Sorry to say, there is no such thing as free unlimited energy. The energy in those home-made batteries does not come from the earth, salt water or potato, it comes from the metals being used - and those metals will oxidize as the process goes on, sooner or later the metals are simply used up. There is no big secret here, this principle has been used for ages, and as always the Chinese excels on doing things cheaply - you can buy "long life" AA-batteries for NOK 3 each at Biltema. It's exactly the same principle applied there, except they have found out how to make batteries without using salty water.

Also, don't confuse volts with watts - to see if a battery actually can be used, try changing the multimeter from voltage to current (amps) and see if you get anything at all. Effect is voltage multiplied with current, you may try to multiply the numbers you have. If you get as much as a full watt from that, still it doesn't mean it's possible to subtract as much as a watt from the battery, the voltage will drop once the current starts flowing.