Here is a simple gadget that tells you exactly what your voltage is. Better than
those RV or Boat battery meters that light up a bar you now have a digital LED
readout. This works on 12 or 24 volt systems and is a great addition to the
portable solar system especially if you are using the car starting battery for
power with the solar kit in am expedition vehicle. This volt meter IS INCLUDED in
all solar kits!
See it here