Replaced alternator, new battery, 1 month later battery gauge dropping low

[tony53220]

Hello all, sorry if my post is a little off but it's my 1st time using this forum. So a month and a half ago i replaced my alternator & battery because my alternator took a dump. the belt was fine and had no reason to be changed. After replacing all that the battery light went off and the gauge went back to being normal again. This morning out of no where i started the truck, got a block away from home & the gauge dropped 3/4ths of the way down above low. After doing a brief look around under the hood i took the truck to autozone & the alternator & battery came back as being 100% in working and great condition putting out all the power they should be. So to do a little more research i did a few things i read online to see if i could somehow figure this out.I know everyone don't recommend this but while the truck was running i disconnected the battery & it didn't die & the truck still ran fine. I realized this really didn't need to be done because the alternator is fine, yet i did it to see if i got the same result from a few things i tried while i still had the battery connected. Yet while the battery was disconnected i got in the truck and turned the heater on overhead lamps & pressed my drivers side window button & the gauge dropped 3/4ths of the way down right before being low & my headlights also became very very dim. After turning all that off and leaving the truck running, battery still disconnected, the gauge went right back up to the middle where it should sit & headlights became bright again. To add to the reason i did these tests is because with the battery still connected i also did these tests & i had the exact same results, but that's why i did it with the battery disconnected to see if i got the same results to rule out the battery. I can change parts but im not the smartest when it comes to the inner workings of automobiles & thats why im here looking for more experienced advice. I just dont understand what it can be. Alternators great, battery is great, yet when at a stop light or when putting any kind of load on the truck from things like the heater, windows etc... the gauge just don't drop a little like what would be normal, but drops down to a critical level right before the light would come on & would be all the way down to almost going under the line to actually being on the letter L & not just the low line above it & the lights become very very dim. Any help would be greatly appreciated. Like i said im not the smartest when it comes to this stuff but i do know a fair amount of what i am doing to know i checked alot of things that could cause this & know this should not be happening. Again thanks in advance for any help.