Gem battery dashboard question

Hello, i have an 2006 GEM E4 and the batteries are a little old.

I keep them all up top Volts, i took it out for a long drive today and after about 6mi i still had 4 green bars. I stopped for a STOP sign and then when i took off again the turtle came up and GEM was slowing.

I thought the turtle came on at the yellow bar not when there are 4 bars?

when i got home, and turned off the key and back on again, i got the error code 15( lower volts, need to charge the bateries).
They charged up to all the green bars again.

My question is that i usually check the green bars to see how to turn back to the home, but now i’m a little confused. Does this happen when the batteries are dying?
Why doesn’t the green bars go down to the yellow bar if the Volts are way down?

thanks
gary

I recommend that you load test the batteries. Voltage tests alone can be deceiving. As a large battery starts to wear out, it begins to behave much like a smaller battery. The voltage will drop off much sooner. This effect causes the electronics to show the battery discharging rapidly.

Daniel