Q-42: I purchased a TUBBY CUBBY by Wattage (Global Hobby Distributors). It comes with a 370 electric motor, 3.7/1 gear box and a 7.5 x 5 propeller. I’m using an FMA Direct M5 receiver and two FMA Direct PS-20 servos. The ESC is a Castle Creations Pixie-7P programmed to cut off at 6.0 volts. Power is provided by a two-cell Kokam 700 mAh Li-Poly battery. The problem is that at about 1/3 throttle, the motor cuts off. What is wrong and how can I correct it?
A-42: You have made some excellent selections in your equipment. These are all items I personally use. The one thing you didn’t indicate in your letter is your motor current. I keep harping on this point time and time again. You can’t make decisions or correct problems without knowing your particular motor current.
I’d have to guess that you are drawing about 6 amps on the 370 geared motor. Most of the present variety of Li-Poly batteries will accept loads up to about “8C” (or eight times the capacity of the battery). At eight times 0.700 the capacity of your Li-Poly battery expressed in amps equals 8.5C. What that means is that when you apply your 6 amps (my estimate) load, your nominal voltage is almost immediately dropping below 6.0 volts. Since that is the cut-off you set in your ESC, the motor will stop.
What is happening is as you increase your motor throttle, you are reaching a point where the load is pulling down the voltage below your programmed cut-off point. Your basic problem is that you need a higher capacity battery. You have two choices, either take a second pair of 700 mAh Li-Poly cells and place them in parallel to obtain a 1400 mAh capacity or buy two new cells rated at 1500 mAh. By doing that your load will now be half as much or approximately 4C. With the extra capacity you should stay above your 6.0 volt cutoff for something like 15 minutes or so. Try doubling your battery capacity and see what happens.