Disable Spotlight indexing, save battery life
Well, the immediate way is:

sudo mdutil -i off /


from the command line (you will have to enter your admin password) to stop indexing immediately.

You might also want to add the following step:

sudo vim /etc/hostconfig


(you might want to use a different editor than vim, YMMV) 

Then, navigate to the entry

SPOTLIGHT=-YES-


and change it to 

SPOTLIGHT=-NO-


SAVE the changes the exit the file.  BTW, have a look at iFixit on how to change the battery, it seems you could have much better use of the laptop that way.