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Old 11-08-2019, 01:07 PM   #1
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Smile Battery monitor

I bet this has been addressed before but here goes. So, I have decided I need to monitor batteries more closely. After spending an hour and a half on batteries due to PGand Eís shut downs, my batteries were at 12.1 V. Idiot lights said down to 1/3. All we did was cook and heat the place for 1-1/2 hours. Of course the refer was on propane and radio backlight and co2 monitor but that seemed excessive. Next morning my suitcase solar panel charged them to full in 20 minutes. Panels were putting out 5.1 amps. I will be doing a parasite test and load testing the batteries checking fluid levels on Two 12v interstates. I would like to install a system, not too complicated. I was looking at renogyís monitor on amazon for around 99 bucks. Donít know a lot but i guess i would need a shunt and some cabling. Any sites where i could get those? and what rating 500 or 350 for the shunt? Iím assuming they need to be purchased separately. Amazon only had one. Donít want complicated but i do want to see percentage left, volts and amps used etc. Iím not real sophisticated in the electrical arts . This is for my 25 rds titanium. The unit is fairly new since April of this year. Suggestions appreciated. Thanks in advance.
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Old 11-08-2019, 03:05 PM   #2
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2, 12 volt batteries are at least 150 AH of capacity, but not much more

12.1 volts is about 50% discharged or 75 AH used. That voltage is only if nothing is on, called resting voltage. If you got that reading with things running its just about meaningless.

20 minutes of charging, at 5 amps, is about 2 AHs. That is no where near recharged.

You need to push in about 100 AHs to get 75AHs of charge back in the batteries.

A battery monitor will show you this, but not fix your under charging issue.

If you get a clear sunny day, you can only expect about 5 hours of 5 amp charging. That adds up to 25 AH of charging. Add a lot more solar or a generator and charger.

What your battery gauge lights was showing was voltage from the panels going into the batteries. You would need to shut down the solar to see battery condition and that would be after at least 1 hour of no charging to bleed off the surface charge.

A cheap monitor will give you a OK picture of your battery.
Unless you have an inverter, a 100 amp shunt is all you need. Everything turned on, in the RV, will probably not overload a 100 amp shunt.
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Old 11-08-2019, 04:15 PM   #3
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We use a TriMetric 2030RV monitor and are very happy with it.
It keeps tabs on our (4) 6v batteries.
I would not install (2) 12v batteries.
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Old 11-08-2019, 05:21 PM   #4
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Quote:
Originally Posted by Lowriders View Post
I bet this has been addressed before but here goes. So, I have decided I need to monitor batteries more closely. After spending an hour and a half on batteries due to PGand Eís shut downs, my batteries were at 12.1 V. Idiot lights said down to 1/3. All we did was cook and heat the place for 1-1/2 hours. Of course the refer was on propane and radio backlight and co2 monitor but that seemed excessive. Next morning my suitcase solar panel charged them to full in 20 minutes. Panels were putting out 5.1 amps. I will be doing a parasite test and load testing the batteries checking fluid levels on Two 12v interstates. I would like to install a system, not too complicated. I was looking at renogyís monitor on amazon for around 99 bucks. Donít know a lot but i guess i would need a shunt and some cabling. Any sites where i could get those? and what rating 500 or 350 for the shunt? Iím assuming they need to be purchased separately. Amazon only had one. Donít want complicated but i do want to see percentage left, volts and amps used etc. Iím not real sophisticated in the electrical arts . This is for my 25 rds titanium. The unit is fairly new since April of this year. Suggestions appreciated. Thanks in advance.
What model of Interstate batteries do you have? Our 23DBS came with a pair of Interstate HD24-DP 12v which turn out to be rather pathetic with a 64ah rating each. If yours are similar I could see a noticeable drain if the furnace was running a good bit of the time you were without shore power.
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Old 11-08-2019, 07:14 PM   #5
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Battery monitor

2801qs -two generic 12 v batteries, when new. One overnight, running a light til bedtime and a fan in the early evening, several hours. In the AM idiots lights said batter at 50%. pretty bad.

Replaced these with Interstates. Have not tested. I always run the generator now.
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Old 11-08-2019, 09:30 PM   #6
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Originally Posted by egwilly View Post
We use a TriMetric 2030RV monitor and are very happy with it.
It keeps tabs on our (4) 6v batteries.
I would not install (2) 12v batteries.

