Bad Battery?

Purchased two GC-2 Interstates back in Sept. of 2013. For a couple months now, my charger (not the onboard converter/charger) has been telling me "bad battery", cannot charge. Thinking the charger is going out, I put it on several other batteries and it does fine. I have been very diligent about keeping these Interstates on a constant charge since I purchased them, never discharging them below 50%. Do I need to check the individual cells? What should they measure?