My indicator only works after charging has stopped for several hours. What it shows is entirely based on the voltage present at the battery so when charging it shows full, and continues to show full for quite a while after charging stops because it takes a while for the surface charge to settle into the battery. When power is being drawn from the batteries, the voltage drops and the indicator will read too low. Still, I find the battery indicator quite useful in contrast to the tank level indicators which are not.
If you have two batteries, they will normally be connected together, in series if they are 6V and in parallel if 12V. Likely there will be a date of manufacture on the top or side of the battery. If carefully used, they can last up to 7 years but discharging below half shortens the life and 2 or 3 years is common. The first sign of old age is the battery being unable to power things as long as it used to.
