Green light

I have a two seater GEM car, which I bought used in 2008. The Gel batteries we’re replace 18 months ago with red 12 volt acid batteries. I watch the water closely, and just topped off a few days ago. After charging all night, the car was pooped out, and only went 10 mph or less. I now have two yellow lights and a blinking green light. Underneath when you look under from left tire. Before, when charging, I only had 2 solid yellow lights. All batteries are 13 V or over, except on which reads 11.8 V. Any suggestions? (Total output voltage is 83 volts, so why is my Gem car in Turtle mode?). Should I buy hydrometer?