Custom building a battery monitor - voltage display unit.

Project date: 2010

This is one of the more detailed projects you'll find on As it is quite involved, I thought I would show a project from beginning to end. This project can be built as a stand-alone monitor, or included in a larger project, which I will do for my NMEA2000 network. Regardless of your application, the monitor shown here is constructed the same.

There are two concepts to determine when deciding on just what to monitor. The first consideration is battery voltage, as shown in the chart below. Note that the battery voltage between fully charged and fully discharged is less than a Volt (wet cell), so some precision is requied for the battery monitor.



State of ChargeWet CellGell CellAGM

The second concept is the notion of a 3% voltage drop, which is a useful measurement as the rule-of-thumb for most electronic circuts states that the voltage should never drop below 3% of the rated voltage for proper operation of electronic equipment. This voltage drop is primarily a function of the wire loss, but the battery voltage cannot be ruled out either.

So where do we put the 3% point at? I like to use 11.64V as the worst-case scenario, which is a 3% loss from 12.0 VDC. If we refer to the battery voltage chart, this relates to a battery having a 25% remaining charge; so in reality, our target voltage of 11.64 means we expect proper operation of electronic devices having a 3% power line loss with a battery having 25% charge remaining. I think this is a good scenario, so we'll include a means of displaying this voltage on our battery/voltage monitor.

To keep the project cost below, as well as provide an easy to view reference, we'll use a LED bar graph to show the voltages. And we'll use different color LEDs to show the voltage, with a Red-Yellow-Green display. And finally, the Red-Yellow transition will be the 3% (11.64V) mark.

To make this project easy, I found two commercially available battery monitor circuits; one a complete electronic kit, and the other a circuit board that requires you to purchaes all of the parts yourself. I'll construct both kits, and provide a comparison between them. However note that both kits are suitable for a battery/voltage monitor, and which one you decide on depends on factors I'll detail later.


On the left is the VM-2 kit by Rainbow Electronics, and retails for about $8.00. On the right is a circuit board from Onstate Electronics and retails for about $2.50. The parts for this monitor is probably around the $12.00 mark. So even though the Onstate monitor is more expensive, neither monitor is going to break the bank. The major difference between the kits is how the LEDs are driven, but in reality the only thing you are going to care about is the Rainbow kit has a 7 LED display while the Onstate board has a 10 LED display. This offers a slightly better resolution, but again, either kit is going to suffice in most cases. The other advantage to the Onstate version is you can adjust both the high and low voltage set points, while the Rainbow kit is less adjustable.

Of course, the advantages of the Rainbow kit are that you have all of the parts in the kit, and the cost is a bit less. If you have difficulty, you can send the Rainbow kit into the manufacturer, and they will troubleshoot the kit for you. For the Onstate version, you are on your own. However, having fewer components, there is not as much to go wrong with the Onstate board.

The Rainbow kit is available on-line from several retailers, including the manufacturer. Onstate Electronics maintains an eBay store, and you can purchase the circuit board from there. I have to state that the Onstate circuit board is very high quality, and its an incredible buy for $2.50. The Onstate instructions include a parts list, and you can purchase all of the parts from, a popular mail-order electronics parts house. The Onstate kit even provides the Digikey part numbers.

I made a slight modification to both versions in the color of LEDs I used, as I'll explain later.

This chart shows the approximate voltage that each LED turns on for each monitor. Note that for the Rainbow monitor, the LEDs turn on in about 0.25V steps, while the Onstate monitor is closer to 0.2V. When you build the Rainbow kit, you have a choice of 1V, 0.5V, or 0.25V steps, determined by the selection of the proper resistor to install, while the Onstate version is set by the relative settings of the low and high point potentiometers (adjustable resistors). Again, in both versions, the exact turn-on voltage of each LED can be varied by an adjustment.

You can see that I marked the 3% point (11.64V) so that it turns on at the RED-YELLOW LED transition, however, the Rainbow monitor will be the last RED LED while the Onstate monitor is the first YELLOW LED. This is a function of the 10 LED vs. 7 LED resolution between the two kits. Also the Rainbow monitor's full scale indication is lower than the Onstate monitor. But as I indicated, you can adjust either monitor so that any voltage points can be measured within the 12V range. I chose the settings as the 11.64V point is more important to me than either the maximum or minimum battery voltage.

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