HELP! Battery/electrical issue - car keeps dying.

PharmDZ

So about a month or so ago I was out somewhere for about 45 mins, came back outside to leave and my car was dead. It wouldn't turn over, kept dying until it wouldn't try to crank at all. This was even while on jumper cables. Eventually the next morning we left it on the cables attached to another car for 20 mins and it started, and was fine after that. I changed my 6 year old OEM battery to an optima yellow top just to be safe, but I still wasn't quite convinced that was the problem. Also, as a side note, about 2 weeks prior I had changed my transmission/clutch. I went back over all the bolts and plugs to make sure everything was tight and/or plugged in and everything checked out.

So fast forward, about a month later I go to start the car... same thing. Car's battery was just dead this time. I went to charge it with a portable jumper this time, tried turning over and died, then after being left on for a good 15 mins or so it started. It's been fine since then, again... but now I'm lost as to what could be causing this issue. Anything else I should check from here?? Thanks everyone