Running computer without battery?

i take my battery out and plug the AC in at all time. but when i shut down my computer and open it again it's wont on until i put the battery back in. i heard taking your battery out and in alot can damage it but if u take it out for a long period of time it's going to be OK. how can i open the computer without having to put the battery back to the computer. im using a HP G21-144DX notebook