Went to start the Jeep at the end of the snow run and battery was dead. Not click-click-click, but dead, everything off.  Got a pull down the hill from Jeff and tried to bump start it the whole time to no-avail.  Borrowed some jumper cables and tried to jump it.  2 minutes connected and the light under the hood still wasn't illuminated.  I then pulled the leads off of Jeff's truck and touched them together to see if my battery had any juice.  The cables did have a slight arc but it was super faint.  I then put the leads back onto Jeff's truck and the light under the hood came on.  A couple of minutes more and we had the starter clicking.  This was enough juice to allow us to hill start it and then the Jeep ran fine.  

Initially I thought the battery had failed.  I have had cells fail in the past and this was similar.  Voltage on the drive home never made it much above 12 and never all the way to 14 where is should be.  I put a trickle charger on the battery over 2 days and tested the battery.  Full charge and over 800 amps, hmm.  So my battery was low but too low to turn on the ECU, prime the fuel pump, and charge the coil? Well yes.  Unlike a carbureted engine, all of those items are controlled by the PCM which has a low voltage cutoff.  I was unable to trigger my compressor which is wired not to need ignition.  But the compressor is triggered by relays and they have a minimum voltage to trigger.  

Ok so my battery was just dead, why?  I left my CB on but I always do that on day runs and I've even left my lights on once all day at work without issue.  Maybe there was a short somewhere or a terminal is loose? Nothing obvious.  At this point I decided to check the trouble codes.  I wasn't holding out much hope because I have an intermittent O2 sensor code that has been around a while.  Plug it in and i get the typical P0057 but there is also a P2503.  "Low charging voltage".  Great, alternator dead.  Go to pull it off and the charging cable nut is finger tight, problem found.  Pull the alternator anyway and have it tested, no problems.

So at some point my alternator charging cable came loose at the alternator, arcing caused the computer to stop charging and throw a code but my existing emissions code prevented me from noticing anything new on the dash.  I was basically driving on mostly battery voltage for who knows how long and it happened to cross over the critical minimum voltage at the top of the hill while parked.  Bouncing on the way back down the hill must have been just enough of to cause the cable to re-connect intermittently to keep the battery charged enough to get home.  

Thankfully I have a deep cycle so battery life hasn't been shortened too much.  I just replaced the batteries in my diesel so Im not looking forward to buying another battery for a while.

Lessons learned:

-Jeff really is the club tow truck

-Batteries are expensive

-Electrical issues are a pain

-Buy mopar O2 sensors so you can actually use your check engine light