One of the most important point about battery source and that is completely overlooked by most of us is the internal resistance of the pack.

This parameter is what characterize the health of the battery and its current capacity. To make an analogy with cars, the capacity of the battery would by the tank capacity of the car, the internal resistance of the battery would be the power of the car's engine.

So sizing your electrical system only by choosing the battery capacity can be very dangerous.
You can find on the market 5000 mAh batteries that will completely collapse ( 5V drop under a constant load of 40A) under load, and at the same time 1100 mah batteries ( like the A123 ) that will keep a fairly constant voltage under a constant 40 A load ( 1 V drop max ).
On that matter, there is a huge spread of quality among Chinese products, even among the same line of batteries.

It is a very good practice to do an internal battery check on a regular basis while doing through a cycle charge. Checking the charge capacity is absolutely not a guaranty that your battery will be able to deliver the required current load without voltage drop. The only parameter that will help you here is the internal resistance of the pack.

On that matter I recommend the Graupner ultra Duo 50 + charger. It is able to display the charge and discharge internal resistance for each element of the pack and log it into its internal memory. So you can track the evolution of your battery health with the time.

As a good practice, I discard my packs from flight use when the internal resistance has doubled from the original value.