The battery has a 120 reserve capacity, which is the number of minutes that the battery will hold voltage over 10.5V at constant current of 25A.
55ah means that you will get 55 amp-hours at whatever current draw. 55ah / 20A = 2.75hours = 165 minutes.
Also, 55ah @ 20 most remains @ 20 degrees Celsius. The amp-hour rating for a battery is highly dependent on temperature.
While considering the above specs may be a correct step 1 in selecting a deep cycle battery, we would never want to use these numbers directly to select a battery and repeatedly run a battery voltage down to the 10.5volts fully discharged level. The resultant undersized battery being repeatedly fully discharged would oversulphate and last only a short time period well below its rated lifetime.
Batteries including deep cycles are evaluated by their makers at no more than a 50% discharge level (about 12.3V) in establishing their expected lifetime.
Without explaining the State of Charge math, RC- reserve capacity in minutes quoted on a spec sheet for any battery should be halved (50%)and preferably be quartered(25%) in any calculations so as to not repeatedly over discharge and damage the battery.
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A battery with an RC of 120 minutes used on a TM drawing 40amps being used 6 seconds of each minute would reach 50% discharge and need recharging after
6hours,15 min.25amps/40amps =.625
.625 X 120minutes X 50% = 37.5min
37.5min X 60second/6seconds = 375minutes
375min / 60min = 6.25 hours
