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[Its manual suggests to not use the extension (which is aimed for the receiver) when charging the mouse.]

I wonder why that is.

Here is a photo of it. enter image description here You can also find plenty of them on the internet.

The cable is a commom USB cable at its ends and the packaging includes an extending USB cable with common endings for the receiver.

The manual: http://www.logitech.com/assets/32643/g700620-002446003403qsg-amr.pdf

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Could be related to electrical resistance which directly affects voltage. –  artistoex Nov 12 '11 at 13:45
    
Was wondering the same thing - thanks! Would be nice if they explained it in the manual –  Frederik May 16 '12 at 11:23
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1 Answer

up vote 6 down vote accepted

Power = Voltage x Current (P = IV)

Voltage = Current x Resistance. (V = IR)

Since the cable has a resistance that is non-zero and a current flowing through it, there is a voltage drop across it, according to the second equation. Because of this voltage drop, the voltage received by the mouse is slightly less than the voltage supplied by the USB port.

Increasing the length of a cable increases its resistance, and correspondingly, the voltage drop experienced. Logitech might be concerned that if you use a long enough cable that the voltage drop will be sufficient to keep the mouse from charging.

Charging a battery depends on a difference in voltage between the power supply and the battery. For example if the battery provides 3.7V at full capacity, you need a power supply with >3.7V, such as a 5V USB port to charge the battery.

All that said, you can usually get away with 6 feet or so of USB extension. It might slow down charging very slightly, but I regularly charge my phone and iPod from an unpowered USB hub which is connected to my machine with a 6 foot extension.

Check what voltage the mouse battery outputs. The closer it is to 5V, the more detrimental the effect of lengthening the cable.

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