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NC in the US of America

With all the talk of putting rechargeable batteries in Gameboy's I figure this is an okay place to ask this question:

Can leaving a LiPo powered device (tablet, smartphone etc) plugged into a sleeping computer overnight cause the device's battery to die?

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Germany

usually a sleeping computer does not provide current via usb. that is, if you are not using an externally powered usb hub.
if tha tis the case, make sure you have a reliable overcharging protection circuit (which you should always have anyways), then you should be fine.
depending on the circuits and batteries quality you might have a small loss of battery amperage, but thats depending on the parts and not the way or duration of charging it.