Car problem.......................…

my car has not been able to start 3 times this week because of the battery dying. i had the battery checked and was told there was nothing wrong with it, the wires are not loose, and the alternator is fine according to a mechanic. supposedly nothing abnormal could be found, and the mechanic would have to be there next time the battery dies to further analyze the situation. because at this point he can not find anything to fix. i commute very much, it is becoming extremely burdensome, any opinions/suggestions? thanks