I first started with the 600 ma/h receiver battery's and always worried about the voltage. My second plane I needed to buy a battery (first battery came with the radio), and went to my LHS. Much to my surprise I could buy a receiver battery that was 2700 ma/h. After flying the new plane for 5 tanks and still having green on the voltwatch I decided big battery's from now on. I replaced the 600 ma/h battery with another 2700 ma/h and have not looked back. My third plane also received a 2700 ma/h battery and I bought a 2200 ma/h battery for my 7c.
I can take all three planes to the field and have quite a few flights before running out of battery. Yes it does weight more but I use it to balance my planes and I haven't added lead yet.