Replacing AC + Battery with DC + Capacitor


I have a 9122B that was modified by Personalized Software to run on AC. They replaced the battery in its shell with a 100 uF capacitor. The input is from an AC/DC transformer with output of 2A at 6V. This is cool, because I don't plan to use the device where batteres are needed.

Now the one working battery pack I had for my Thinkjet 2223B has gone to electrolyte heaven. I disassembled the pack (and another, non working one I have) and figured out that the bateries were 6 "C" sized NiCads wired in series, with a resistor soldered between two of the cells. A rectifier was included on a PCB to produce the required charging current. A quick Google revealed a source for replacement batteries, but not the packs, for $49.95. I'd rather do something like was done for the 9114B, and replace the battery and rectifier with a transformer and a capacitor.

I am mostly ignorant of basic electronics, being a software guy and all. How can I figure the right value for the dummy load capacitor, given the nominal output voltage of the battery pack?

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