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Old 24-01-2023, 12:06   #1
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Calibrating Battery Monitor

So I have just installed a proper battery monitor with shunt for the first time. I'm confident I have the wiring all correct, with all the amps in or out passing through the shunt. The device is working as expected, with a very good read on charging from solar, battery charger, and alternator, as well as a good read when discharging, so I'm quite happy.

My question is about properly calibrating the device so that it knows when the batteries are full/empty, and what Ah capacity to use.

For a start, I told the device that I have 150Ah capacity. I have 4 105Ah AGM batteries, so effective capacity should be 210Ah. The batteries are 3 years old, so I discounted that value down to 150Ah as a pure guess. I'm wondering if anybody smarter than me has a way to refine that guess and get a little more confidence behind that number. I've read a little bit about ways to test that number, but they're all pretty extensive and I'm not sure I have the capability to perform the test accurately, ie I don't think I can provide a consistent load for 20 hours, or anything like that.

Secondly, I have to tell the device where to start counting amps when the batteries are either full or empty. I set the monitor to full after running the genset and charger for a couple of hours, and the unloaded voltage was up at 13.2. But I had an idea on how to do a bit better. If I let the batteries get nice an full on the solar, shouldn't the monitor tell me when amps stop flowing into the battery? And once that happens, shouldn't I be confident that they are full? Right now, the monitor reads 10 amps flowing in, and voltage all the way up at 13.5. I'm surprised that I don't see the amps flowing in coming down at this point. Any thoughts? Surely amps can't flow into the batteries indefinitely.

Thirdly, the device wanted me to input several additional values, listed below. I've included notes on what I've done and how I'm thinking about each one, but input and advice on where to set each of these is appreciated.

1. Full Voltage. When the voltage is higher than this value, the device will reset to 100% capacity. I initially put this in as 13.4V, thinking the batteries would certainly be "full" if the voltage got that high. However, when I run the battery charger, it gives an input voltage of 13.8, so it reset the capacity to full as soon as I turned the charger on. Should I just set the Full Voltage value up to 14 or so to avoid this? I don't think the batteries are ever meant to have that much voltage but I'm not sure. I also have a new charger to install that I think will provide an input of up to 14.2, so I might have to go even higher? I'm unsure both about how high an input voltage is safe for these batteries, and how to interpret the charger input voltages vs where I should set this value on the monitor.

2. Zero Voltage. When the voltage is at or below this level, the capacity will be set to 0. I really have no idea where to set this number, since the voltage might drop pretty sharply under a big load. My understanding is that the voltage can safely go down to about 10.5 under load, but only if there is sufficient Ah capacity remaining to provide the power. If it gets down below about 11.5 or somewhere around there without a load, then that would be too low. So where should I set this number to avoid false alarms but also protect the battery?

3. Power Off Voltage. The voltage that will trigger the battery monitor to power off to avoid the monitor itself using too much power and damaging the battery. I set this at 0.1V above the Zero Voltage value so that the device will shut off just before the batteries reach zero voltage.

4. Alarm. The Ah remaining value where the device will start yelling at me to charge. I put this at 10Ah. This seems pretty straightforward, but I may want to give myself a little more cushion than that in case my total capacity estimate is off. Not sure how many layers of conservatism I should build in here. If I'm conservative on my total Ah capacity estimate, then I should be able to set this quite low safely.

5. Attenuation Ratio. This is a percentage that the device will automatically reduce my total capacity by every cycle. I have it at 0 for now, but I'm hoping someone here will have an idea of how fast AGMs degrade?

Thanks very much to anyone that read all the way through!
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Old 24-01-2023, 15:20   #2
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Re: Calibrating Battery Monitor

The only way to know the capacity of the batteries is to test them, and yes it can be involved. But it doesn't have to be. If you have a load close to 10A (close to the current for a 20 hour rate for your bank) then from full charge run them to 10.5V. Then do some math.

Guessing as you have isn't going to be very accurate, but at least is better than assuming they still have full capacity.

