I just wanted to share a few pieces of information about the Mobi. First of all if you need a battery for the Mobi pen, you can buy it at Radio Shack for $5.99. Their catalog number is 55027736. It's a GP (Gold Peak) 1.2 volt battery. To get to the battery, locate the square little button on the back of the pen just above the 2 silver rings. Use a key or something to press the button in a little. Slide the pen apart and you will see the battery. I have also found that if your pen is not working and you have definitely charged it, you might just need to take this battery out and use a pencil eraser to clean the contacts. When you are charging the Mobi and the pen, be sure you see the little pen charging icon. This is a little square with a line at an angle through the top. If you do not see the little square constantly filling and emptying, then the pen is not being charged. It is very possible to charge the Mobi and not charge the pen. You may just have to push the pen down all the way into the pen slot of the Mobi. You also may just have to turn the pen around a bit in the slot. In any case, be sure you see that the pen charging icon is displayed. I have also noticed that leaving the Mobi in your car (in the cold) the day before you want to use it is not a good idea. I did this the day before I went to a school to do a training and the fully charged Mobi and pen did not work the next day. I hope this helps.