The exact voltage at 100% varies by chemical composition. A quick look at Ultracell's website didn't lead me to their specific documentation
(which you should request to get the correct charging
voltages and amperage rates) but I did find a reference to
AGM batteries which are sealed should be brought up to 2.35 volts and then discharged to a rate of 1.75 volts per cell...
so the 13.15V seems to be a reasonable open circuit voltage (2.35*6cells = 14.1V)