I seldom bother to use my car. In fact I am not sure why I have it. So of course the battery gets flat every now and then. Then I mount my spare battery and recharge the flat one. It is a pain to see to it that the one in the car does not get too flat.
Therefore I made this circuit I found on the net and saved a long time ago. I changed it to use a 10-bar LED, since it takes less space than the original with 10 separate leds.
All you need is:
2x pots 5k
LED with 10 bars (I used Kingbight DC-763HWA)
Resistor R1 4k7
Resistor R2 1k2
For adjusting the tester you need a voltmeter and a variable source from 10 volt to 15 volt.
Step 1: The PCB
This is the PCB I made for it.
As you can see, I have cut one leg off from the right pot
If you can not make your own drawing, here is also my file, which opens in ExpressPCB (free).
Step 2: Adjusting
After you have built the circuit you need to adjust it.
Put 10.5 volts into it and adjust the right pot until first bar just lights up.
Put 15 volts into it and adjust so that the last bar lights up.
Only one bar will light at one time.
Mount it to suit your needs.
Now you have a 10 bar indicator which show 10.5 volt to 15 volt in 0.5 volt steps.