Gould, it's 2.1 x 6, you get 12.6.

If it were 2.1, you'd get 12.6. Corect.

However, the potential capacity of a cell is
2.2 volts.

Consider the "Ideal Charge Curve" described by Rick Proctor of Cruising
Equipment Co:

Bulk phase: Chare at a rate up to 20-40% of amp-hour capacity to a voltage of
about 14.4 volts. (Gel: 14.1). For example, a 200 amp-hour battery would be
charged at 40-80 amperes. This will bring the battery to about 75% of full
charge.

Aceptance phase: Maintain battery at 14.4 volts (Gel, 14.1) while the amperage
is steadily reduced. This restores the next 25% of capacity at a declining
rate. Your battery can be considered fully charged if it will accept current
equal to 2% of cpacity at 14.4 volts. Example: A 200 amp hour battery accepts
only 4 amps at 14.4 volts.

Float phase: The voltage is reduced to 13.3 volts, (gel 13.7 volts) to maintain
the battery without losing electrolyte from the cells. *THIS IS A MAINTENANCE
PHASE, NOT A CHARGING PHASE*

If you test a battery aboard a boat and it reads only 12.6, it is not fully
charged.
Fully charged to "float phase" is over 13 volts for flooded batteries and 13.7
for gel cells.