Hi Subh4n
Our BlackBerry Batteries are lithium ion which do not have memory , It attains a full charge within 2 to 3 hrs of charging.Also we should note that it should go below 10 % ,Continous draining and using below this level may actually reduce our BlackBerry battery life , Also what you friend is doing is not a good Idea , while reboot it takes a lot of battery resourses and rebooting at 5 % is not wise, his on device battery meter might be out of sync that is the reason he might be getting extra but I am sure his battery may show drainage at faster rate .
There are no such battery backup for any Bold or any other BlackBerry Smartphone.On your BlackBerry 7 smartphone you can unable battery saving mode , set it to 20 % so that it may warn you when it reach that level which I think is a best practice to maximize our BlackBerry Battery.
In addition you can read this Article from Battery University which might help you How to Prolong Lithium-based Batteries
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