My 2002 Xterra wouldnt start the other day so my husband dropped a new battery in the car he tested the battery with a volt meter and the battery read at 11 so then he dropped a new alternator into it it still reads at 11 and thats with the car running. We have had all the belts ,fuses and checked all the dash lights to make sure everything is working what else could it be ?

i'd test the volt meter on another car, just to make sure it's not giving a misread

Have your alternator checked at an autoparts store. Most of them will do it for free.