X2, only 2-6v instead. Very happy and it interfaces with the matching solar controller if you have or upgrade to solar panels.
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Old 11-08-2019, 10:04 PM   #7
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found this helpful

Volt % charge
12.6 100.%
12.5 90.%
12.42 80.%
12.32 70.%
12.2 60.%
12.06 50.%
11.9 40.%
11.75 30.%
11.58 20.%
11.31 10.%
10.5 0.%
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Old 11-09-2019, 09:38 AM   #8
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If you got that reading with things running its just about meaningless.
No, it can tell you a lot if you know the merit of your batteries.

Terminal voltage follows SOC over a very repeatable curve based on internal impedance. If you know what that is (easily measured empirically) you can discern SOC at any current level. Fancier battery monitors do this very method to determine SOC on the fly.

To the OP, I find there's a point of diminishing return to the fancy battery monitors. Yes, they collect and display all sorts of valuable an interesting data and trends, but at the end of the day all you really care about is running your stuff. As much as I "get" batteries and can digest the minutia of this data, in an RV I kinda don't care. I just want to know if I'll make it through the night running the furnace or how long to run the genset to bring them back up north of 80%. Believe it or not, I've gotten used to just using the 4-LED status panel. I spent a bit of time testing it to know just what level each LED represents. A very coarse test but often close enough. I also installed a cheap chinese battery monitor near the battery compartment which can tell me a bit more accurately where things are at:

https://www.amazon.com/bayite-6-5-10...eter-Voltmeter

The downside to this one is it doesn't count charge Ah, so the displayed Ah used isn't something you can go by except from a fresh charge. But still useful to see a snapshot of terminal voltage, current and watts. A Victron, Trimetric or other battery monitor can give you instantaneous SOC and other curious and interesting data but they're pretty nerd-centric devices to use and interpret. Great if you're interested in that stuff or you think you can get something out of your batteries you aren't already. I've done that enough in stationary and mobile applications to know that I don't care in my RV, so a basic voltmeter/percentage indicator is about all I want to know for regular use. Something like this checks the box:

https://www.amazon.com/dp/B01LSBH7Y6

After a few outings you'll know what percentages shown reflect what you consider "full" and "empty" for your battery bank.

Mark B.
Albuquerque, NM
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Old 11-09-2019, 02:00 PM   #9
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Battery Monitoring

Battery monitoring is important to RV and TT owners who dry camp or boondock. Of course it does not have to be precise like medical or aviation may need.

To understand the standard battery state of charge (SOC) meter found in the broad range of low end TT, you must learn some electrical battery facts. Without this understanding, the error between what you think you have and what you actually have is extreme.

This type of meter measures battery voltage and assigns a % charge to it. The problem is if the system is being charged by shore power, generator, solar, wind, or any other power source, the gage is completely wrong. % of charge will show much higher than actual. It is showing mostly the rate of charge. The charger profile starts out with high current charge limited by the charger and with low voltage. This shows as low % of charge. As the charge progresses the voltage rises to one of 13.2, 13.6, or 14.4 volts. All of these voltages show as 100% on the meter.

If the system is under load every thing is the opposite. Discharging the battery at a low current will show a little lower % of charge than it actually is. If the system is under high load it will show much lower than it actually is.

The SOC meter you have may be biased to be more accurate when discharging a little. You don't know what the manufacturer has done unless you measure the battery voltage and compare it to the static voltage chart for lead acid batteries. Static means it has not been on charge for 4 hours or more and has not been on discharged for a while.

If you measure the battery voltage at the battery terminals using a digital voltmeter, you can learn to interpret the approximate SOC under all these conditions.

To get a battery monitoring device to do this will require a much more sophisticated measuring and computing device. Different brands of devices will perform differently, but most do a much better job than the standard meters installed by TT mfg's.

The cheap after market meters like the one you are considering may or may not be an improvement. If so, it will be minimally better even if it does show a three digit display. Some of the other monitors cited by posts above will do a lot better.

What you need:
The device must measure battery voltage, charge, and discharge currents. It must provide calculations based on both measures plus keep track of accumulated charge and discharge currents. The better ones will "learn" as you use the battery and compensate for other changes that are important over time. Some of them become highly complex. All should have a calibration procedure for your specific battery bank.

