researched generator battery

Did some generator research after downloading the manual (again). Tony and I were curious how battery warning worked. I learned the Control Panel does take a reading. Voltage below 11.0 volts for a minute will trigger the warning. I also tried to determine when we last changed the lead acid battery. I know we cooked the first battery from October 2010 by running the blanket continuously. But I couldn't find notes about a subsequent change. That'd be impressive if we've received 4 years of life...