Building a voltmeter to monitor deep cycle batteries

I mentioned in previous posts that we have a popup camper.  We do as much camping as possible and the entire family really likes it.  Popup camper owners are known as being almost obsessed with making modifications to their campers, and I’m no exception.  This post is about one of these modifications.

Lately, we have been camping without electrical hook ups, so I need to keep a better eye on our electrical usage.  Since our battery is in need of replacement, I want to start the new battery’s life off right by properly maintaining it.

I always knew about this great site about 12 volt batteries, The 12 volt Side of Life.  The site is great for campers, because it has so much information about maintaining and using 12V batteries.  It may also be just as useful to homesteaders that want to generate and store some of their own electricity.

I decided that a voltmeter was what I wanted to do in the camper.  This will make it easy for me to keep an eye on the state of charge of my battery.  The 12V Side of Life site showed how to go about making a voltmeter, so I went this route.

Parts for the voltmeter

I did some additional research and decided that I could save a couple of bucks and have a custom build if I built the meter myself.  I ended up buying the parts from a company called Qkits.

I bought a PMLCD panel meter, the precision resisters to “program” the meter and a 9V battery connector.  I will deal with Qkits again if I need any other electronic parts.  I placed my order and then discovered that I ordered something wrong.  I contacted them via e-mail and got it all straightened out in a couple of minutes.   They have good customer service.

Voltmeter circuit board

The instructions for customizing the meter are pretty good.  The meter comes set up to measure 200mV but can be customized to measure 20V, 200V or 500V.  Actually, it can be configured to measure any voltage by using a voltage divider circuit.  The decimal point can also be set by soldering in some jumper wires.  Just above and left of the center is a potentiometer to calibrate the meter.

Voltmeter customized

Here’s a shot of the customized circuit board.  The 2 resisters at the top of the board are the voltage divider, these allow me to measure up to 20Vdc.  On the upper right corner are the jumper wires that set the decimal point and there’s a jumper that turns the negative sign on or off.  I set the jumpers to have 2 decimal places and to show me the negative sign if the voltage is negative.  Of course since I’m measuring my battery, it should never show the negative sign.

Testing the voltmeter

It took about 30 minutes to solder in the resisters and jumpers.  I also soldered in the 9V connector and the probe wires.  I hooked it up to a 9V battery and it was off by about 0.1Vdc when compared to my digital multimeter.  So I adjusted the potentiometer and now it is off by 0.01Vdc, I think that’s close enough for my usage.

I still need to mount the voltmeter onto a panel so it looks good in the camper.  This panel will also hold the water level indicator for the under floor fresh water tank.  I will build that in the next week or so.  Once it warms up so I can open up the camper, I will wire the voltmeter into the camper and install the panel.  I’ll make another post when I build that panel.