ORIGINAL: Matt Kirsch
You need to match Volts for Volts, that is, if the stock battery is 3V, you need to come as close to 3V as you can to avoid burning out the motor and other onboard gear in the plane. Being an RTF, its capacity is certainly very limited.
Replacing a 3V with a 6V will practically guarantee a burnout.
According to the writings on the stock NiMh battery, it actually says 6V (though its mah is not specified), so I'm just guessing a 7.4V 2 cell Lipo may work...
But an additional question - I notice that a ordinary Lipo battery has 3 wires coming out while ordinary batteries like the NiMh only has two i.e. + and -. What's the third wire for?
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