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Old 03-13-2019, 11:45 PM   #11
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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 Ö 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 Yesterday, 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 Yesterday, 10:21 PM   #16
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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 Doís and Doníts 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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