Don't confuse the "rated voltage" of an airborne back (4.8v for a common 4-cell pack) with the actual measured voltage. As mentioned, 5.3v is pretty normal for a 4.8v "rated" pack at full charge.

If you fully charge your RX pack, and it only reads 4.8v on a loaded voltmeter, you've got a problem.

You can get a larger capacity pack if you wanted to. But with proper charging, 600mah on a trainer is usually plenty.

Also, the "wall wart" charger that came with your system, it's designed for a 600mah pack. If you go buy an 1800mah pack, it won't charge it properly. It would take at least 30 hours to charge, maybe more. It might not even fully charge it ever, since the losses in the pack might totally consume the low charge current that was meant for a 600mah pack. So, you'd need a new charger for a larger pack.

So, I suggest you not worry about exotic or extra batteries, and concentrate on getting your 600mah packs charged correctly.

However, going to a 6v RX pack will not help matters. In fact, a 600mah 6v pack will actually drain FASTER and give LESS flight time than a 4.8v pack of 600mah. The reason for this is that at 6v, the servos draw more current. It's Ohm's law v=ir in action.

I agre with rctrax, I try to toss my stuff on charge the night before flying, regardless of how much charge I think it might have. (note, "night before" does not equal midnight, when you're getting up at 7:30am for an early flight. That 7.5hrs of charge time isn't going to cut it. Get it on charge at dinner time.)

However, leaving your batteries on charge all the time on the come-with charger is not good for them, it will over-charge the packs.