Battery keeps dying...

[DieHardBigRaw]

Hey everyone, new rider here. I have a 2000 GZ250 which I bought used about a month ago. Everything was fine until one day I left the keys in the ignition (got distracted, yeah I know, bad me) and killed the battery. I charged it on a trickle charger over night and it fired right up, but ever since then it will randomly die and and wont hold a charge. I took it to AutoZone and it wouldn't get over a 75% charge even after being plugged in for over 4 hours. It sat at 75% for 2.5 hours! I am just wondering if the battery is gone or if there is an issue in the charging system that is causing the battery to die. I only drive it about 10-15 miles a day. I live on a military installation and only drive it to work. I have taken it for a few long rides (about an hour) but dont want to take any chances until I figure out why it will die randomly. I also dont have the money for guesswork? Thanks guys!