How long will a battery last? - Page 4 - Fiberglass RV


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Old 06-15-2016, 07:04 AM   #43
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..I recommend a digital multimeter (DMM). Avoid anything with an analog readout. It's likely to have a lower input impedance which could effect the accuracy. A $15-$20 meter is fine and can be purchased in most hardware or auto stores. You want one with its own battery. Meters that draw there power from what they are measuring can effect the measurement and can cause parasitic drain if left plugged in. Raz
To give you an idea of just one of the challenges of using a voltmeter to figure out how much charge is left in your flooded battery, I bought two meters with digital displays and both read about .10 volts different from my quality digital multi-meter. Now, .10 volts but not sound like much but according to Mark S. Nemeth (The 12 Volt Side of Life guy), there is only a .21 volt difference between a battery that is 75 % charged and one that is 50% charged (and ideally should not be discharged any further). So being off by only .10 volts could easily give you and inaccuracy of 12% or more.
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Old 06-15-2016, 07:33 AM   #44
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To give you an idea of just one of the challenges of using a voltmeter to figure out how much charge is left in your flooded battery, I bought two meters with digital displays and both read about .10 volts different from my quality digital multi-meter. Now, .10 volts but not sound like much but according to Mark S. Nemeth (The 12 Volt Side of Life guy), there is only a .21 volt difference between a battery that is 75 % charged and one that is 50% charged (and ideally should not be discharged any further). So being off by only .10 volts could easily give you and inaccuracy of 12% or more.
As I said once before. I used to take 5 bench quality meters and connect them to the same source. All would read different. In the lab our tech would use a very expensive temperature compensated voltage standard (sent out yearly) to calibrate meters. Still the meters will read different.

Calculate you usage. Choose a battery to meet that demand. Use your meter to check on things once in a while. Get used to "close enough".
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Old 06-15-2016, 10:56 AM   #45
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A better quality multimeter is something I will need to invest in then. Right now all I have is one of those big harbor freight meters that they give away free with any purchase and one of the prongs already fell off.
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Old 06-15-2016, 12:16 PM   #46
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"approximately correct"....

or "close enough" by another name.....

I don't think no one will ever know exactly how much power is in their "tank"... and as somebody said the nature of the numbers means that a small change in them means a large amount...12.6+ to 12.2 doesn't sound like much but it is 100% of the power at your disposal (below 12.2 you are not being "nice" to your battery and will pay for it later)

I got tired of taking out a meter every time...so I installed a permanent one...and I really like having it.....it gives me good info like how fast I'm using up available power....as a for instance: when the furnace (biggest power hog in our trailers) kicks in the displayed voltage drops like a rock into the elevens...when it shuts off the meter climbs back up to within one or two hundreds of a volt from where it was before...this sort of tells me that if I run the heater all night (10-12 cycles) I will spend .2 volts during the night...again doesn't sound like much but that is HALF of available power IF my battery was right FULL when I started.....

a hydrometer reading of a battery at rest for an hour is probably the best measure....but nobody wants to do that every day/night...

as said before...there's a lot of close enough, sort of, gut feel and approximately in this business...good voltage info will make you comfortable and less susceptible to "power anxiety" fully using your trailer unplugged.....just my opinion of course
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Old 06-15-2016, 12:31 PM   #47
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or "close enough" by another name.....

I don't think no one will ever know exactly how much power is in their "tank"... and as somebody said the nature of the numbers means that a small change in them means a large amount...12.6+ to 12.2 doesn't sound like much but it is 100% of the power at your disposal (below 12.2 you are not being "nice" to your battery and will pay for it later)

I got tired of taking out a meter every time...so I installed a permanent one...and I really like having it.....it gives me good info like how fast I'm using up available power....as a for instance: when the furnace (biggest power hog in our trailers) kicks in the displayed voltage drops like a rock into the elevens...when it shuts off the meter climbs back up to within one or two hundreds of a volt from where it was before...this sort of tells me that if I run the heater all night (10-12 cycles) I will spend .2 volts during the night...again doesn't sound like much but that is HALF of available power IF my battery was right FULL when I started.....

a hydrometer reading of a battery at rest for an hour is probably the best measure....but nobody wants to do that every day/night...

as said before...there's a lot of close enough, sort of, gut feel and approximately in this business...good voltage info will make you comfortable and less susceptible to "power anxiety" fully using your trailer unplugged.....just my opinion of course
I was also looking at those meters but from what I saw is that they all come with bright blue displays that are on all the time and i just wasn't sure of how much battery it would eat just running a meter like that and also if it would be too bright in a scamp 13.

