Question about which battery

Ok so its a 60" p-40 top flite red box and i have a stinger 15cc RE which says its cdi takes 7.4 - 14 volts I was at my local hobby shop and asked the guy working there what kind of battery i need for it and told him I think I need a 7.4v and he tells me no that will ruin my servos with to many volts. That I need something 6.6v or less. Now im very confused can anyone help