You'll probably be better off in the long run to buy a true deep cycle battery for the purpose you've indicated. As has been mentioned, you'll need two of those golf batteries to get 12 volts.

B.O.

I haven't bought one in about a year now, but Wal-mart/Sams Club has a 12V deepcycle rated for 115amp hours for about $55 unless things have changed since I bought my last one. These have been doing me fine, I have 3 of them, but I don't have them in constant fulltime use, but I do use and even abuse them alot. So far I like them fine myself. They still work good, but who knows for how long they will live?

As for the 6V golf cart batteries, I don't know which would be better, those or these 12V ones. Probably 2 of the 6V, but then again you have to add 2 at a time.
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nothing to lose

Text lifted (with thanks) off of...
http://www.windsun.com/Batteries/Battery_FAQ.htm

A battery "cycle" is one complete discharge and recharge cycle. It is usually considered to be discharging from 100% to 20%, and then back to 100%. However, there are often ratings for other depth of discharge cycles, the most common ones are 10%, 20%, and 50%. You have to be careful when looking at ratings that list how many cycles a battery is rated for unless it also states how far down it is being discharged. or example, most golf cart batteries are rated for about 550 cycles to 50% discharge - which equates to about 2 years.

How depth of discharge affects cycle life on batteriesBattery life is directly related to how deep the battery is cycled each time. If a battery is discharged to 50% every day, it will last about twice as long as if it is cycled to 80% DOD. If cycled only 10% DOD, it will last about 5 times as long as one cycled to 50%.

Lead acid batteries go to heck if they dont STAY charged too, something about them "sulfating", like i care what it is called, i just realized to late, that letting my car battery drift down was killing it. (i dont drive much for the "environment")
SO
add to the horrible charge/discharge life of a Lead Acid, and tack on the death from staying discharged to it :-(  and prefer that the battery be charged, you know in that fantasy world :-)

now that i keep the LA batteries charged properly i am getting MANY more years out of them, then the former stupid way i dealt with them.

some gell cell info i learned (also the hard way)
overcharging these creatures causes bubbles in the darn gel (electrolyte) and they never leave.  and the further you discharge them (or any LA) the less cycles you get, as bad as 20 cycles for full discharges.
so with gell cells the same thing is true, you want to keep them charged, and the charge control is even more important (bubbles).

then i learned (i did not want to know) that they ship the lead and acid off to 3rd world countries to "recycle" the materials. it was a few years ago, but they showed this village living in its usual squaler conditions, with a acid creek and lead lined children.  it was not a good thing to do to people.

i just thought i would comment, this stuff ALL sucks, NIMHY and LI-ION are much more expencive, but they love to be discharged and charged.
there really is no totally practical way to store human type energies, without caveats.