The bulk mode voltage can't be fixed as the CC is trying to increase the battery current which raises the voltage. In other words, when discharged, the battery voltage is low, say 12.2 v. The CC can only charge at a certain current based on voltage (12.2v) and internal battery (depending on battery makeup) resistance and available light to the CC. So, the CC will try to put as much current into the battery that it can, until he battery reaches the absorption voltage. At that point the CC will not let the voltage increase, but will allow amps to charge the battery until full.
Voltage doesn't charge a battery, current does. But voltage is needed to push amps (current) into the battery. Voltage is used by the CC to determine the state of charge, thus what it (the CC) should be doing
2000 Cherokee 29BH with 6V batteries, LED lights & 400 watts of solar power, flipped axles and raised. 2007 Tundra 5.7L DC-LB with lots of mods. C-co, 8/158th AVN Maint.