What bat. and speed controler for twin 300's?
Here are a few basic concepts:
1. In series, current stays the same and voltage adds up.
2. In parallel, voltage stays the same and current adds up.
3. Rechargeable batteries have strict limits on how much current they can produce. If you draw too much, they get very hot and can become damaged.
4. The more current you draw from a battery, the lower its voltage goes.
5. Voltage and current are dependent on each other. Raise the voltage, and current must increase proportionally. Double the voltage, double the current.
6. Power is voltage times current. Double the voltage, and current must also double. This means that by doubling the voltage, you get four times as much power at the propeller.
Okay, here's what happened:
When you have the motors wired in parallel, the full voltage from the battery goes through both motors. Together, the two motors draw twice as much current as a single motor. The two motors running together are drawing more current than the batteries can produce, causing the low-voltage cutoff circuit in the ESC to shut off power to the motors.
When you had the motors in series, the voltage from the battery was split equally between the two motors. This means that the motors were only running at about 1/4 power, as compared to the parallel wiring.
All you should need is a different battery. AA cells are not very good for electric airplanes in general. You should get an 8-cell pack of 2/3A cells, like KAN950s or HECELLs.