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Old 07-04-2020, 10:39 AM   #1
kenmoffat
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Battery level meter

When reading the battery level meter (on the panel with tank levels), is 2 bars as low as I should allow the batteries to discharge? or is one bar still not hurting the life span of the batteries? (I ordered a plug in voltage meter, but haven't recieved it yet, so need some guidance). Thanks!
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Old 07-04-2020, 11:08 AM   #2
chuckster57
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Welcome to the forum

The battery meter on the monitor panel is not accurate, you need a true voltage meter. You’ll notice if your not plugged into shore power, as you add load the number of lights will go down, turn the loads off and the lights come back on. When you say you got a “plug in” meter, where does it plug in?
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Old 07-04-2020, 11:11 AM   #3
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to a cigarette lighter plug.
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Old 07-04-2020, 11:14 AM   #4
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Does it show voltage?

Some information you might find interesting:

https://www.energymatters.com.au/com...age-discharge/
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Old 07-04-2020, 11:16 AM   #5
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Yes, it's a spitter with usb charger ports and a voltage indicator.
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Old 07-04-2020, 11:27 AM   #6
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Hoping to do a few days off grid, have 2 12 volt Interstate batteries, 2017 Springdale, no slide, LED lighting, not much drain except my CPAP machine and my wife's electric fan. maybe a little radio, charging cell phones and laptop. I have a 100 watt solar panel that provides charging. (AcoPower)
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Old 07-04-2020, 11:35 AM   #7
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$10 volt meters (or the standard OE control panel "idiot" lights) are not reliable in an active RV coach. The disparities between load and resting voltages makes them absolutely useless.

The better route (and for not much more $$) would be to install a volt/amp meter with a battery shunt and measure current flow (amperage). Use the battery's amp hour capacity to determine what the state of charge (SOC) is at any given time, no matter the loads.

If the coach has the typically flooded lead acid battery, assume a 50% SOC would be the lowest a battery can safely be discharged to. For example, if the battery has a capacity of 80 amp hours, using 40 amp hours would put the battery at 50% (or empty).

Using the resting voltage, typically that would equate to 12.1V. it's just next to impossible to get an accurate representation of the resting voltage when the trailer is being used.
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Old 07-04-2020, 11:44 AM   #8
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Interesting. So you would say that if I turn off everything and measure the voltage, that would not help. What if the voltage at the cig plug says say 12.5 volts when stuff is turned on, like running an inverter to power the electric fan and a LED lamp and having overhead LED lights on, and maybe a CPAP machine running off 12 volts. Would there be a low limit on voltage, or is that still a useless measurement? And isn't measuring at the cig plug the same as at the battery? And thanks again for answering!
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Old 07-04-2020, 11:47 AM   #9
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Sad that they don't install useful gauges for battery and tank levels.
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Old 07-04-2020, 12:22 PM   #10
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Your comment, I have two 12 volt Interstate batteries and want to boondock 3 or 4 days with only my CPAP, the wife's fan and a few cell phone chargers....

First, two GP24 Interstate batteries have about half the power capacity of two GP31 Interstate batteries. So what batteries do you have?

Second, Your CPAP will completely discharge a couple of GP 24, 12 volt batteries overnight "IF" you have the humidifier turned on. Add your wife's 12 volt fan (or inverter and a 120VAC fan) and you're sure to be waking up to dead batteries. A couple of GP 27 or GP 31's might make it, depending on other use, but you aren't going to get a "3 or 4 day stay with a CPAP with the humidifier turned off and a fan running all night, every night"...

Third, a 100 watt solar system on a "sunny day" likely won't keep up with your trailer's "normal electrical use" (refrigerator on gas, water heater on gas, lights, water pump and the radio)... On a cloudy or rainy day, even less likely you'll make it through the night.

Have you considered a small generator to "augment/replace" that 100 watt solar panel? Otherwise, you're probably going to be disappointed in trying to go more than overnight with a couple of GP 24's and probably won't make it 2 nights with GP 27's or even GP 31's.

ADDED: To answer your question about measuring "at the battery" and "at the cigarette lighter plug" being the same. No they aren't. There's a voltage drop in the wiring between the battery and the lighter plug. Depending on the amperage draw of the parasitic drains (CO monitor, LPG detector, refrigerator door heater circuit and control board, etc) there may be a 1-3 amp drain that you can't shut off, that's enough to make a 2 or 3 volt drop "at the lighter plug". Plus, you can't disconnect the battery (to remove all the load) and measure it at the lighter plug. You can only do that measurement "at the battery terminals"...
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Old 07-04-2020, 01:08 PM   #11
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Thank you for your replies. I have GP24's, but have run my CPAP overnight without draining my batteries, no humidifier. and lasted another night using the solar panels. I will be in touch after our 4 night trip to let you know how it went. I'm now a bit nervous, but we will make do either way. Thanks again.
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Old 07-04-2020, 01:30 PM   #12
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Interstate recommends a discharge level "not below 70% for their hybrid batteries" (that's the ones with a CCA rating on them). 70% is a resting voltage of 12.32 VDC. Most true deep cycle batteries (like golf cart batteries) do not have a CCA rating. They can usually be discharged to 50% without damage. 50% is a resting voltage of 12.06 VDC.

