I use this one from AMAZON for around $10... It pivots and has a small graph scale on the edge and also has three lights that tell you your approximate charge status...
I went for a long time with my truck without being able to monitor the 12VDC battery in it...
This sure has helped me in this regard
INNOVA 3721 Battery and Charging System Monitor
google image
My 12V socket is in reach so when I stop my truck and shut it down I reach over to the plug-in Voltmeter and slight pull it out of the socket where the display goes off. Then when Iget back into the truck to use this is the first thing I do is push the DC Voltmeter back into the socket before I start the engine...
Roy Ken
