Moe, go to radio shack and buy #22-112 $5.99
vehicle voltage tester/uses LEDs page 238 2002 catalog
remove the test probe and alligator clip install an on/off switch and say an inline 1/2 amp fuse to be safe and connect to the battery or even your acc terminal in the car's ignition switch that way it will turn off if you forget.
it has 6 leds to tell you battery condition and various charging conditions.
It's small fairly accurate, cheap the auto supply store sells this for $10.00+
it has a magnet built in so it can stick to your dash or do double stick tape or velcro..whatever
I use this in my vehicle and even on the motorcycle as a voltage monitor
regards, Marty email@example.com