SC2Mapster

I'm trying to create some skills that have a charge, like the vultures bombs, but will recharge once a certain condition is met. So for example I have vultures that have bombs. Once they drop all 6, if they go to the command center, they can get a bomb recharge. More specifically I'd like it to be triggered by something that happens in skirmishes. So for example, a skill would recharge after a the skilled unit kills 5 other units. Or it triggers once the unit has half his life. Etc. Is there also any way to make it trigger randomly? Like if a unit is attacking it has a random chance of triggering this ability to gain a charge? Thanks for any help.