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Old 03-12-2019, 03:02 PM   #1
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Battery panel question

We have talked a lot about batteries and how FLA batteries should ideally stay above 50% SOC to avoid damage. Other than a fancy battery monitoring kit, many trailers like mine come with a cheap looking control panel which shows the approximate battery level.

So, my question is this: when the control panel is showing the battery level as empty, is it denoting 0% SOC? Or is it programmed with FLA cells in mind and showing empty as 50% SOC?
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Old 03-12-2019, 06:33 PM   #2
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Quote:
Originally Posted by Hart_family View Post
We have talked a lot about batteries and how FLA batteries should ideally stay above 50% SOC to avoid damage. Other than a fancy battery monitoring kit, many trailers like mine come with a cheap looking control panel which shows the approximate battery level.

So, my question is this: when the control panel is showing the battery level as empty, is it denoting 0% SOC? Or is it programmed with FLA cells in mind and showing empty as 50% SOC?

Good question.
It might be a better solution to add your own battery level meter to match your new battery(s), that way you know for sure.
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Old 03-12-2019, 07:21 PM   #3
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Totally understand and agree. But I was still curious to know how those basic factory panels are set up.
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Old 03-12-2019, 07:57 PM   #4
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I'll add to the confusion, my panel appears to show 3 different voltages under the lights, although they are completely contrary to every chart on batteries I've ever seen. 12.3 is around 70% and 11.2 is near dead. I put that meter in the category with the holding tank lights, there's no reason to ever push those buttons.

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Old 03-12-2019, 08:41 PM   #5
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Haha! Good to know.
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Old 03-12-2019, 10:24 PM   #6
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It's also good to understand that after you turn off a load the batteries will seem to recover a bit. This is known as a surface charge and will cause those cheap battery indicators to show a higher charge than you actually have. FWIW, those battery level indicators are useless. The only real way (practical) is through the use of a good battery monitoring system.
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Old 03-13-2019, 07:47 PM   #7
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State of Charge Voltage

Measuring battery voltage in order to determine its state of charge is not a simple task. There is one voltage profile when charging. There is another when the battery has been stable for 4 hours. Stable means neither charging nor discharging. Many people are impatient and wait only 30 min. The third profile is when the battery is being discharged. There are formulas for this third profile, but they include measuring the current as well as the voltage.
The overly simple control panel lights seem to assume the batteries are being discharged, but the lights do not account for the current flowing either into or out of the battery.
There are many stable voltage profile tables available. I will try to attach one. This table assumes there has been no charging or discharging for 4 hours or so. You need a digital voltmeter to measure this ($20 at local hardware store) The battery should be disconnected from the TT circuits since you can’t know it is stable without.
You can install better battery monitoring systems that make all the necessary calculations. The better BMI would measure both voltage and current and keep track of current in and current out. It would have a microprocessor to calculate the SOC. I am at a loss to recommend one.
Attached Files
File Type: pdf Charging SOC Table.pdf (161.0 KB, 29 views)
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Old 03-13-2019, 08:01 PM   #8
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I use this battery monitoring system and it's pretty good!
This is a Canadian site so prices are in Canadian dollars.

TM2030RV KIT Includes Monitor, Shunt 500 mA and 20' Wiring Harness - We Go Solar Canada
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Old 03-13-2019, 08:03 PM   #9
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Or this guy
https://www.solar-electric.com/blsedivoampm.html
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Old 03-13-2019, 09:37 PM   #10
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Am I all wet?

I've been thinking about this for 6 months. Did buy a unit to protect the trailer's systems from bad ac coming in. Just haven't mounted it yet. But I also bought the meter that reads all sorts of stuff that's been happening with the entire system.
But that will be a story whenever I get around to installing both.

I know I have 210 Amp hours in the batteries. I know that 50% is 105 amp/hrs. So I have only 105 amp hours worth of electricity to use before charging.

OK...now here is where I may get into trouble...you seniors can help me out.

If I use a radio that draws 2 amp/hrs for 15 minutes. Then I've used 1/4 of 2 which is .5 amp/hrs of juice. If I use 6 lights for 4 hours, that's 1amp each X4X6...which is 24 amp/hrs. With me so far? If I keep an approximate record of usage...just a little note throughout the day. By the end of the day I should have a general idea of how much juice I've used. When I reach the approximate 105 amp/hr figure, I know it's time to charge regardless of what the little meter says.

