Deep cycle batteries-----
Number One battery care must: never, ever, leave a battery in any state of discharged. Recharge it as soon as you stop using it.
Number Two battery care must: never, ever, discharge it completely.
If you haven't already screwed it up, it may recover from your lack of proper maintenance.
A voltmeter and/or specific gravity meter are the two items to check you battery's condition.
Read this: http://www.windsun.com/Batteries/Battery_FAQ.htm#Major%20Battery%20Types