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Old 01-13-2017, 07:26 PM   #1
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Cheap 12 V outlet voltage meters

Are those cheap voltage meters that plug into a 12V outlet (cigarette lighter) any good?

The ice storm blowing through Missouri got me thinking that it would be helpful to know the actual voltage, in case of an campground outage, so you don't rely on the 'idiot' lights to decide when to run the generator. I went through a 3 day ice storm outage a couple of years ago and I'm sure I ran the generator way too much but I was 'driving blind'.

I'm not a boondocker, so I question if it would make sense for me to invest and wire a more expensive one, and only use it every couple years, if the cheapo will serve the purpose.
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Old 01-13-2017, 07:37 PM   #2
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Well, I have one with the digital readout and 3 light (red yellow and green) and it works well and has for years (2 campers now). I've checked the reading against my Fluke Digital voltmeter and it's real close all the time...

I think it was about 8 bucks online and plugs into a cigarette lighter socket.

Works for me.
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Old 01-13-2017, 07:40 PM   #3
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You can get a free multimeter from Harbor Freight, way better than those cheap ones you're asking about.
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Old 01-13-2017, 07:48 PM   #4
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For the convenience and simplicity, YES those little cig lighter socket voltmeters are great.
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Old 01-13-2017, 07:56 PM   #5
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You can get a free multimeter from Harbor Freight, way better than those cheap ones you're asking about.
So, you leave the leads stuck in something all the time? I'm not following your thought train at all....

My cheap one works fine and it's illuminated too. Plugged in right now, in the barn. I checked it an hour ago when I watered the cattle... 13.7V.
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Old 01-13-2017, 08:25 PM   #6
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Thanks to all for your helpful replies, looks like it will fill the bill for a low tech user like me. I'm going to spend my (gulp) $10 and give it a try.
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Old 01-13-2017, 10:08 PM   #7
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So, you leave the leads stuck in something all the time? I'm not following your thought train at all.....
Sorry, thought this device was for testing voltage temporarily.
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Old 01-14-2017, 03:10 PM   #8
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Originally Posted by TLAK View Post
Thanks to all for your helpful replies, looks like it will fill the bill for a low tech user like me. I'm going to spend my (gulp) $10 and give it a try.
I cross-checked my 12V plug-in with my digital multi-meter and got the same readings. You're good-to-go.
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Old 01-14-2017, 03:56 PM   #9
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keep in mind what you read will be lower than actual when discharging and higher than actual when charging. But you will get a good feel for what you have with these plug in testers. Your batteries will love you don't let them go below 11.9 -12.0... then fully charge them.
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Old 01-14-2017, 05:09 PM   #10
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Voltage charts for charging are based on STATIC unconnected batteries that have neither been charged or discharged in 24 hours.
The voltage reported on ANY voltmeter in a unit in operation is innaccurate...especially if it has been plugged in and the park suddently goes dark. You're gonna see readings in the 13+ Volt range which is WAY over "full" which is 12.7 static.
12.1 static is 50% full and where you shouuld start a full recharge BUT ...if you have stuff running in the coach...you will see 12.1 ...possibly when you are nearly full.
A true battery monitor is what is needed for accuracy...rather than a voltmeter ...but if you don't boondock...it is overkill...if you do...it is needed or you'll pay for it in batteries and generator time.
Anyway...I don't see any value in a voltmeter...portable or plug in... to measure batteries installed and active in an RV. More valuable would be a turkey baster type hydrometer so you can measure cells and charge more accurately...and detect bad ones. 10 bucks on Amazon. Also handy for adding distilled water to the cells when they need it.
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Old 01-14-2017, 05:11 PM   #11
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Sorry, thought this device was for testing voltage temporarily.
No problem. I didn't quite get my mind around your comment. Was just in my unit in the barn at the cheapo meter is reading 13.2 with 3 bars. Hard to beat a heads up display for 10 bucks.
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Old 01-14-2017, 05:28 PM   #12
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No problem. I didn't quite get my mind around your comment. Was just in my unit in the barn at the cheapo meter is reading 13.2 with 3 bars. Hard to beat a heads up display for 10 bucks.
Since your cheapo meter AND any other meter would read 13.2 volts you can be sure you are NOT reading your batteries voltage. You are either reading the charging voltage from your converter...or the surface charge left over after charging since a 12V wet cell battery CANNOT be more than 12.7.
In cold weather...this reading may be above 12.7 on a disconnected battery for several weeks since when self discharge stops...so does the dissapation of the surface charge. Applying a load tester may be the best way to get a good reading. Harbor Freight...about 20 bucks.
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Old 01-14-2017, 07:15 PM   #13
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Since your cheapo meter AND any other meter would read 13.2 volts you can be sure you are NOT reading your batteries voltage. You are either reading the charging voltage from your converter...or the surface charge left over after charging since a 12V wet cell battery CANNOT be more than 12.7.
In cold weather...this reading may be above 12.7 on a disconnected battery for several weeks since when self discharge stops...so does the dissapation of the surface charge. Applying a load tester may be the best way to get a good reading. Harbor Freight...about 20 bucks.
Have one of those as well but it's a Snap-On. My unit is inside the barn and it's shore powered as it stays all winter but thanks for the concern, it's unfounded. Just out there, kicked on the furnace and made a coffee.... I may be reading my charge rate, no issue with that. I just gad the batteries out last week and load rested them. All fine.
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Old 01-14-2017, 07:20 PM   #14
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Have one of those as well but it's a Snap-On. My unit is inside the barn and it's shore powered as it stays all winter but thanks for the concern, it's unfounded. Just out there, kicked on the furnace and made a coffee.... I may be reading my charge rate, no issue with that. I just gad the batteries out last week and load rested them. All fine.
Gotcha...misunderstood. So if you see anything other than 13.2 then you'd be worried!
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Old 01-14-2017, 07:33 PM   #15
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Nice to have my own inside storage. My unit 'sleeps' inside with the tractors, all the implements and my pickup truck too. I've found that RV's especially, stay nice a long time when not out in the weather. Even in the summer, I keep it inside when not using it.

