@p900: You may charge at 1200 mA, so yours "thin naked wire" has to tolerate 600 mA. If it is too thin, it won't.
I also didn't break either battery (see my previous posts on this thread), but soldered two wires between (+) and (-) of the battery pins. You do not have to connect the third connector of the 2nd battery.
Mine has worked great also and having ~2600 mAh makes use of N900 much more fun when you do not have to restrict how much you surf on the web or use GPS during the day.