quote from onrecess:

I bought a battery testing ESV for less than $20 and check my batteries before every flight. Cheap insurance- spend the $18 bucks.

Great advice ...

I use the Volt Watch type indicator to test my battery between each flight. However I load it down with a 20 ohm resister because this indicator uses only the servo loads you have in the plane. Not enough in my opinion. You can get them under 10 bucks now. I have info on my site to explain how to do this if anyone is interested. The bottom line is to test your battery before every flight. (even the first flight that you think you have a fully charged battery)
Ace