This thinking eliminates the surface charge that can creep into the calculations and really isn't or shouldn't be taken into account anyway as it's not "real"...at least as far as determining available charge.

And...it should give a pretty good indication of how you are doing in saving energy, very useful if you like to boondock.

So why don't we do the math more often? It's not hard to do...??

Curious....

P.S. you could even look up the equipment you have, and make a chart of what they each use per hour.....go from there.
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Old 03-13-2019, 11:45 PM   #11
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Cool

Quote:
Originally Posted by videoarizona View Post
I've been thinking about this for 6 months. Did buy a unit to protect the trailer's systems from bad ac coming in. Just haven't mounted it yet. But I also bought the meter that reads all sorts of stuff that's been happening with the entire system.
...
And...it should give a pretty good indication of how you are doing in saving energy, very useful if you like to boondock.

So why don't we do the math more often? It's not hard to do...??

Curious....

P.S. you could even look up the equipment you have, and make a chart of what they each use per hour.....go from there.
It sounds like fun … for you. I have a cheap little meter that does that. It accumulates watts, but that is the same.
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Old 03-14-2019, 04:09 PM   #12
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Quote:
Originally Posted by videoarizona View Post
I've been thinking about this for 6 months. Did buy a unit to protect the trailer's systems from bad ac coming in. Just haven't mounted it yet. But I also bought the meter that reads all sorts of stuff that's been happening with the entire system.
But that will be a story whenever I get around to installing both.

I know I have 210 Amp hours in the batteries. I know that 50% is 105 amp/hrs. So I have only 105 amp hours worth of electricity to use before charging.

OK...now here is where I may get into trouble...you seniors can help me out.

If I use a radio that draws 2 amp/hrs for 15 minutes. Then I've used 1/4 of 2 which is .5 amp/hrs of juice. If I use 6 lights for 4 hours, that's 1amp each X4X6...which is 24 amp/hrs. With me so far? If I keep an approximate record of usage...just a little note throughout the day. By the end of the day I should have a general idea of how much juice I've used. When I reach the approximate 105 amp/hr figure, I know it's time to charge regardless of what the little meter says.

This thinking eliminates the surface charge that can creep into the calculations and really isn't or shouldn't be taken into account anyway as it's not "real"...at least as far as determining available charge.

And...it should give a pretty good indication of how you are doing in saving energy, very useful if you like to boondock.

So why don't we do the math more often? It's not hard to do...??

Curious....

P.S. you could even look up the equipment you have, and make a chart of what they each use per hour.....go from there.
Peukert effect makes it more complicated than that. I'm no electrical engineer but I've been surfing RV websites, particular the solar part, for several years and have read about the same topics over and over. Peukert effect comes up a lot. My understanding of the Peukert effect is that basically low draw items don't drain the battery as much as high draw items. This may seem intuitive and obvious, but what I mean is that 50A load for 30 min (50x0.5 = 25Ah) will drain the battery more than a 5A load for 5 hours (5x5 = 25Ah), despite the math suggesting that these are equivalent drains on the battery.

As stated, estimating a SOC is more complicated than a quick voltage reading. Having said that, I never bothered with a fancy battery monitoring system. You can still get good info from that voltage reading and that's been good enough to enjoy camping. If you are currently charging the battery, then you aren't going to know the SOC but you can tell from the voltage what phase the charger is in (assuming it is a multi stage smart charger). In my low amp solar charging situation while camping, if I'm not at a steady state and voltage is slowly climbing, I'm in bulk stage and batteries are not at all full. If they are steady state at the bulk/absorption setpoint, then that means they are getting there. Not at all fully charged, but probably at least 3/4 or better? If I'm at float voltage steady state, then they're pretty much full.

First thing in the morning with no charge applied is a good indicator of battery SOC based on voltage. I always hope to see something greater than about 12.3V. :-)
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Old 03-14-2019, 05:11 PM   #13
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Originally Posted by ewarnerusa View Post
Peukert effect makes it more complicated than that.

<snip>

First thing in the morning with no charge applied is a good indicator of battery SOC based on voltage. I always hope to see something greater than about 12.3V. :-)
Makes sense...appreciate the explanation. And I agree...check it in the morning and go from there...
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Old 03-14-2019, 08:00 PM   #14
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That would work yes but is a bit convoluted. Don't forget the draw from your fridge circuit board and you LP gas detector and anything else you may use but forget to write down and... well, you get the point.
One other thing just to complicate the issue further. Battery ratings are not what you can use but what the battery is theoretically capable of holding. The rule of thumb is to go by only 80% of the total amp hours your battery is rated at.
So, a 210 amp hour battery is really a 168 amp hour battery which the usable portion is 84 amp hours (50%).
Using a battery monitor such as has been recommended the 80% is part of your programming of the monitor so what you read is what you have.
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Old 03-19-2019, 04:25 AM   #15
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I answered my own question by...

