I recently constructed a max size 32x32x32 Big Battery (cause why not) and I have some obsevations.
Storage is dependant on electrolyte volume. The battery will store 150 million rf per bucket of sulfurous acid contained in the battery. My max size battery can store over 4 trillion rf [((30x30)-4)*30)*150 million].
Input and output from an electrode requires the electrode to have contact with the electrolyte. For each face of an electrode block in contact with electrolyte you can transfer 2500 rf/t. In my max sized battery it maxes out at 300,000 rf/t.
To set a power tap to accept energy you must both, set it to +ve, and click on it to increase the transfer rate as the default is set to 0 rf/t. Te extract energy from a power tap, you must set it to -ve and click on it to increase the transfer rate.