The remaining voltage readings for either internal batteries (right reading) or external batteries such as Solar or external connected batteries (left reading) are written to every image such as in the example below.

Power Reading

What voltage should should I replace the batteries?

Internal AA

10v - You should make a plan to replace the batteries.

External Batteries

Lead Acid - 12v -  This is approximately %50 consumed, any further you'll damage the battery.

Li-ion -  9.4v - The battery management system will protect the battery down to this voltage.

Solar

The normal operating voltage for the Solar panel is between 12v and 12.5v, if the voltage is consistently lower than this then the panel is not getting enough sun.