What is so special about six volt batteries might be a better question.
SO, I will try to present the answer and why I recommend them.
Six Volt (GC-2) style batteries are designed for Golf Car (hence the GC designation) use, which is a long slow (Compared to total capacity) discharge to a level as low as 50 percent state of charge.
They are also the single. MOST PRODUCED Lead acid battery in the US and likely the world as golf courses order 'em by, depending on the course, the pallet load or the truck load.. Now that's a lot of batteries Each golf car takes at least six.
RV use is the same, a long slow discharge (Compared to total capacity)
NOw starting batteries on the other hand are designed to dump current at 2 or 3 or more times their C/20 capacity rate (I will explain) But only for 30 seconds, THey need to be kept fairly full, like 80% or more state of charge This means there is not a lot of power to use.
MARINE/deep cycle can go a bit lower, say 75% but that's still only half the DEEP CYCLE GC-2
Finally capacities
C/20 means the 20 hour capacity if you discharge the battery so it hits DEAD exactly 20 hours from start, HOW many amps did you use... Times 20
Understand not all companies rate their batteries that way Wal*Mart for example uses the 1 amp rate.. This gives their batteries a much LARGER number when in fact.. they are not any better than others.
Capacities
Group 24 is about 75 AH at the C-20 rate
Groups 27 and 29 are just below and above 100
Group 31 is about 120-130
"
But these are MARINE/deep cycle so you can use 25%
Now. the CG-2 is 210-230 and you can use half of that.
I mentioned earlier the it is the single most popular lead acid battery.
Well that means the production lines do not shut down and re-tool as often so the unit cost is far smaller per unit due to the cost of re-tooling frequently to make different sizes.
Now you have the reason I LIke GC-2 Most BANG for your BUCK.
Home was where I park it. but alas the.
2005 Damon Intruder 377 Alas declared a total loss
after a semi "nicked" it. Still have the radios
Kenwood TS-2000, ICOM ID-5100, ID-51A+2, ID-880 REF030C most times