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Catalina/Capri 25/250 Sailor's Forums

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cheap battery monitor

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Arlyn Stewart
Master Marine Consultant

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USA
2984 Posts

Initially Posted - 03/02/2003 : 20:02:19

Cheap ways of monitoring the battery have often turned to the Harbor Freight digital volt meters. However...buying a battery for one is almost as much as the meter and if the meter is left on... then a few days later the battery is gone.

I investigated the possibility of supplying the meter battery from the test load. While the meter will function very well on 12 volts instead of 9v, the display locks up if trying to use the tested voltage to power the meter. It was a good idea that didn't work.

So, its back to Harbor Freight VOM or one of the relatively more expensive products available....or is it?

Today, I replaced the led in the Marinco plug on my solar panel that quickly blew out a few years ago when I installed it on my panel. As I expected, there was no resistor in line to protect the led from the much higher than 12 volts the panel puts out when not hooked to the battery.

So, I placed a 470 ohm resister in line...and then I decided to add a 12v zenier diode in line as well so that the led didn't turn on under 12 volts.

The result is that at 12.3 volts which is nominal for a wet battery, the zenier has turned on just enough to provide low intensity on the led. At 12.8 volts...the led glows bright. Below 12v, the zenier stops triggering and there is no light and the led won't drain the battery further.

So... if wanting a very cheap and easy battery monitor...its a good way to go as it requires very little space to drill a small hole and insert an led into the power panel.

The parts are at radio shack and probably not much more than a single 9v battery... of which I bet I have replaced 10 of in the last 5 years and which are always dead right when I want some idea of what is going on with the batteries.

Not that I will throw the vom away...but I won't have to turn it on very often anymore and if its not on...I can't forget to turn it off.

Wiring is simple...the long leg of the led goes to + as does the black bar of the 12v zenier. The 470 ohm resister makes no difference. I used a clear led, if using a dark led, use a 220 ohm resister or so. Wire all three in series, the order is not important.

<img src="http://www.stewartfam.net/arlyn/led.jpg" border=0>

Arlyn C-250 W/B #224
<img src="http://www.stewartfam.net/arlyn/rr.jpg" border=0>
N/E Texas and Great Lakes

[url="http://www.cox-internet.com/arlynstewart/"]Arlyn's C250 Mods n Cruisin Stories[/url]

Edited by - arlyn stewart on 03/02/2003 21:16:49

Edited by - arlyn stewart on 03/02/2003 21:29:43

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CatalinaGuy
1st Mate

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USA
55 Posts

Response Posted - 03/02/2003 : 21:03:50

Great tip! I'm going to add this to my boat ASAP. Thanks Arlyn.

Doug Hibberd, 'Shiraz'
C25 SK/SR 1983
Austin, TX

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Arlyn Stewart
Master Marine Consultant

Members Avatar

USA
2984 Posts

Response Posted - 03/09/2003 : 08:34:09

Ooooops <img src=icon_smile_sad.gif border=0 align=middle>, this appears not to work.

I was preparing to install a permanent version in my power panel yesterday, and when pretesting, it didn't work. The turn on voltage was 13.4 (low light) and 14.2 before the light got bright. These voltages are not helpful.

I'm not sure what happened with my initial installation in the Marinco plug. I've got several meters and may have used the Harbor Freight one and got bad numbers or simply misread what I thought I was seeing or the zenier I got from my parts supply may have been out of tollerance. An 11 volt Zenier might provide numbers very close to those wanted and might be in fact what I put in the Marinco plug...but I'm not sure. Sorry if I have caused anyone to build this and have it not work.

There is a way to use an led as a voltage meter and be able to adjust it for the desired voltage needing to be indictated. That circuit is called a comparator. While not very complicated, it does use an IC.

If any one wants that circuit...email me

Arlyn C-250 W/B #224
<img src="http://www.stewartfam.net/arlyn/rr.jpg" border=0>
N/E Texas and Great Lakes

[url="http://www.cox-internet.com/arlynstewart/"]Arlyn's C250 Mods n Cruisin Stories[/url]

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