adex44yrs

Hello, I am try to design a battery simulation to test a lead acid battery charger. My design is similar to one found here: http://www.accelinstruments.com/Applications/TS200/Battery-Simulator-AppNote.html (figure 2A). Other than I used MOSFETs instead of the BJT and alot PMOS is connected in collected parallel (the current sink). I am having some of problems which I can't really figure out why. The charger will turn on for few seconds and then turns off without then repeat the cycle. Any help please?