So, discharging your "GP 24's" below about 12.3 volts is significantly shortening their lifespan. Your CPAP and the wife's fan will still run at 11.5 VDC, but your battery charge state at that voltage is around 20%, so you can see the damage you "might be doing", even though the CPAP and fan are still "humming along".

Here's one place to get a "battery voltage/charge rate chart" https://www.rvtechlibrary.com/battery/bat_volts.php
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Old 07-04-2020, 05:38 PM   #13
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Thank you for the information. We'll see how long we can last. Might have to invest in a power pack of some sort for the CPAP. A generator isn't in the cards. I wonder if a supplemental solar panel, another 100 watt, would be enough, assuming sunny days.
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Old 07-04-2020, 05:53 PM   #14
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If it’s roof mounted, you can get 200W in a single panel.
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Old 07-05-2020, 08:57 PM   #15
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I've decided to get a second 100 watt add on solar panel. That should give me sufficient charge to survive 4 nights, hopefully. Will report the results. Also, wife getting battery powered fan.
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Old 07-12-2020, 07:53 AM   #16
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Ken, the replies above are giving you good advice. You will soon learn that discharging any battery below the factory recommended level (50% to 70%) will result in very shortened battery life. Thus, when you need to replace your 12 volt batteries, buy good deep cycle batteries such as a Trojan T105 or USB 2200. These are 6 volt batteries but you just connect in series. Your 12 volt batteries are connected in parallel. Trojan recommends discharging to 50% for long life and good performance.

Regarding solar charging, Your 100W panels at full sun, will give you about 8 amps for a few hours. Thus, assume full sun last for 5 hours, thus, the maximum you can get is 40 amp-hours under near perfect conditions. Depending on your location, time of year (angle of the sun), etc, you might get as low as 20 amp-hours. Add cloud cover and you will rapidly discharge your batteries with the power load you describe. Watts=VoltsXamps. So add up the amps or watts on all connected items and convert all loads to amp-hours. Add the parasitic loads, and you can estimate the needed daily amp-hours. Now you know the amp-hours per day you need. Compare that to the battery storage rated capacity and you will have a betty idea of your solar power generation.

A second 100 Watt panels is a good idea as your single panel is marginal at best. If you have a good charge controller, I would connect the panels in series so you supply power to the charge controller at a higher voltage. This will lessen your amperage to the charge controller and the wires you have installed can accept the higher amps with acceptable voltage drop.

So, what probably seemed simple, is more complex. Nevertheless, lots of us have done this. I have a single 280 Watt panel on my trailer but I keep my relatively quiet Honda generator around just in case. However, a properly designed solar generator combined with proper batteries is quiet and effective.
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Old 07-12-2020, 08:56 AM   #17
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We picked up battery powered fans and a second 100 watt panel that plugs in to the original panel. We are predicting full sun for 3 of the 4 days. So the only essential load other that trailer maintenance is my CPAP, which I will run without humor and heated house. Pretty low draw, so we shall see what happens. We are in western Washington, so not famous for internet sunshine. 🙄


It's it necessary to disconnect the batteries in order to determine the state of discharge?


Thanks again!
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Old 07-12-2020, 10:50 AM   #18
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To get the state of charge, it is best/necessary to disconnect. However, accuracy of the voltage vs charge depends on the parasitic load. You likely do not know this. If very small, then not much error in leaving them disconnected. So, since disconnecting is a PIA, I would recommend to "calibrate the error" by measuring the voltage before and after disconnecting. Then you can see the effect of the parasitic load.

For the Trojan T105 Battery, 0.06 volt is 10% Look at this data sheet as it will help you understand batteries better. It will be different for your batteries but there is a similar effect.

http://www.trojanbattery.com/pdf/dat...ata_Sheets.pdf

I would suggest doing that a couple of times as the parasetic load my vary depending on conditions. You can also buy and install a disconnect switch to make the measurement easier. DO NOT TRUST the OEM Disconnect switch if you have one as it does not disconnect all loads.

Note that every battery manufacturer recommends a different charge and float voltage for the battery. This is shown on the data sheet. These settings are adjustable in your charge controller if it is a high quality model. Check you owners manual.

Good Luck.
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Old 07-12-2020, 11:48 AM   #19
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My charge controller is a Victron Energy Smart Solar MPPT 100/20. This unit is very small but is really "smart" . I connect to it via a Bluetooth App and I get lots of information on my solar system including the instantaneous energy being produced, battery voltages, total energy produced per day, etc. Here is a link for the manual.

https://www.victronenergy.com/upload...R-DE-ES-SE.pdf

You could get by with a slightly smaller one but in case you might consider adding a third panel, then get the one I have. In your geographical area, solar in the summer months is great but falls off RAPIDLY in the fall and spring and is almost nothing in the winter months.
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Old 07-19-2020, 10:11 AM   #20
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Update, we were able to boondock for 4 nights using my CPAP without humidity and using some lights. The two solar panels kept us charged fairly well, in spite of having full sun for only party of the day. We will be trying it again in a month. Thanks for all the replies.
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