I bought 2 of these portable chargers because of discount. They came with mini usb to micro usb cable, they fitted but the phone itself got no responds (not charging). They worked with all other cell phones in my house. Is there anything I can do to make it work? The output is 5v 500mha.
sorry for my broken English.

You've got to make an adaptor where the two middle pins(d- and d+) are shorted together. Power and ground should be left alone.
It should charge then, though you'd really want a higher-spec'd charger - 1A at least.

You have two...you could make a Y cable, shorting the middle pins together (outer pins are ground/+5, inter two are data), and combining both chargers to make a 1 amp charger.

Really though, a 500mA charger should do the job, it will just take longer. I used a 500mA charger for months, since I left my Nokia at a relatives during a trip. It takes a bit longer, but it should still charge. If it's not charging at all, I would check that the converter cable (micro to mini) works. Those tend to have a high failure rate when it comes to being manufactured right. I bought 3 before I found one that worked.

Thanks for help. When I connected the wall adapter to the P790 and other side to my N900, it works, but when I disconnected the power from wall adapter it stopped charging, therefore the converter cable is working. It feels like the P790's output does not match the power for N900, but it is a regular USB output.