you can't - I was recently told by a canon telephine rep that there is no battery, apparently there is a capacitator on the main board that controls the date/time functions - once it goes you have to replace the whole board - I'm located in Canada and was quoted Can$225.00 for the replacement board which would then still have to have it installed.
you can well imagine my reaction and opinion of this Canon product!!!!
what I did to resolve the issue was to do the the 24 hour full charge and then every 2-3 days later I turn the printer on for an hour or so in "energy saver" mode - this seems to charge the battery - sorry the capacitator - enough to keep the clock happy