To set full, yes, you look at the charge current, also called tail current. Roughly 4% of the batteries capacity while charging at absorption rate. So, 8.4A for a 210Ah battery. As batteries age, it will increase, so maybe 6% if not new. Note that this needs to be at absorption rate, so charging by solar isn't a good way to do this, as solar voltage and current change with sunlight.

What brand battery monitor is this? The settings you have available are odd. There should be things like tail current, efficiency, and peukert.

Here is a very good reference and explanation:
https://marinehowto.com/programming-a-battery-monitor/
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Old 24-01-2023, 15:22   #3
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Re: Calibrating Battery Monitor

Assuming the settings on your charge controllers (solar, shore, alternator) are correct, you can use any one of them to charge to full, and then set the SOC on the monitor to 100% after.

AGM batteries take a long time to actually get to 100%. The voltage will show higher when charging than at rest. And your battery monitor is smart enough to know that.
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Old 24-01-2023, 16:06   #4
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Re: Calibrating Battery Monitor

Quote:
Originally Posted by wholybee View Post
The only way to know the capacity of the batteries is to test them, and yes it can be involved. But it doesn't have to be. If you have a load close to 10A (close to the current for a 20 hour rate for your bank) then from full charge run them to 10.5V. Then do some math.

Guessing as you have isn't going to be very accurate, but at least is better than assuming they still have full capacity.

To set full, yes, you look at the charge current, also called tail current. Roughly 4% of the batteries capacity while charging at absorption rate. So, 8.4A for a 210Ah battery. As batteries age, it will increase, so maybe 6% if not new. Note that this needs to be at absorption rate, so charging by solar isn't a good way to do this, as solar voltage and current change with sunlight.

What brand battery monitor is this? The settings you have available are odd. There should be things like tail current, efficiency, and peukert.

Here is a very good reference and explanation:
https://marinehowto.com/programming-a-battery-monitor/
Thanks for the response! I do have several follow-up questions:

How precise should the 10A load be if I try to do the test? I can probably come pretty close to that with my computer if I keep it cooking all day and all night, but it will definitely fluctuate, probably erring on the high side. Say 10-14 amps. That seems like a pretty big range and might give me crap results. I'm not sure what other combo I might come up with for a 10A draw though.

1. If I do run the test, I know I can't charge during that time. I would probably just throw a blanket over the solar panels vs. disconnecting them. Any reason to think that's not good enough?

2. For setting full: I want to make sure I'm understanding you correctly. I need to supply voltage to the batteries at the Absorption Charge level. I'm reading that AGMs want 14.5-14.8V for absorption charging. I then watch the battery monitor until I see 4-6% of Capacity in Ah incoming in A. At that point, I can tell the monitor the battery is full and begin tracking. Is this 4-6% value of effective capacity or nameplate capacity? In your example you said 8.4A for a 210Ah battery, so I'm assuming effective capacity, but I wanted to double check.

3. I'm perfectly willing to accept your expertise on this, but I'd still like to understand why I shouldn't use the solar for setting full. It seems like as the battery approaches full, it should accept fewer amps from any charging source. So on a long sunny day with no loads on, I should get up to the point where close to 0A comes in. But today I charged via the genset until my rough-set monitor thought I was at 100% charge, then let the sunny solar put ~10A in for maybe 8 hours. So theoretically, I put another 80Ah into the batteries through the day. It seems unlikely that the batteries were really only 60% (or less) full when I switched off the genset, but I never saw any decline in the rate at which the batteries accepted charge, which seems very odd to me. Since you seem very fluent in all of this, I wondered if you could help me understand.

Finally, to answer your question: This is a Renogy monitor using a 500A shunt. The manual makes no mention of tail current or peukert.
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Old 24-01-2023, 16:08   #5
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Re: Calibrating Battery Monitor

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Originally Posted by SV Confianza View Post
The voltage will show higher when charging than at rest. And your battery monitor is smart enough to know that.
I actually observed the monitor jump from ~85% to 100% the moment the 40A charger turned on and supplied a voltage above my "Full Voltage" setting. So clearly the battery wasn't full at that point, but the way I had set the settings forced it to read full. I guess you're saying that it's smart enough to know that I will set the "Full Voltage" setting up in the 14.xV range and handle that input correctly?
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Old 24-01-2023, 16:45   #6
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Re: Calibrating Battery Monitor

It would be helpful to clue us in to what "device" you have. Why keep it a secret?