The Battery University website contains more information than I can learn in a life time. I have extracted some pages that may be relevant if you wish to learn. Feel free to look at other articles shown in the extensive index.
Battery University

https://batteryuniversity.com/
How does the Lead Acid Battery Work? https://batteryuniversity.com/learn/article/lead_based_batteries
Charging lead acid batteries https://batteryuniversity.com/learn/article/charging_the_lead_acid_battery
Charging with Solar, Turbine https://batteryuniversity.com/learn/article/charging_with_solar_and_turbine
How to Charge and When to Charge? https://batteryuniversity.com/learn/article/how_to_charge_when_to_charge_table
How to Store Batteries https://batteryuniversity.com/learn/article/how_to_store_batteries
Summary of Do’s and Don’ts https://batteryuniversity.com/learn/article/do_and_dont_battery_table
There is a lot more...
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Old 11-09-2019, 02:06 PM   #10
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The reference I made to the monitor you cited in your original post was wrong. Renogy makes many monitors. There is one on Amazon for $99 that may be one of the best.

It is complicated, isn't it. If you buy it, let us know how well it works for you. I would like to get a good one also.

Renogy
500 Amp Universal Battery Monitor, High and Low Voltage Programmable Alarm
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Old 11-09-2019, 06:11 PM   #11
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Mark, when you're using your dmm to check the battery voltage I assume that the propane detector inside the trailer is on. Do you turn it off so there is no load on the battery or just let it be a constant in all your readings? Thanks
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Old 11-09-2019, 07:10 PM   #12
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I use a Victron BMV712 and like it a lot. Not cheap at about $200+, but it measures amps used and amps put back into the battery!! and then displays %SOC based on you telling it the Ah that you have available (ie. battery bank size). So if you are running Lead Acid batteries you can tell it 50% of the actual battery so you don't go lower than 50% SOC and hurt the lifetime of your battery(ies). You can watch the amps go in/out of the battery. It will also tell you your current (no pun intended) draw in amps, watts, etc., and will predict at that draw how long you have before you drain to your set point. Pretty nice. Takes a bit of set up and install is definitely some work. It requires installing a shunt on the negative lead and running a wire from the shunt to the BMV monitor that you can put somewhere in your rig. It also runs off of bluetooth to an Victron app on your phone.
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Old 11-10-2019, 10:05 AM   #13
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Battery monitoring

Whew, what a complicated subject. Thanks to all your replies. I know a lot more now then before. I have figured out that my batteries are interstate 64 ah batteries. No wonder they discharge so fast. I did put the volt meter on them the next morning so that is how I came up with the 50% discharge. It measured 12.1 V with minimal discharge happening at the time (co2 and refer on propane) so I am pretty confident they were down to 40 or 50%. In addition, that 100 watt solar panel was probably putting out the maximum it could, so the idiot lights would have read full almost immediately. That fooled me, but now I know better. It was reading the % of change due to the panel blasting away at the batteries. So grateful. I will be upgrading to at least two, mabe 4 220ah 6 volts once I kill these. Guess a generator might be in my future too. But, first, I’m going to have to install a generac at my house, thanks to PG& E here in lovely Californian (read sarcasm here). Again, thanks to all, I always learn so much from you folks. Great forum
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Old 11-10-2019, 10:10 AM   #14
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Mark, when you're using your dmm to check the battery voltage I assume that the propane detector inside the trailer is on. Do you turn it off so there is no load on the battery or just let it be a constant in all your readings? Thanks
It can be whatever constant you want. That's where having an ammeter you can see can help you interpret the voltage measurement you're getting. A "typical" load of a refrigerator and some lights can be an easily repeated scenario and the voltage you get with that load can tell you pretty closely what your present SOC is. Of course, the greater the load the more the terminal voltage and SOC will drop, so knowing the load is an important variable in estimating SOC with voltage. It's not practical to let a pack rest with no load for a few hours to know SOC, so learning how to interpret whatever battery monitor employed while in use is where you want to be.

I'm in the camp where I don't really care what the SOC is, if it's enough to run my stuff. I've gone through everything in the RV to see what it draws and I capacity test my batteries to know what I've got. So it's with a "trained eye" I can look at terminal voltage along with what's in use and have an idea where I sit. One can buy a spendy battery monitor that does this for you and they work great, but you either need to understand what variables to plug in for your system, or base it on faith that the defaults will work for you. Better than nothing for sure, but like most things no free lunch. I get that my process is anything but foolproof to the uninitiated but just planting the seed that even basic system monitoring can go a long way towards having reliable power.

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Albuquerque, NM
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