....opening the owner's manual. These cheap battery level panels, if worth anything at all, show the levels based on voltages for FLA cells.

And yes - the lowest light showing a low battery level corresponds to something below 20%, which is not advised for FLA batteries.
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Old 03-19-2019, 10:21 PM   #16
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Originally Posted by Wanderer View Post

Battery ratings are not what you can use but what the battery is theoretically capable of holding. The rule of thumb is to go by only 80% of the total amp hours your battery is rated at.
So, a 210 amp hour battery is really a 168 amp hour battery which the usable portion is 84 amp hours (50%).
Using a battery monitor such as has been recommended the 80% is part of your programming of the monitor so what you read is what you have.
50% of 80% is going too far. Neither is a requirement, only a rule of thumb.

When battery manufactures do test their batteries they use a procedure like this:
Charge to 100% (takes many hours of low current to finish the charge.)
Discharge, perhaps at the 20 hour rate down to 10%.
Repeat until battery capacity no longer exceeds 80% of new capacity.
The number of cycles to 80% of new is the cycle life.

The better battery manufacturers also test using a draw down from 100% to 50%. Each battery design is different, but typically this test gets twice as many cycles or slightly more.

The best battery manufactures also test using multiple draw from 100% to many different low levels. The results can be plotted on a nearly straight line with a 45 degree slope. A straight line would mean level of draw down is proportional to number of cycles.

Cycle life is not useful battery capacity. If you get twice as many cycles drawing down to 50% instead of 90%, it means you need twice the initial weight, space, and number of batteries. If weight, space, and initial investment is not an issue, go for the 50% draw down scenario.

So, you have batteries. The cost a significant amount. You want to preserve them, but you also want to USE them. How can you protect your investment?

The answer is, follow the battery maintenance procedures. The standard battery maintenance procedures matter far more the small gain using 50% rule of thumb.

The 50% draw down rule is a rule of thumb that protects against many other issues. The main one is that when flooded cell batteries are drawn down below 10%, bad non-reversible chemical things begin to happen inside the battery. Making an effort to not draw down below 50% protects against accidental draw below 10%. It is not a battery maintenance procedure needed to protect your batteries.

That said, it is not a good idea to plan to use 90% of battery capacity in your TT. Planning has a way of going wrong. Leave room for error. Even measuring the point of 10% is rough and is easily off significantly. On the other hand it is ok if you did use 90%. That did not hurt your batteries. If you went further and drew them down flat, that would be a bad sign.

AGM batteries are different. They are lead/acid batteries, but have major chemical and physical differences that allow them to be drawn down flat. Drawing down flat is not good for them, but two or three times in their lives is not a problem. On the other hand they are sensitive to overcharging. Overcharging causes venting. Any venting is permanent. Water can not be replaces.

Battery University
https://batteryuniversity.com/
Charging lead acid batteries https://batteryuniversity.com/learn/article/charging_the_lead_acid_battery
Summary of Do’s and Don’ts https://batteryuniversity.com/learn/article/do_and_dont_battery_table
There are many more good articles on the Battery University web site for those who want to know. You can spend a few days or a few months learning it all. I tend to forget as much as I learn.
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Old 03-21-2019, 12:27 AM   #17
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battery monitors

I love VideoArizona’s answer! A man after my engineer’s heart! Of course, if he had a current meter attached to the battery, he could make his calculations even more accurately.

Having a good battery monitor system will help keep your batteries in better condition, and make them last longer. I exclusively use LiFePO4 batteries now, and still use a high quality battery monitor. I expect my LFP batteries will outlive my 5er. I have had good luck with the Bogart 2030 (but use the 200A shunt, the 500A shunt is not accurate enough) and the Victron 702. Some of the Victrons actually have bluetooth that you can monitor your battery from your phone! Here is a low cost Chinese battery monitor that I’ve had good luck with, but you are always taking your chances on Chinese products.

https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

Remember, lead acid batteries have bad charge and discharge inefficiencies. This means that if you use 100AH out of your battery, you may have to put 115AH back in to fill them up again. Good luck and let us know what you finally settle on.
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