I use a "desulphator" device to extend the life of old SLA/AGM batteries. It provides a very slow trickle charge on the average - but in short pulses that are high-current. Seems to help, if kept running for months. Great for the old batteries I use for running LED lamps at home in case of a power outage. But I wouldn't use a battery that is that old and tired in my glider. A new SLA is cheap and is good for several years of flying use, as long as you only discharge it halfway, recharge right away (slowly) with a good smart charger, and never store it where it is hot (as in a glider trailer).