If you have the money to spend, (like $250) this is the deluxe meter to get:

http://www.solar-electric.com/e-meter.html

The E-meter (now called the Link-10) monitors every aspect of your battery. You know how much current is going in (or out), what the voltage is, the temperature, how many amp-hours have been removed (or replaced), the state of charge, how much longer the battery will last given the present discharge rate, how many charge-discharge cycles have occurred, what the average depth of discharge was, what the greatest depth of discharge was, the charge efficiency.......... There is even a bargraph "gas gauge" to let you see how "full" the battery is at a glance.

There is an optional serial port so that you can send ALL this information to a computer (or palm pilot) to log every second.

There is an optional "Lift Lockout" feature that closes a set of contacts when the state of charge goes below some value you have set. This would let you set off an alarm if the battery became, say, 1/2 discharged during flight.

The unit weighs just a few of ounces. I have one in my electric car and on my electric drag bike. http://www.killacycle.com They are common on ships, sailboats, solar power systems, electric cars, and fork lifts.

The E-meter draws about 20 mA when it is "sleeping", so you might want to provide a way of turning it completely off if you store the plane for more than a week or two without a trickle charger in place. When you turn it off completely, it "loses its place" in terms of state of charge (and blinks to let you know this has happened.) However everything else stays nicely in non-volatile memory. The very next time you come to full charge, the E-meter "knows where it is."

If you are going to be ultra anal about your electrical system, this is the meter to have. (You would want to add a separate ammeter just for the alternator to get ALL the information you would ever want about your electrical system.)