The USB port is an amazing thing. You can plug almost anything into to do a variety of tasks, one of which is recharging batteries.
While this may not seem useful at first, it can come in handy for situations such as the airport. If your MP3 player runs out of battery power, you can just charge the batteries from your laptop. Of course, you will be trading power between your laptop and the rechargeable batteries but in this situation it is worth it.
There are lots of devices out there which can accomplish this (here is an example) or you can go the USB battery route with no charger required. Alternately, you can build your own USB power source to power/charge your devices.
Personally, I use a solar battery charger for recharging my batteries, but I can definitely see the appeal of these devices.
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Keep in mind that when you use one battery to charge another that it is not a one for one trade off. Charging any battery is very inefficient process and much of power to the battery being charged will be lost as heat energy.
Good point. Additionally in my scenario above, the laptop would consume battery power required to operate it.
I agree it is not the most efficient method, but is a viable alternative with legit uses.
One of the best instant battery voltage has risen to the maximum allowable voltage of the charge source, current through the battery begins to decline. This simultaneous event of reaching maximum voltage and the start of current decline marks the beginning of the absorption step.