It has been done, even though PowerTanks aren’t designed to have replaceable batteries. Please see these astronomy forum postings for more information:
http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/3217148/Main/2474152
http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/3243632/Main/3241838
NOTE: replacing batteries and parts or entering the PowerTank housing voids the Celestron warranty. Celestron assumes no responsibility for such homemade power supplies, modified PowerTanks or batteries and parts from other manufacturers or for improper use of such power supplies with our or other products.