Another issue I ran into was they all seem to ship from China and I don't have time to wait around until July or August because of their slow shipping.
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Old 06-15-2016, 01:10 PM   #48
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..

Another issue I ran into was they all seem to ship from China and I don't have time to wait around until July or August because of their slow shipping.
I already have a few of these from China.. New Auto LED Cigarette Lighter Voltage Digital Panel Meter Voltmeter Monitor Car | eBay

Just plug it into your cigarette lighter socket when you want to get a reading. If you don't have such a socket, maybe add one.. they are very handy.

If you want to PayPal me $8 you can get one in a few days (CONUS only).

Its off about .1 volts at 13 volts.. but like everyone says, that is close enough.

Of course you can add a switch to any panel type always on meter too.. I use a momentary contact one, so it works only while the button is pressed.
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Old 06-15-2016, 01:42 PM   #49
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I already have a few of these from China.. New Auto LED Cigarette Lighter Voltage Digital Panel Meter Voltmeter Monitor Car | eBay

Just plug it into your cigarette lighter socket when you want to get a reading. If you don't have such a socket, maybe add one.. they are very handy.

If you want to PayPal me $8 you can get one in a few days (CONUS only).

Its off about .1 volts at 13 volts.. but like everyone says, that is close enough.

Of course you can add a switch to any panel type always on meter too.. I use a momentary contact one, so it works only while the button is pressed.
That's a cool gadget, I was wondering if they made those. I do have two of those sockets wired up in my Scamp so I will probably buy one from you. Just give me a few days and i will pm you for your PayPal info, I just have to take care of a few other items for my camper first. Thanks.
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Old 06-15-2016, 02:00 PM   #50
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on all the time....

that's what the switch below it is for....to turn it off...

about the cig. plug meter mentioned....before this panel meter I had another one that was three digits (like the cig. plug model)...unfortunately I did not feel like it gave me enough information....about as much information as if your fuel gauge in your car only told you if your tank was full, half full or empty.....(I reused the three digit meter for voltage from solar panels) four digits way more useful IMO

shipping for the 4 digit was a couple of weeks IIRC
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Old 06-15-2016, 02:03 PM   #51
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that's what the switch below it is for....to turn it off...

about the cig. plug meter mentioned....before this panel meter I had another one that was three digits (like the cig. plug model)...unfortunately I did not feel like it gave me enough information....about as much information as if your fuel gauge in your car only told you if your tank was full, half full or empty.....(I reused the three digit meter for voltage from solar panels) four digits way more useful IMO

shipping for the 4 digit was a couple of weeks IIRC
How do you have it wired, right to the battery or somewhere in between?
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Old 06-15-2016, 02:06 PM   #52
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and about the pic....

IIRC the picture was taken with charger off....voltage from panels reads 16.6....the batteries appear full as the solar charge controller is supplying 13.3 volts to the system (maintenance charge)
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Old 06-15-2016, 02:11 PM   #53
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IIRC the picture was taken with charger off....voltage from panels reads 16.6....the batteries appear full as the solar charge controller is supplying 13.3 volts to the system (maintenance charge)
I will look into both types of voltmeters more closely, realistically I only need it to tell me when the battery is half full, if it even gets to that point.

We went camping for two days and when I checked my battery it read 12.4 after using everything besides the TV. I would say the longest I would dry camp would be 3 days.

I will have to test again after I get a 12v fan and media player for the TV.
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Old 06-15-2016, 02:15 PM   #54
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close to the battery....

it's wired to a circuit I ran to/for my "home made dash panel thing"... fused 10A...from the battery cut-off switch

the 15A fuse is on the solar panels circuit
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Old 06-15-2016, 02:23 PM   #55
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And then there is loading.

Go to your car. Turn on the lights. Now start the car. The lights dim because the battery voltage drops due to the large current drain of the starter motor. That's called loading. Any time you draw current, the source voltage drops only to raise again when the current is turned off. The more current, the greater the drop. The ability to maintain a constant voltage over a range of current is called regulation. Electronic power supplies, (AC to DC converters ) are designed for good regulation. Batteries don't have very good regulation. So when using your meter to check the state of charge of your battery it's not just the variation in meter readings that can cloud things. Loading can too.

And then there's noise....
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Old 06-15-2016, 06:16 PM   #56
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Those 12 volt coolers will drain your camper battery or you tow vehicle battery like a big hog on corn at a trough. My brother offered me one and I said no thanks. I like a good reasonably priced cooler and don't mind a little ice bill. If you're only out for three days make good hard ice at home and most any "5 day" cooler will get you by. Especially if you precool your drinks in your home refrigerator. Having a cooler drained battery is not good. Spoiled food, nothing else will run and worst of all warm beer. The ice company's kids need shoes too.
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