Six months if abused could indeed see a battery almost at the end of its service life (I have seen many fail after only a month or two when severely abused by people who were massively overdischarging them (and yours is a SLA- they have charging current limits that can see them failing quickly if not followed as well)- most gel/SLA batteries have an initial maximum charging current (usually less than 30A for a 100Ah rated one) which limits the wattage of panels you can handle unless you have a quality charge controller that limits inrush current on SLA settings... and often have a relatively low output current (which limits the inverter size they can handle)