If you do a search in Ebay with keywords like: <LCD Voltage Current Watt Power Capacity Digital Meter> you get several possibilities like the ones in the following pictures:
They are very low cost battery monitors. I mounted one of this in Margarida and I am quite happy with it. Together with the solar panel I keep the batteries under control and never left them to go into a lower state of charge as it happens once in July of 2013. We were anchored in Croatia in a small island and, when arriving to the boat after a visit by ferry to Dubrovnik, we heard the alarm sound from the built in monitor. It was showing 10,5V for the service battery bank! As a result we had to replace the batteries, two weeks later, in Brindisi. So I installed one of these monitors following this diagram.
The first and the second lines of the display show the voltage of the battery and the current coming out or going into the battery. They take these measures several times in a second and so they also keep a record of the charge that remains in the battery. Charge is measured in Coulombs so they are also known as Coulomb meters. The third line shows the charge remaining in the battery. When the battery is fully charged we set the display to show 100% or 280AH (that is the value of my battery bank). The way I use it to look to the third line and make sure that it does not go lower than 70%. When I see 70% in the monitor, I switch on the engine and run it until the value increases up to 90% or more. These normally happens in non sunshine days when I am anchored for several days. The final aspect of the system is shown here:
I had some problems in calibrating the system but was able to accomplish that with the help of the module's vendor. More complete information about this project can be found in CruisersForum and in Taberna del Puerto (spanish) where I post my experiment.