Waiting on a new amp meter to arrive before I diagnose, but thought I'd ask for some opinions in the meantime. '05 Envoy 4.2L L6 (134,000 miles); couple of days ago, after the truck sat for five days, the engine cranked very slowly and would not immediately start. The headlights and reverse lights, which turn on when the doors are unlocked, were bright as usual. Generally, once you click the ignition switch to the start position, it auto starts. But the allocated second or two for auto start wasn't long enough to fire up the engine. It needed a second attempt, holding the ignition switch in the start position for roughly 4 seconds before the engine finally fired up. The engine cranked over much slower than usual. I did not pay much attention to whether or not the headlights dimmed excessively, but the backlighting on the gauges dimmed considerably. Since the dome lights are LEDs, they didn't change much. For some reason, however, my fog lamps turned on once the engine started. My first thought was battery; a Motorcraft BXT-78-E (800 CCA) sourced from East Penn Manufacturing and installed on October 14, 2017 with roughly 18,250 miles on it. Average battery life around here is 12-18 months, although I still have 8 months left on the warranty (36 months). Grabbed my battery booster, and once I arrived at my destination, the truck sat for a little over 90 minutes. Before using the booster, I was curious to see if it would start - it fired right up with auto start! It cranked strong and took less than 2 seconds before the engine was running. When I got home, I hooked it up to my OptiMate battery charger with the battery in the truck. It did not go into "Save" mode (severely low voltage/sulfated battery) after doing a self test of the battery, and proceeded to "Charging" mode. About an hour and a half later, the battery was fully charged and optimized, and it passed the test - the charger stops charging for a specific period of time, monitoring voltage, before testing the battery again. Decided I would drive the truck daily for the next couple of days. The following morning, it cranked over slower than usual but started. The truck then sat for a little over 9 hours at work, firing right up with zero issues. I did not hook it up to the battery charger this time but measured battery voltage when I parked - 12.7V. I checked battery voltage again after 4 hours - 12.5V. After the truck sat overnight for 14 hours, ambient temperature was 54Â°F and battery voltage was now 12.22V. All lights were on nice and bright. It cranked over very slowly, and refused to start on 3 attempts with backlighting on the cluster dimming considerably. Since this is a side post battery, with nowhere to secure the multimeter leads and no assistant, I couldn't measure cranking voltage. I hooked up the booster, and let it sit for a minute. Although it turned over a little faster, albeit still slow, the engine started. Got to work, the truck sat for 9 hours and fired right up again with no issues. Although I was supposed to measure resting voltage, I forgot. Parked the truck when I got home, measured battery voltage and it was 12.65V. Checked again after 4 hours, voltage was now 12.5V. The truck sat overnight again for 14 and a half hours. This morning, battery voltage was 12.26V and ambient temperature was 57Â°F. The engine turned over real slow, and auto start disengaged. Holding the ignition switch in the start position for a couple seconds and noticed cranking got slower and slower. Pulled the booster out, and hooked it up. After a minute, it turned over slow but started. The foglights came on by themselves again, and now I noticed the headlights were out. Turned auto lamps off and turned them back on again, but the headlights wouldn't turn on. Manually flipped the light switch to the on position, and the headlights turned on. Turned off the lights completely and set them back to auto lamp mode, now the headlights were on. Now I'm going to see what resting voltage is after it sits for 9 hours, but I'm wondering the possibility of high amperage draw by the starter causing havoc with other modules on the truck. Will be interesting to see what the amp readings will be once my new one arrives. Going to check all connections, grounds, etc. in the meantime.