I made friends with the guy who came out to service the REALLY BIG power backup (UPS) unit at work. He told me that his company replaces a battery after three years, just in case. He gave me one for free. It is 100 Ah, and I have it connected to a small solar panel. Works great. If you do get a used battery, be sure to check the date code (usually stamped into the top of the battery) and do a simple load test (attach an old headlight, and check for voltage drop over time). These batteries are also usually available at ham radio/electronic flea markets for $20 or so.