The original question of a simple method to detemine "how much is left" requires a little homework on the part of the OP. There are two ways of approaching the question. First, if your setup is a 1500 mah 4 cell nicad or NIMH battery on a typical 40-90 size plane, 4-5 servos, it is probably drawing around a 400 mamp rate. At that rate, the battery should last for a total of over 3 hours total flying time (300 mamp x 3 hours = 1200 mah).

The typical flight lasts around 10 minutes, but your style may differ. With 10 minute flights, that's 18 flights. I don't think I've ever made 18 flights in one day on a single plane.

Now the question of voltage. The most accurate way to establish the "no fly voltage" on your set up is to create a battery discharge curve. You can do this from just about any charge state on the battery (as long as its not fully discharged). Fully discharged state for nickle based batteries is 1.1 volts per cell (4.4 volts in the OP's case). The curve is easy to create. Plug your loaded volt meter into the charging plug and leave it there. You can use the loaded volt meter to discharge the battery (it should be around 200 ma rate) or just turn the plane on with all servos connected. Either way, you want a constant discharge rate. Now record the voltage every 5 or 10 minutes. When the voltage drops to 4.75 volts, check it more frequently. You'll eventually see the point where the voltage drops off quickly. You can go down to 1 volt per cell without damaging the battery, but there is no point in going so low.

Now make a graph of time and voltage. Look for the "knee" of the graph, and then go back 15 minutes. That voltage is now you're "no fly" voltage. In otherwords, that point where you are no longer confident that you have at least 15 minutes left. That graph can be laminated and kept in your flight box for reference. If you're really worried, do this annually for each of your batteries.

In my experience, that drop typically occurs between 4.5 and 4.6 volts for Nickel batteries. I usually set my no fly voltage at 4.7 volts.

Brad