12v battery gage

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AzRon

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Feb 6, 2008
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Hello all, I have looked thru the library and have done a search and found nothing on how to put a 12v battery gage onn my pup.
My pup is a 2007 Fleetwood Williamsburg and want to put a Battery gage on it, so i know how much juice i have left on my battery, when away from shore power.
Can this be done.  Thanks  AzRon
 
The easiest way to monitor the battery state of charge is a digital voltmeter.  You can either install one permanently or just use a digital multimeter from your favorite store, like Radio Shack.  This article in our library has some information but there are numerous sources for complete charts showing state of charge vs. voltage.
 
Thanks, Tom, I missed that one when I did my search of the library.  Do we have a table of voltage vs. state of charge when under load?
 
Do we have a table of voltage vs. state of charge when under load?
Click to expand...

I don't recall seeing one Ned. Maybe we should twist Gary's or Karl's arm to put one together.

BTW I probably threw you a curve by creating a new library area for Batteries and DC (12V) stuff.
 
Here is the Charge Table for 12V batteries (also applies to 6V batteries in series/parallel for use as a 12v system).  The data is consistent with a similar table on the Trojan Battery web site, so it ought to be good.
 

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AzRon...
Does your rig have a 12v outlet (lighter receptacle) anywhere? If so, the easy way is to buy a 12v meter that plugs into that receptacle. They are often found in auto parts stores, boat shops, fishing tackle stores (used with trolling motors on fishing boats), and some RV stores, etc. JC Whitney sells them mail order too.  I have a digital one which is fairly accurate.
 
That's the same table we have in the library. I think Ned was asking about an equivalent table for a battery under load.
 
That table is the same as in the library article which is voltage under no load.  It's not usually practical to measure voltage when boondocking with no load, so a table of volts vs. state of charge while under load would be more useful.
 
Negative on the 12v outlet. I'm looking at putting a gage somewhere in the trailer, maybe near or on the breaker box.
 
Karl,

You just need someone who's a whiz at complex math and physics to show you how to do it  ;)
 
The TM-500 I use gives me an indication based on the current being drawn, the voltage at that instant and the size of the battery that I have entered.  It immediate readouts are %, Volts, Amps being drawn, and the AH less than full charge.  There are many other available measurements but those are the ones always displayed as I have mine set.

Wally is the expert on this box.  It is a little pricey for a popup though!! 

The combination of a DC amp meter with shunt and a DC Volt meter will do adequately.
 
I think Ned was asking about an equivalent table for a battery under load.
Click to expand...

As Karl says, you would have to assume a specific load to build such a table. On the other hand, amp-hour and reserve capacity ratings assume a mythical steady load, so why not a voltage table under similar conditions?
 
We're looking for a quick measure of the state of charge.  It's not practical to remove all loads before measuring the voltage.  My Link 1000 shows battery voltage while under load and it's a reasonably good measure, good enough to tell when to start charging.  No need to compensate for all the possible loads.  In this specific case, we're talking about a popup, which should have a limited range of load on the battery.
 
OK guys I'm dry camped down in Key West.  Eleven days so far & ten to go.  It was a low of 70 this morning - brrr

Thanks to Lou, Bob & all my 700 w inverter setup is working great.  I have the trailer plugged into the inverter with the converter plugged into the generator.  We use lights, including a 40 watt 110v reading light & a couple of 6 watt 110 night lights, 12v exhaust fan and 110v comfort fan, computer, tv etc. 

I've been using the Charge Wizard & idiot lights.  The idiot lights are C G F and L (Charging, Good, ? and Low?)
I also have a DVM in the bathroom cabinet with the DC panel.  Early on I checked it a lot and logged voltages - now I check it occasionally.  It is consistent with the lights.

The Charge Wizard acts a little differently than the instructions would lead you to believe.

After a good charge and stopping the generator it stays on for a while and the G light is on.

Then slow blink (supposedly this means 12.8 to 11.8 so solid on must mean above 12.8 )

Then it turns on solid again and the idiot light says F.  I interpret this to mean go crank the generator. If the fans run much of the night 'cause it's had to sleep at 80+ degrees with no air movement - then it goes back to green at 3AM so I shut the fans down & wait to be the second person to start their generator.  Without the fans I can go 'til mid day.

Do I really need something like the TM-500 or the TRI-METRIC 2020?
 
Do I really need something like the TM-500 or the TRI-METRIC 2020?
Click to expand...

No. You really only need a figure of merit, not a precise value. After a bit of use you work out a routine (as you have) and know when it is time to run the generator to maintain whatever lifestyle you have chosen.
 
Would a voltmeter from a lawn tractor be close enough?  It may not be the most accurate but at least give some kind of indication.  Would just have to drill about a 1 1/2 hole and run 2 wires to it.
 
Well. Roamer.. I might argue with you on this in some cases.

But in this case what he said.... And frankly you might find the voltmeter more accurate than you think.  Put a good digital of known accuracy up against it when you install.

Something to think about: All readings are relative, accuracy is NOT as important as consistancy

So using a calibrated digital, note the error, and when you think you should again use the calibrated digital, and if you are at the 50% charge point, Mark the dial.  Then your accuracy will be much better
 
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