Having a charging problem with my XL lately, I think the alternator is working intermittently. It'll charge the battery while driving, but if the truck sits for more than 2 hours, my voltage is down to under 10, and I have to use a jump-pack to get it going. So, did a little research on alternators: 2004 GMC Sierra 3500 trucks have the exact same part numbers as ours for the factory 105 and 145 amp alternators. Advanced Auto and RockAuto both have an optional 253 amp alternator, new, for less than some of the remanufactured units at either place. Found mine on E-Bay, no core required and with shipping included, for $115 - at Advanced it was about $250! https://www.ebay.com/itm/H2-Suburban-ALTERNATOR-253-AMP-97-98-04-HIGH-AMP-HD-High-Output/352212758226?ssPageName=STRK:MEBIDX:IT&_trksid=p2055119.m1438.l2649 I'm starting with the alternator, as I'd replaced my battery back in August. If the alternator is good, then I might have a bad battery. If that doesn't cut it, then I'm going to disconnect all my accessories and then reconnect them one at a time, until I find the culprit.