Colin's Astronomy Pages

Power box
Battery Monitor circuit

Contents

As this is quite a large project, I've decided to split this project into several pages. Each page dealing with a different aspect of the project.

Overview

The Autostar unit provides a battery level meter that's accessable via the mode button (press and hold). This is all very well, but I've found that it's not sensetive to detect the levels on a 12v battery, until it's almost completely discharged. To that end I wanted to add a circuit to my control box, that allows me to instantly see what the battery level is.

Circuit diagram

Here's the circuit diagram for the battery monitor.

I can't take credit for the basic design of the circuit. As I got the design from another website. I did make a small modification to the original circuit to correct the dot more quirk, that everyone seems to miss.

Final adjustsments are simple and the only thing needed is a digital voltmeter for the necessary accuracy.
Connect an input voltage of 12.65 volt between the positive and negative poles and adjust the 10K trimmer potentiometer until Led 10 lights up. Lower the voltage and in sequence all other Led's will light up. Check that Led 1 lights up at approximately 11.89 volts.

At 12.65 volt and higher the battery is fully charged, and at 11.89 is considered 'empty'.
The green Led's indicate that the battery capacity is more than 50%, the yellow Led's indicate a capacity of 30% - 50% and the red Led's less that 30%. This circuit, with the components shown, uses less than 10mA.

Again I laid out the components to fit nicely onto a Size 1 PCB (100mm*75mm), This was so that I could stack it with the dew heaters bar graph. Here's the layout.

Note: Since creating this project, I've found that there's a couple of issues.
First, deep cycle batteries can be run down to about 10.5V, Since I use an 85Ah battery, I re-calibrated my circuit to show fully discharged at this voltage. As this is used for an indication more than a mark to recharge the battery at, I'm not worried if this voltage is close to the absolute minimum that the battery should be allowed to read.
If I were to remake this circuit, I'd spend a little more time altering the track layouts on this and the Dew heater monitor circuit, so that I could drill a hole in the Dew heater circuit to allow a screwdriver to gain direct access to the trimming preset. This would help to make the calibration a little easier, especially after I upgraded the box with the extra sockets. The sockets also have an LED to indicate that they are connected.

Power box Battery Monitor circuit

Power box
Battery Monitor circuit