My t450s comes with an internal battery. The comment on this guide records the process for disabling the internal battery temporarily.
I also used this guide from Lenovo as I found removing the back plate rather difficult.
crwdns2858784:0crwdne2858784:0
crwdns2858785:0crwdne2858785:0
crwdns2916922:0crwdne2916922:0
My t450s comes with an internal battery. The comment on this guide records the process for disabling the internal battery temporarily.
I also used this guide from Lenovo as I found removing the back plate rather difficult.