2002 Mitsubishi Montero Battery not staying charged

My battery warning light is on. I have replaced the battery with the suggested specification and I have replaced the alternator. The car work well for a few weeks and not the light is coming on again, when I tested the battery in was at 9.6 volts, so I took out the alternator and had it tested and it passed the test at two different auto stores. I don't know how to trouble shoot it further, perhaps, change the belt?
Thank you in advance,
Richard