Battery Voltage Monitor





Introduction: Battery Voltage Monitor

This voltage monitor has been designed to remotely display battery voltage so that people monitor and maintain their batteries easily. It was designed for use on off-grid reneable energy systems (such as my solar PV system), but it would also be useful to anyone who has a lead-acid battery, such as caravans, boats and cars.
It is designed to plug in to a car 'cigar lighter' socket and display the voltage on a remote LED display. Here are the plans to make your own.

Step 1: Parts and Tools Required

Parts required
  • 3mm plywood sheet (around 1/3 sheet of A4)
  • Wood glue
  • 2 x M3 machine screws (around 12mm long)
  • 2 x M3 nuts
  • 4-digit LED display
    • I used this one from SunTekStore (but available at other places including eBay):
  • On/off switch
    • I used this one from SunTekStore (but any switch would work):
  • 2m of twin (red/black) cable
  • 'Cigar ligher' type plug
Tools required
  • Access to a laser cutter - such as at your local hackspace
  • Soldering iron and solder
  • Side cutters or wire strippers
  • Posidrive screwdriver

Step 2: Cut the Enclosure

For this step you will need access to a laser cutter. I use the one at my local hackspace in Nottingham (
The .dxf file is attached here. It is also attached as a .pdf for viewing. You are cutting 5 pieces (rather than the 4 shown in the photo) which will all stack on top of each other to create the enclosure.
Please feel free to adapt this as you would like.
(Edit 15/11/12: New version of the .dxf files has been uploaded)

Step 3: Glue the Enclosure

The enclosure is made from a stack of laser cut pieces. The bottom three pieces are glued together and this holds the two M3 nuts in place. The top two pieces are held in place with the two M3 machine screws. This means you can open the enclosure again, for any reason.

Step 4: Solder the Display and Switch

The next step is to feed the cable through the small hole in the enclosure.
Also push the small slider switch into the smaller hole in the front of the enclosure.
Strip the cable back around 5mm. Remove the black cable on the LED display and solder the black wire (of the red/black cable) to the same point on the LED display.
The red wire of the LED display is the power supply and the green wire is the voltage monitoring wire - in this case they are the same thing, so we connect them together and wire them to the switch.
Wire the red cable of the red/black cable to the switch and solder.

Step 5: Fit in the LED Display

The wires and switch are soldered. We now need to use the last two laser cut pieces to close up the LED display unit.
Apply some hot melt glue to hold the display into place. Ensure not too much glue is used as it could stop the unit from closing correctly.
Use the M3 machine screws to clamp the unit closed.

Step 6: Solder the 12V Plug

Open the plug by removing the black screw and the front metal connector (this unscrews and then the metal disk pulls off)
Push the red/black cable through the cable grip. This is hard work as the hole is only just the right size. Use some long nose pliers to pull the cable through.
Put the red wire through the spring and solder on the connector.
Solder the black wire to one of the side connectors.
Screw together the plug and put back the metal disk and fuse and end connector.

Step 7: Plug in and Monitor Your Voltages

You are now finished - easy eh?

Now you can remotely monitor your battery voltage. Please have a look at these articles for more detail on how battery voltage relates to state of charge of your battery:
I also can supply the parts for this project (within the UK) in a kit available here, if anyone is interested:



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U inspired me to build an voltige indicator on my rc car, thanks for shearing! :D

14, 00:13.jpg

I really like this that you made!!

I am very new to the solar power type things and wanted a voltage display to show what my little 12v battery was at in the process of charging or what was left on the actual battery... I like that you added the on/off switch. In this case when you turn the switch on it would display whatever source or power you have it connected to right? Then flick the switch OFF and it's not draining or using any power right?

Just want to make sure I understand it first before doing it.

Thanks again,

Hi Nigel,,
Thats exactly right - I added the switch so that it does not drain the battery (the system I had it added to was very small). There are three wires from the voltmeter that I bought: There are red and black wires which are for the power supply to the unit and there is also a green wire, which is the voltage sense wire. This can go to wahtever you are trying to measure. In the case of this insrtuctable I wire both the red and the green together so that the power supply and the voltage sense are the same thing, so you can just plug it in and read the voltage.
You could also use a push switch, rather than a slide switch. This would mean you never leave the display switched on.
Let me know how it goes,


Nice & easy to build :)
there may be other application's to it than the car battery(computer up's,electric vehicles , in inverter's etc. etc.)

sadly i can not get the 4 digit led display voltmeter near me :( ,will have to search around a bit

XxHDxX, unfortunately you have not mentioned your geographical location. But I am assuming that you are in United States. If that is the case, then you'd find the exact unit with AMAZON costing about $7 with free shipping.
Copy the below link into AMAZON search bar and you'd get it. Cheers.
0.36inch 4 Digit Digital Panel Voltmeter 0-33V Red LED Voltmeter

Sreeci i appriciate your help,if you would have scrolled down a bit you would have seen the reply where i mentioned that i live in India,sadly buying online would multiply the cost of this unit,so am trying to find it locally
Have fun

Thanks for the comments - you are right, this could be used on any supply within the voltmeter range (which is 5-30V), so on 24V supplies.

Not sure where you are based, but a seach on eBay brings up quite a few in different places (mainly China):

You might have to change the .dxf files to fit a different sized LED display. Hope that helps.
Cheers, Matt

just saw that you you have this instructable in the design competition,so took no time to vote for it :)