isn't good, but could a bad starter cause everything to short out? I got underneath and the starter wires don't seem loose or anything, just dirty from the small oil leak. My next step is to put it on a trickle charger and I guess replace the starter and fix the oil filter leak. My friend who did the install lives 6 hours away so we can only troubleshoot so much over the phone. Any ideas on what else I can try or check?

I would start by checking all battery terminal connections. I had similar issue with another build that burned the contact area of the terminal at post of isolator due to poor connection. sounds like the main power to the isolator has poor connection to it. Just my opinion

Thanks for that info, and all connections and cables look ok, none are corroded or loose. Sometimes its hard to tell i know that.

So I got chargers on both batteries...both were fully charged so low or drained batteries are not the culprit it seems.

I put a positive end of a jumper on the main starter battery and got under the Jeep and put the other positive end on the positive post on the starter. It came to life but doesn't sound like the normal starter. It sounds like i just turned on an angle grinder or a drill. Just a loud single pitched whirring noise. So that means it's bad right? I mean it should sound like it does when I turn the ignition on and it cranks the motor right? Silly sounding question I know but I want to make sure im not missing anything. If it is bad, then it is shorting out my entire battery system and the dual battery setup could possibly be unrelated at all?

Sounds to me like like the starter's the problem - but it is an odd coincidence.

Think I'd pull it, try turning it my hand to see what it sounds/feels like.

I would pull the starter and open it up. If the angry pixies are misbehaving you will probably be able to see the reason why.

This thread may be useful: https://www.naxja.org/forum/showthread.php?t=1115001

UPDATE: So I got it started. I was about to pull the starter but decided to tinker a little with grounds and terminals before doing so. I had the secondary battery system disconnected from the regular starting battery to eliminate the new setup from theist of suspects. I was cleaning the grounds and terminals yet again and noticed that the positive terminal was kinda crackling and subtly sparking everytime I disconnected and reconnected it. Now, it actually did this once during the install before we ever combined the batteries and my friend told me to turn the key to off because this happened and he thought the ignition was on, so I went to turn it to off but it was already off. I should have spoken up then. So this afternoon I decided to remove the starter battery and I replaced it with the secondary battery and there were no little sparkys when I connected the positive terminal this time. Then she started right up! The original battery would not jump start at all, and sat on a charger until the charger showed it was fully charged so I guess I don't understand car batteries at all.

So one mistake I think I made, and my buddy warned me about this, was that the secondary battery was brand new and has 40 more cold cranking amps than the starting battery which was a year old, so if it was going bad maybe we accelerated that by asking it to charge a larger battery. But it was only a year old Interstate Battery so I didn't think it would be going bad, but since it was sparking before we ever added the second battery something was clearly wrong. Hopefully there isn't a parasitic draw in my electrical system.

Im going to go pick up an identical battery for the second battery in my dual setup and hope for the best. It's not my daily driver so I'll give it another shot with the dual batteries and hope that this was just a case of a bad battery. Any thoughts in this development are very welcome!

I am not sure I am totally following your explanation, but if you have a battery with a dead short in it then I think the situation makes sense. The crackling/sparking would be a measure of the degree to which the bad battery was shorted out.

On the plus side, it is much better to discover a bad battery at home than on the trail.