I have started using the new, Quanum Pocket Vibration Telemetry Voltage Meter With Alarm from HobbyKing.
It uses a very small monitor/transmitter plugged into the balance port of the flight battery.
The airborne part is 1-1/8" x 1-1/16" x 1/8", it only weighs 4.3 grams about 1/7 of an ounce. It has a antenna that is 4.5" long, soft wire. Range is said to be 300-400 meters.
My plane never went beyond range even though I tried to do so. It just got too small to be comfortable, even with a 42" wing span.
The receiver can be in your shirt pocket although I have mine on the metal handle on the back of my JR 9303, when it vibrates, you will NOT miss it.
Bottom line; I love this thing, it is well worth the price, for telemetry of the most important variable in an airplane. On my first launch I realized I had a weak battery. This unit monitors each cell, not the total for the battery. so if you have one bad cell in the middle of a good pack, it will tell you. then you can do some ground testing to find the bad cell.
I don't have any connection with HK, it's just that this is great. I will not fly without it.
Quanum Pocket Vibration Telemetry Voltage Meter With Alarm