Regardless of your power source, I would recommend using a constant current circuit to power the electrodes in any electrolysis experiment. Try the L200C power regulator ic. The place where you buy it from should be able to provide the data sheet which contains the recommended circuit for using it as a current source.
Batteries should not be connected in series unless specifically designed to be. Never mix different types of battery. Don't mix old batteries with new batteries. A battery may explode if,
a. a reverse voltage is applied to it
b. too much current is drawn from it
c. an excessive voltage is connected to it.
If one of your electrodes became coated with a conductive substance, it might have caused the electrolyte to act like a battery, causing a reversal of the current flow.