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Here is a chart from the Trojan website:
Percentage of Charge Specific Gravity Corrected To Open-Circuit Voltage
6v 8v 12v 24v 36v 48v
100 1.277 6.37 8.49 12.73 25.46 38.20 50.93
90 1.258 6.31 8.41 12.62 25.24 37.85 50.47
80 1.238 6.25 8.33 12.50 25.00 37.49 49.99
70 1.217 6.19 8.25 12.37 24.74 37.12 49.49
60 1.195 6.12 8.16 12.27 24.48 36.72 48.96
50 1.172 6.02 8.07 12.10 24.20 36.31 48.41
40 1.148 5.98 7.97 11.89 23.92 35.87 47.83
30 1.124 5.91 7.88 11.81 23.63 35.44 47.26
20 1.098 5.83 7.77 11.66 23.32 34.97 46.63
10 1.073 5.75 7.67 11.51 23.02 34.52 46.03
I thought I would include > 12v as a demonstration to show it is not the voltage that matters i.e, 6 vs 12v since lead acid batteries are combinations of 2v cells.
So with a good brand deep cycle battery wether you get a 6v or 12v doesn't matter. They are the same basic battery one with 3 cells the other with 6 cells.
Trojan makes a very nice size 150 Ah, 20 h rating in 12v. Three of these and you have 450 Ah. They make a very nice 360 Ah 6v. Two of these and you have 360Ah. The chemistry is the same. The battery, Trojan, construction is the same. There is no difference between 2 6v and one 12v in a well regarded name brand battery. The weight of the battery in pounds for Ah produced at 12v is the same.
If you don't see the 20 Hour Ah rating but something else such as CCA it is not a deep cycle battery. There are combo batteries like the Harris which are a little sturdier, heavy duty, than a starting battery but not the same construction as a deep cycle. Starting batteries, combo batteries are designed to give a rapid discharge of high amperage current. Deep cycle batteries are designed to give up a slow long discharge of low amperage current irregardless of the voltage. The chemical engineer designs the battery to control the chemical reaction that he/she is seeking.
The determination of which deep cycle is best for you is determined by what your needs are and the space that is available to you. Then you have to be sure you have a charging system that is capable of maintaining these batteries.
BTW I seem to remember that I once calculated the Harris battery to be about 65Ah or so. So a 50% discharge limit is "ideally" 30 amps at 100% efficiency, which doesn't happen. You realistically probably have about 20 to 24 amps to work with for each Harris battery. Which is what most of you seem to be seeing, some less. As the battery ages, sulfonates and not properly maintained it slowly says goodby, adios.
I hope this helps, it's a combination of chemistry, chem engineering, physical space available, amps required per unit time (cycle), judgement and personnel preference.
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