You seem to misunderstand your bank capacity. For a coulomb counter you use the total battery capacity, not half, and usually not discounted until you're really sure they're bad, and if so, time to replace anyway.


You may be interested in these:


For everyone installing a battery monitor: The "Gotcha Algorithm" thread, a "MUST READ"

http://c34.org/bbs/index.php/topic,4922.0.html

DEFAULTS are factory settings that are made to be modified to suit your setup.

Also read this one:

Programming a Battery Monitor (by Maine Sail)

https://marinehowto.com/programming-a-battery-monitor/
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Old 24-01-2023, 18:52   #7
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Re: Calibrating Battery Monitor

Quote:
Originally Posted by Stu Jackson View Post
It would be helpful to clue us in to what "device" you have. Why keep it a secret?
It's a Renogy battery monitor with 500A shunt. I don't have any kind of specific model number, but I think they really only make the one. I should have put that in the original post, but the reason I keep referring to it as "device" is because calling it a "renogy battery monitor" over and over would be annoying.


Quote:
Originally Posted by Stu Jackson View Post
You seem to misunderstand your bank capacity. For a coulomb counter you use the total battery capacity, not half
For AGM (and most lead acid types) you only want to discharge to 50% DOD, resulting in a usable Ah capacity of half the nameplate capacity. I programmed my monitor to use this usable capacity, which means I can run it down to near 0 Ah capacity remaining (if I get everything calibrated, which is a challenge). Alternatively I could program the full nameplate capacity and only run it down to 50% Ah capacity remaining. I'm not sure there's really much difference between the two methods, but I feel like 0% = "out of gas" is more idiot proof.


Quote:
Originally Posted by Stu Jackson View Post
and usually not discounted until you're really sure they're bad, and if so, time to replace anyway.
This is contrary to all the other advice I've seen here and in the very articles you've provided for me to read. In the marinehowto article, it has a nice in depth discussion on how an AGM's capacity may NEVER be as high as nameplate, because lead acid batteries have to "cycle up" to full capacity, but by the time they do they've often been abused enough to already be somewhat worn out. My batteries are 3 years old, and going off the examples in the marinehowto article, they are likely somewhere around 25% less capacity than when they were new.

Also, the marinehowto article is the same one that Wholybee provided in their much more helpful reply.

Quote:
Originally Posted by Stu Jackson View Post
DEFAULTS are factory settings that are made to be modified to suit your setup.
...yes... which is... why I posted this thread...
Also, my charger came with all of the inputs default to zero, except Ah which was default set to 100Ah. These are so ludicrously wrong as to make the readings nonsensical until I change the defaults, so really no risk of me leaving them alone. Which is I guess better than if Renogy had set defaults to reasonable guesses for most users. Then it might not have been immediately obvious to the user that these inputs need to be changed.
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Old 24-01-2023, 19:14   #8
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Re: Calibrating Battery Monitor

Quote:
Originally Posted by JebLostInSpace View Post
Thanks for the response! I do have several follow-up questions:

How precise should the 10A load be if I try to do the test? I can probably come pretty close to that with my computer if I keep it cooking all day and all night, but it will definitely fluctuate, probably erring on the high side. Say 10-14 amps. That seems like a pretty big range and might give me crap results. I'm not sure what other combo I might come up with for a 10A draw though.

1. If I do run the test, I know I can't charge during that time. I would probably just throw a blanket over the solar panels vs. disconnecting them. Any reason to think that's not good enough?

