Another HELP my car wont start thread.

[1993MTXSHO]

well, my car wont start. I parked it 2 weeks ago when i went off to school. I just came home this weekend to get it to bring ot school, I get in no interrior lights, ok mayeb a fuse is blown go to start it nothing. Battery is dead... wrong, has a full charge. there is NO power to anything at all in the car nothing wil lturn on. I turned the key and the dash lit up very dim for about 3 seconds then died. The car has done this before when im out and i go to start it it cranks for a split second and dies no pwoer at all. I wait a minute and try again and it starts fine ad if nothing happened. I check negitive battery wire and all the starter wires. Also looked at the ground on the tranny (assuming its a ground just saw a wire on the case and checked up , everything is tight. I really would lie kto have the car tomorrow to take to school thats why i came home so anyone have anyideas for me? Any help is much appriciated.

 

[1993MTXSHO]

ok im dumb, i just checked the battery and its 6v so its dead. The reason I measure 13V originally was that I assumed the battery died and hooked the charger up for a few seconds, the charger read full charge so I checked the voltage and it read 13 because I just had the charger on it. I tried it again and watched the voltage go from 13 down to 5 or 6. So now my question is is what could all of a sudden make my battery die like that. the car has sat for months before and then started right up again. I do have subs but they have not been a problem and iv had them for a year.