The ESV is an meter we use to check the condition of the batteries in the airplane. They're anywhere from $8 on up. They're small and easy to use. Some guys check before every flight.

As I said before: The transmitter is going to last through a couple of airplanes in a day. It's also got a readout that'll make monitoring it easy. Your TX does show battery voltage, right?

Now.........
The DX7s I've been flying (a couple of students have them) have 1500mAh batteries in them. How long are you flying in a day? 1500mAh ought to outlast the pink bunny that plays the drum.

Fully charge that DX7 and then go fly. Watch the voltage reading and count your flying time. If the reading goes below 9.6, put the TX on the Triton. If that doesn't happen, when you get home put the TX on the Triton then. Either case, look at how many mA the TX battery takes from the Triton to fill 'er back up. Then divide that by the number of flights you made on that TX. You'll know roughly how many flights you should expect from that battery next time out. That Triton (as does the AccuCycleElite) gives you the tool to watch your battery performance. They'll tell you when a battery is starting to crap out on you. They'll also help some batteries by cycling them, but that's a different deal. The same technique works for your flight batteries.

Wall warts come in all manner of ratings. A buddy of mine uses 6v packs in his airplanes and bought wall warts to suit them. He understands electricity and knew what to buy. If I remember correctly he had a hard time finding those warts.