Batteries are rated by amp-hours (not amps). It is a measure of the discharge rate the battery will provide for one hour. It is not exact and is only a measure fo the battires capacity. Without continuous recharging a 9 amp-hour battery is suppose to discharge at a one amp rate from full charge to nothing in 9 hours while a seven amp-hour batter will do the same in seven hours.
Several bikes that I have built or repaired run with "batteryless" or "battery eliminator" Boyer
Power Boxes. While It seems nice to run without the battery, on the street little things like the brake lamp being on at an idle will kill the motor. The voltage is so low at idle (less than 9 volts DC) with this system if the 3 amp brake lamp is illuminated that the Boyer
Ignition stops working. The fix is a battery of course. In these instances I have added a computer gel cell battery of (2.2 amp-hours) and the problem is solved. That battery was chosen because of its small size (about the size of three pack of cigarettes stacked end to end).
The point being that if your charging system keeps your battery fully charged, the Boyer
Ignition will work just fine, no matter the amp-hour rating of your battery. The battery limits how long you can run with the charging system that is in a reduced output stage of repair. Add to that point that with no lighing load the Boyer
works well enough to propel my Rickman Triumph in MX races without a hiccup as well as my dual sport Triumph TR5T (lights off). Choose that battery that fits the best and is of the type you wish to run (gel or reg or ?). The Boyer
will work just fine either way!