My Tx pack is still the original one that came with the DX7 which (from memory) is a 9.6V 1500mah NiMH pack. I've thought about upgrading it a few times but haven't yet. I guess the difference is that I can see the battery gauge on the radio, and it'll beep to warn me when it's low whereas I can't tell what the planes are doing when they're in the air. I've done about 8 flights in a day without the Tx battery getting dangerously low.

I fly three planes - a Great Planes Super Skybolt that runs five servos, a Seagull Models Dual Ace with seven servos, and a Seagull Models Harmon Rocket 3 (modified to have flaps) with eight servos. In all cases the servos are the DS821's that come with the DX7 (they're a reasonably priced digital servo that's a bit more capable than average) except for throttles and flaps where I use standard analogue Hitec servos. All my planes have "lost model alarms" and LED volt meters on board, and the Dual Ace also has an on-board glow driver to help with the twin engines (with its own independent battery). Don't forget though, it's not really how many servos you have on board, it's how many you're likely to use at once and what load you're going to put on them.

As CGR has pointed out some 2.4Ghz receivers (most notably Spektrum ones) have issues with low voltages "rebooting" the Rx. It's never happened to me even when I was flying 4 servo trainers on 4.8V packs, but I'd rather avoid it where possible. CGR's also pointed out that that higher voltages also equates to higher current draw, which is why I won't use less than 2200mah packs where I'd be happy to use a 1700mah pack in a 4.8V system (if I was willing to go with 4.8V of course. ) Having said that I was able to use the 4.8V 1100mah pack that came with my DX7 without an issue in three different (all now defunct) planes - I just tend to err on the side of caution. I should also say that none of my planes have ever finished the day with flat batteries, and I'd like to keep it that way.

[link=http://cgi.ebay.com/B6-LiPo-Li-Po-NiMH-2S-6S-Battery-Balance-Charger-Black_W0QQitemZ270427522693QQcmdZViewItemQQptZRadi o_Control_Parts_Accessories?hash=item3ef6bc8685&_t rksid=p3286.c0.m14&_trkparms=65%3A12|66%3A2|39%3A1 |72%3A1205|293%3A1|294%3A50]Here's a link[/link] to one of the cheap battery chargers I mentioned. If you search for "battery charger" in toys & hobbies/radio control you'll find plenty that'll work, probably cheaper than the one I've listed. You usually have to get a 12V power supply too, or run it off a car battery. I got it because finding a charger for a single NiMH cell for the on-board glow was proving more difficult than it should have been, and the digital charger was going to cost about the same as the plug pack.