What bat. and speed controler for twin 300's?
 

I 've got a swift racer with a pair of Graupner speed 300's installed and I need some advice on what kind of batteries and speed controller I could use with this combo. I've got an 18 amp Kotronik speed controller and a 9.6v battery pack of sanyo nimh AA size at 1850 mAh. I've tried these but the speed controller cuts the power just above half throttle and the batteries get really hot. That happens when the motors are wired in parallel. When the motors are wired in series everything works fine but power seems marginal for the model. I have no idea what is going on. I'm really new to electrics and none of this makes sense to me. Any suggestions would be very much appreciated.

Thanks

George C

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.

What bat. and speed controler for twin 300's?
 

Thank you for your input, it was really helpful. I think I’ll keep the speed controller I’m using now and I’ll go for a HECELL pack. I did a quick search and found them in a site called edogfight.com They say they can give up to 16 amps. I hope that is sufficient. Further more it seems that the 8 cell pack is about 50 grams lighter that the one I’m using now.

Thanks for the help

George C

What bat. and speed controler for twin 300's?
 

HECELLs are good batteries, which is probably why I recommended them, eh?

If you're still running into that low-voltage cutoff problem, I would re-think the propellers and/or gearboxes on the motors. Speed 300 motors shouldn't be drawing more than 8 Amps each.