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I'm using a 2S-350 LiPo for my backup battery, and drawing 2 cells off my 10S-5000 LiPo motor battery thru the balance plug for the main radio power.
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No switches, just plug everything in each flight since I'm taking the canopy off to plug in the batts anyway . . . two less components to fail or carry around.
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My planes lost 44 grams going to this setup vs. carrying a separate receiver battery and one power source.
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