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Old 09-10-2012, 04:32 PM   #1
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Solved: Inexpensive, accurate battery monitor

Below see a picture of a DC voltage display and matching switch that I put in this weekend at the same time as a solar panel installation project.

Total cost for parts was $10.

Power is coming from the 12V feed to the refrigerator (which is just behind this wall) but could come from any 12V circuit in your trailer. Now I can always see exactly how the batteries are doing. Whole setup uses almost no power - all lighting is LED - and I like the ability to switch it off.

The meter is far more accurate than the simple monitor built into my airstream - I measured with a fancy fluke digital multi-meter and the little blue meter accurate to better than 1%.

I've used several of the blue LED switches in my trailer - like them a lot. The LED only illuminates when the switch is on, which makes it hard to accidentally leave things on sucking your battery.

DC Voltage display costs $7.79 from Amazon
Amazon.com: LCD Digital Volt Voltage Panel Meter Voltmeter 7.5V-20V: Electronics

Matching switch costs $2.50 from Parts Express
SPST Automotive Round Rocker Switch w/Blue LED 12V 060-770

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Old 09-10-2012, 04:42 PM   #2
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Clever! And it looks good, too!
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Old 09-10-2012, 04:59 PM   #3
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Jealous of your rocker switch

Very clever indeed... I must agree, since I bought the very same digital voltage meter! I have to admit that I'm jealous of your lighted rocker, you found a much sexier switch for yours.

I spent a little more overall because I'm building mine in a project box... I didn't want to put a hole in a bulkhead for it, I'm going to attach it with adhesive (at least initially) above one of the reading lights on the street-side bulkhead that walls off the bathroom. I'm going to tap off the light for the voltage reading as long as having the light on doesn't affect the reading too much. If I have to run a line for the voltage I'll tap off the fuse block, which is in the closet on the other side of that bulkhead.

I set mine up with a toggle switch for constant-on, and a momentary button to take a reading on the fly so I can switch it off to conserve those few milliamps when I'm on battery power, and have it on to monitor the converter voltage when on shore power. I put a fuse inside the project box as well, just for extra overkill.

I haven't installed it yet, so the display is dark, but it's clearly the same display, I recognized it instantly. Nice choice! The matching blue light on your toggle is just perfect, I may have to upgrade to that switch later.

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Old 09-10-2012, 05:11 PM   #4
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The meter uses almost no power -maybe a couple of milliwatts. No chance it will affect your lighting in any way.

Funny you picked the same one - I was looking for something with two digits of precision to the right of the decimal point and this one jumped out at me. I was very pleasantly surprised that it was accurate too...
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Old 09-10-2012, 05:34 PM   #5
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Nice work.
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Old 09-10-2012, 05:38 PM   #6
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Originally Posted by ddruker View Post
The meter uses almost no power -maybe a couple of milliwatts. No chance it will affect your lighting in any way.

Funny you picked the same one - I was looking for something with two digits of precision to the right of the decimal point and this one jumped out at me. I was very pleasantly surprised that it was accurate too...
I am more concerned that having the light on might affect the local reading more than the overall battery voltage, since it's a 3-lamp 1156 fixture with a relatively small supply wire.

And I agree with you that the likelihood that I'd ever drain the battery by any significant amount with the meter is tiny, I just figured I was less likely to turn it on and leave it on when storing the trailer (for example) if I had a button for momentary readings. Nerd that I am, I already thought about putting a line for turning this off on my departure checklist, and I haven't even installed it yet.
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Old 09-10-2012, 06:42 PM   #7
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Very nice. Thanks for the parts links, too.

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Old 09-20-2012, 10:53 PM   #8
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So, this DC volt display.....does it show the dc voltage running through that one circuit only (so I'd need one display for each circuit)? Could I hook it up on the wires between the battery and the inverter to show what the batteries are doing?
Love that switch too...
I am doing a full reno of a 57 Overlander and am just at the wiring stage...looking for ideas...

Mark
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Old 09-20-2012, 11:21 PM   #9
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So, this DC volt display.....does it show the dc voltage running through that one circuit only (so I'd need one display for each circuit)? Could I hook it up on the wires between the battery and the inverter to show what the batteries are doing?
Love that switch too...
I am doing a full reno of a 57 Overlander and am just at the wiring stage...looking for ideas...

Mark
As long as the voltage isn't measured after the load in the circuit, all the circuits should be about the same. There'll be some small variance based on the resistance of wires of different length, etc. If you want the definitive reading you could run a wire back to the main fuse block that has the main lines from the battery attached.
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