Nokia Vibrating Battery (5100/6100 series) ! Perfect for high noise environments and great for meetings and restaurants when a phone should be seen but not heard. Incoming calls trigger the battery to vibrate in a firm insistent manner, even if you have the ringer turned to "silent ring" mode. Just slide the battery on the back of the phone, then select "silent ring" mode from the phones menu. You now have the ability to choose between ring and vibrate modes. You're the only person who has to know you have a call coming in!Note: new Ni-MH battery's full performance is only achieved after 2-3 complete charge and discharge cycles.