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It all depends on the output of the charger and the internal charging circuitry of the Sony reader.
Most cell phones these days use the same battery type of the readers do. They are nominally 3.7 volts, but fully charged at 4.2 volts. Fully discharged is 3.0 volt. Preferably you don't want to fully discahrge or fully charge them. Many cell phones charge to 70 - 80% to increase the cycle life (number of charges) that battery can handle.
Since they all use the same battery, and USB is speced to put out 5 volts, that is sort of the standard these days.
But I really like the iGo system, each tip sets the proper voltage and polarity for the devices.
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