2. For setting full: I want to make sure I'm understanding you correctly. I need to supply voltage to the batteries at the Absorption Charge level. I'm reading that AGMs want 14.5-14.8V for absorption charging. I then watch the battery monitor until I see 4-6% of Capacity in Ah incoming in A. At that point, I can tell the monitor the battery is full and begin tracking. Is this 4-6% value of effective capacity or nameplate capacity? In your example you said 8.4A for a 210Ah battery, so I'm assuming effective capacity, but I wanted to double check.

3. I'm perfectly willing to accept your expertise on this, but I'd still like to understand why I shouldn't use the solar for setting full. It seems like as the battery approaches full, it should accept fewer amps from any charging source. So on a long sunny day with no loads on, I should get up to the point where close to 0A comes in. But today I charged via the genset until my rough-set monitor thought I was at 100% charge, then let the sunny solar put ~10A in for maybe 8 hours. So theoretically, I put another 80Ah into the batteries through the day. It seems unlikely that the batteries were really only 60% (or less) full when I switched off the genset, but I never saw any decline in the rate at which the batteries accepted charge, which seems very odd to me. Since you seem very fluent in all of this, I wondered if you could help me understand.

Finally, to answer your question: This is a Renogy monitor using a 500A shunt. The manual makes no mention of tail current or peukert.
The 10Amps doesn't need to be that precise. But it can't fluctuate. Whatever it is, needs to stay as stable as possible. A computer is a poor choice. If you have an inverter, a 100W light bulb should get you in the ballpark. Make a note of the current it uses, and use that.

1. A blanket is probably fine.

2. It's a moving target. I would go with nameplate capacity. But understand that as the battery ages the tail current will go up. If the batteries are a few years old, you may never get to 4%, regardless of if you use nameplate or actual capacity. Some judgment is needed. Try it, watch it, and figure out what the tail current is.

3. The issue is that you might not reach full absorption voltage because of not enough sunlight, or if you do, then a cloud goes by and the current drops, and it looks as if you have reached 100%, but you really have not. You could use solar to charge as much as you can, but would need to start the generator to get the last bit in, and to read the voltage and current. This is similar to the Gotcha that Stu posted about. Another Gotcha.

I am not familiar with the setup of the Renogy meter, but it should NOT reset to 100% at a programed voltage. You should need to program the voltage AND tail current. So, when you connect your charger and the voltage jumps up, it doesn't reset to 100% because there is a high current flow. Then, when it reaches the tail current you programmed in, it resets to 100%.
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Old 24-01-2023, 19:33   #9
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Re: Calibrating Battery Monitor

So after reading the marinehowto article a couple of times, I think I'm going to forgo the capacity test for now. Even though it's probably possible to get it close enough to right to be useful. It requires at least 2 days where my house bank is not in use. Since I'm living aboard at anchor, that's really not very feasible. That would be two days with no fridge, no water pressure, no anchor light, no phone or laptop charging, no cabin lights or fans etc etc etc. That's a crappy 2 days right there. I'll probably try to conduct a test the next time I'm dock-bound for a few days. At least then I can run things through the AC panel without touching the house batteries.

In the meantime, I think I have enough understanding about the tail current now that I can periodically manually re-set the counter to full when I'm using the genset. Then at least counting errors won't be able to compound for very long. Combine that with an even more conservative estimate of my battery's actual capacity - say 130Ah - and I should at least be able to avoid damaging the batteries. And I'll get all the benefit of having a much more accurate measure of discharge rate and consumption, which will certainly help.

It seems like this Renogy monitor is significantly less smart than the latest and greatest monitors available. That's probably why it was half the price of a Victron. Mine definitely does reset to 100% when it sees the "Full Voltage" value that I input. So I'll set that threshold super high, maybe 15V, and stick with manual resets to 100% instead. It doesn't seem to have any of the smarts around temperature, peukert, or charging efficiency either. So I'll have to keep some of that info in my head, and just try to avoid discharging too close to my calculated 50% DOD. That way the inevitable errors that do add up hopefully won't cause me to damage the batteries.

I guess the upside is I'm learning all this on 3 year old batteries that are probably halfway killed anyway. When I replace batteries in the next year or two, I'll have a much better understanding of all this and be able to take care of my new ones better.
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