There's somewhere on the net , that goes through the different batteries type....car ..marine..forklift..etc. Car batteries , when used for "off grid" where there charged over and over again, don't do very well. I think the number is round 40-50 recharges before there NFG. The plates in a car battery are very thin, and will not take very well to deep cycling. A marine battery, like the ones you'd uses on a trolling motor, have the plates inside that are a little thicker, making...more usable as an off grid battery.
Car batteries are designed to hit hard/high short loads ( your car starting) and charged fast, for short periods of time. Ever tried to start you car and it just wouldn't start.....if you turn it over and over again , it doesn't take long to kill your battery. And if you do it enough times ...the plates in the battery will, warp...over heat and you'll need a new battery. Car batteries are a little different on the way they rate them, amps, cold crating amps and reserve amps. The amp rating is how much the battery can put out for a very short time ( about 5-10 a temps ) at trying to start your car, with out damaging your battery. Cold cranking amps is an amp rating @ a given temp (- something) , in very cold temps, a car battery or any battery with only have about 40-50% of it total amps available. And reserve amps (the one you want to look at) is the amps available after you've tried to start your car over and over again, and leave it for a while( the battery cools down) is really what you have for amps/deep cycle.
If the batteries are free, then have some fun!! It better to try something, then to never have done it before, because you listen (to me) and maybe other that say done do it. It will work for a short time...week... a month...who knows, but its a good starting point, where you can learn , with out spending a lot.
Right now, my system...inverter (48 volts) will provide 4400watts of 120 or 2200watt of 220, I have 8 solar panel @ 325 watt per panel (2600 watts) 8x 6 volt Interstate L-16dual batteries rated at 380 amps@20 hours( these batteries weigh 110 pounds per battery, very heavy!! MPPT PVA charger and wiring...disconnect boxes fuses...etc.
Altogether I think I have about $14000.00, not including the Lister SR2 with a 10Kw ST head.
But I make all my power for my home...no grid tie in here....will there is, just down the road, but I don't want anything to do with it!! :-) I like the way it works, there is maintenance,but very little. And if the light go out, down call the hydro hot line...it all you.
Cheers
Brian