Quick tips for increasing your flight duration in your electric model airplane.
I got a short email the other day asking about tips when upgrading your battery pack. I am going to make an assumption. I will assume that the current battery pack is meeting the power needs of the model. The goal would then be to increase the flight duration.
Volts and Amps
For the model to keep the same flight performance with the new battery pack, it must supply the same voltage and at least the same current to the rest of the power system.
Keeping the voltage the same is easy. Just use the same number of cells in the new pack.
You then need to figure out the electrical current needs of your power system. One way is to use the lowest current capacity from the battery pack, speed control, or motor. Here is my reasoning. Your model is flying around okay and it is not burning up any of your components. Then the actual amperage of the system is probably less than what the weakest component can handle.
A much better way is to actually measure the amps using a wattmeter. Just fire it up and go to full power. Propellers unload in the air, so you can assume that the reading on the ground is the maximum the battery needs to be able to supply.
Finding a Match
I would aim at increasing the battery capacity by 25-50%. The weight increase will be small and the handling qualities of the airplane should be close to what they were before.
Figuring out what capacity you want is the easy part. What C rating you need is a little harder. Of course, you could just use the same C rating as the old battery pack. But the higher the C rating, the more you will pay for the pack.
Current capacity in amps is (battery capacity in amp-hours)*(C rating). As a simple rule of thumb, if you double the battery capacity, then the C rating can be cut in half.
Suppose you are flying around with a three cell 1200 mAh 30C battery pack. That means your old pack can supply up to 36 amps (1.2*30). Using a wattmeter, you measure the maximum current at 30 amps. If you upgrade to a 1800 mAh pack, then the C rating just needs to be 20. This is because 30/1.8 = about 17 amps.
Battery packs decay over time. Their voltages remain relatively steady, but their ability to supply current will decrease as the pack ages. Hopefully the C rating from the manufacturer has a built-in safety margin big enough to account for this eventual decrease in performance. If you want to play it safe, then use a C rating a little bigger than you really need like the example above. I recommend that you do this.
Battery packs always come with connectors soldered on. Try and find a new battery pack that uses the same connector as the old one. If it is different, you have two choices. You can do what I used to do and cut it off. A lot easier and almost as good is making yourself or buying an adapter that converts between the two plug types.