No mice issues either. I have 3 resident Tom Cats, excellent mousers. They whizz on the tires but I can deal with that.....

I need to get after a wax job soon.
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Old 01-15-2017, 12:24 PM   #16
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Heres what I installed. Cheap on Ebay, works great. Mounted it above my stereo. usb, power port and digital volt meter. I included a pic showing how I wired it. Real easy to install. Volt meter runs constant, but not extremely bright so not a distraction. Very low power consumption. I have a main battery kill switch that I turn off when storing the trailer.
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Old 01-16-2017, 08:45 AM   #17
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Heres what I installed. Cheap on Ebay, works great. Mounted it above my stereo. usb, power port and digital volt meter. I included a pic showing how I wired it. Real easy to install. Volt meter runs constant, but not extremely bright so not a distraction. Very low power consumption. I have a main battery kill switch that I turn off when storing the trailer.
Nicely done, WD!

Staying with the OP, a volt meter has some value in his situation, but its limitation has to be understood. When autonomously camping, and the battery / batteries are under very small load, what it reads gives one a good idea when to recharge ... maybe. Is it the method to know exactly the state of charge? ... certainly not. But if it was me and all I had was a voltmeter, seeing a flat 12.0 vdc, its time to charge. Running a generator to properly charge batteries via a converter is costly; technically it could take a day to days to fully charge. The key is under light load and it has been under light load for several hours, see what the voltmeter tells you. Camaraderie is correct, in that, that to really knows requires a hydrometer; and easier than that is with a battery monitor. A volt meter will also tell you, given the same understanding of its limitation, that if you read 13.xx or 14.xx while charging that you are indeed charging. A volt meter cannot tell you when to stop charging. I always recommend a good battery monitor such as the Trimetric TM-2030, which requires a shunt and a some wiring performed. I can't understand how folks can buy a $25K to whatever $$$ RV, who need to know battery status; but can't afford another $250 to do it right.
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Old 01-16-2017, 08:09 PM   #18
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Any meter or display works if you understand it, and check it often under charge/discharge conditions.
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