The duratrax you set one time and thats it. It will set the fail safe when any battery pack reashec 3.8v. It dont matter if its 6v pack or 4 AA pack. They are reallt small and work great. I have several trucks using them and never had a run off. This is the one you want. Only about 20.00
http://www.duratrax.com/caraccys/dtxm4000.html