Homemade Battery Monitor

From Honda Trail Bike Wiki

Working page with details of a Battery charge monitor, a circuit for which has been designed by a very nice chap called John Fields on the usenet group alt.binaries.schematics.electronic.

The design is included below. I do intend to build this when time permits, and report back as it if and how well it works. The component cost is very low, and should outperform similar devices that retail for £40ish.

If anyone beats me to building one of these, please post the results to this wiki page.

Well, here's a built version of the battery monitor -

There are a few of problems with the circuit diagram above -

1) The main one being that there are three resistors with the designation "R2"! All have different values - 2000 Ohms, 1500 Ohms and 649 Ohms.

2) Also, the resistor values shown are from the E24 series and are not too common. I have re-calculated the resistor values for the more common E12 series (easily available from, say, Maplin's).

3) There is no protection against accidentally connecting the supply the wrong way round. This would result in a very dead battery monitor. In the modified circuit I have added a diode to prevent any damage should this happen.

4) The circuit as it stands, above, would not work as expected. In particular the "High" condition LED would never light. It needs to be connected to pin 12 of U2 (74HC138) and NOT pin 13.

Proper circuit diagram as well as a much smaller built version to follow. (Lutin)

Before I forget -

Red LED denotes voltage "High", ie >= 14 volts.

Green LED denotes voltage "Okay, ie >10 volts and <14 volts.

Orange LED denotes voltage "Low", ie <10 volts.

Updated circuit diagram -

And here's the completed smaller version -

The circuit board is 66 x 38 mm, or 2 5/8ths by 1 1/2 inches. The coins give some idea of the size.

The hole in the middle of the board is deliberate as it is needed to allow fitting into the small box that I found in Maplin's -

If anyone is interested, I can provide a drawing of how to prepare the strip board for making up the circuit. This one ended up being modified as I went along and is therefore not as neat as it could be.

The completed gizmo looks like this -

I have priced up the components from Maplin's (date of price check Oct 2008) and it comes to around £13 which includes the box. I also checked another retailer called ESR (www.esr.co.uk) and their prices come to about £7.00 including delivery but without the box. I think the box is priced at £1.59 in Maplin's.

I can provide a full list of parts, and any other details, should anyone be interested.

To make things even neater, I have created a design for a PCB for the circuit that will fit into the Maplin box. Here's a 3D view of what it would look like -