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Old 17-01-2014, 08:15   #1
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Technical question - bank state-of-charge question

I've got a strange battery bank question, it is a mix of the Xantrex battery monitor and battery technology.
My battery bank consists of 8 Exide A412/120Ah batteries in a 24V configuration. Because I'm a liveaboard and have things running, I can't let the batteries just rest for several hours, but after running through the night at 3A drain (which is equivalent to a discharge rate of 480Ah/3A = C160), which is a low drain, the voltage shows 24.55 after turning off everything for 10 minutes. This equates to 24.55/12 = 2.045V per cell.
The Xantrex tells me that the bank is almost 50% discharged (227Ah removed), which I am not sure is correct. I've sent a mail to Exide/Sonnenschein asking them for some open voltages vs. DoD data to ensure that I am getting correct data. I have a feeling that my battery bank has more charge than the Xantrex is displaying.
The A412 series is a VLRA dry gel battery that uses (if I read it correctly) calcium technology. When I charge the bank from the generator the system starts in Bulk mode until the voltage hits the recommend level, which in this case was 10 minutes, then the absorption phase at 28.1V sets in, starting at 80A and decreasing until, an hour and half later, the charger switches to float mode at 27.1V and is still pumping in about 20A.
At a constant discharge rate of C80, what should my battery voltage be at 50% DoD? I cannot find the Uf value for the A412/120 battery in the Docs, I think that value denotes the final voltage at 100% DoD and that the voltage to DoD state ratio is a linear one. The technical specifications for the A412/120 are at
http://www.sv-zanshin.com/manuals/so...ina412120a.pdf
http://www.sv-zanshin.com/manuals/so...n400series.pdf
http://www.sv-zanshin.com/manuals/so...book_part2.pdf
I'd welcome suggestions as to figuring out what my true battery stage-of-charge is.
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Old 17-01-2014, 09:02   #2
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Re: Technical question - bank state-of-charge question

Are you sure the battery monitor is resetting to 0 amps removed after you are charged? My link 2000 shows lower than actual voltage reading (according to my fluke multimeter), you can change the parameters on the monitor for when it resets to 0 amps removed (fully charged). Or you can reset to 0 manually. If you verify that the shunt is working properly (reads the correct amperage for whatever is running), and that the monitor is sensing the 24 volt bank, then amp hours removed should be correct, maybe you have a load cycling on that you don't realize. The monitor doesn't calculate state of discharge from the voltage, but from amp hours removed. It does use a combination of voltage and charging amps to determine when to reset to 0 (fully charged). You have to change these parameters according to your chargers set points and (in my case) the voltage reading error from the xantrex. I hope this helps, I just reconfigured my battery banks and got my link 2000 working so I have been studying the manual and learning to change the settings.
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Old 17-01-2014, 09:19   #3
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Re: Technical question - bank state-of-charge question

Quote:
Originally Posted by Zanshin View Post
I've got a strange battery bank question, it is a mix of the Xantrex battery monitor and battery technology.
My battery bank consists of 8 Exide A412/120Ah batteries in a 24V configuration. Because I'm a liveaboard and have things running, I can't let the batteries just rest for several hours, but after running through the night at 3A drain (which is equivalent to a discharge rate of 480Ah/3A = C160), which is a low drain, the voltage shows 24.55 after turning off everything for 10 minutes. This equates to 24.55/12 = 2.045V per cell.
The Xantrex tells me that the bank is almost 50% discharged (227Ah removed), which I am not sure is correct. I've sent a mail to Exide/Sonnenschein asking them for some open voltages vs. DoD data to ensure that I am getting correct data. I have a feeling that my battery bank has more charge than the Xantrex is displaying.
The A412 series is a VLRA dry gel battery that uses (if I read it correctly) calcium technology. When I charge the bank from the generator the system starts in Bulk mode until the voltage hits the recommend level, which in this case was 10 minutes, then the absorption phase at 28.1V sets in, starting at 80A and decreasing until, an hour and half later, the charger switches to float mode at 27.1V and is still pumping in about 20A.
At a constant discharge rate of C80, what should my battery voltage be at 50% DoD? I cannot find the Uf value for the A412/120 battery in the Docs, I think that value denotes the final voltage at 100% DoD and that the voltage to DoD state ratio is a linear one. The technical specifications for the A412/120 are at
http://www.sv-zanshin.com/manuals/so...ina412120a.pdf
http://www.sv-zanshin.com/manuals/so...n400series.pdf
http://www.sv-zanshin.com/manuals/so...book_part2.pdf
I'd welcome suggestions as to figuring out what my true battery stage-of-charge is.
Yes! This is the issue I am also asking about... amps to voltage doesn't make sense.
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Old 17-01-2014, 09:33   #4
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Re: Technical question - bank state-of-charge question

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Originally Posted by Thumbs Up View Post
Are you sure the battery monitor is resetting to 0 amps removed after you are charged? My link 2000 shows lower than actual voltage reading (according to my fluke multimeter), you can change the parameters on the monitor for when it resets to 0 amps removed (fully charged). Or you can reset to 0 manually. If you verify that the shunt is working properly (reads the correct amperage for whatever is running), and that the monitor is sensing the 24 volt bank, then amp hours removed should be correct, maybe you have a load cycling on that you don't realize. The monitor doesn't calculate state of discharge from the voltage, but from amp hours removed. It does use a combination of voltage and charging amps to determine when to reset to 0 (fully charged). You have to change these parameters according to your chargers set points and (in my case) the voltage reading error from the xantrex. I hope this helps, I just reconfigured my battery banks and got my link 2000 working so I have been studying the manual and learning to change the settings.
I synchronized the Xantrex (which is correctly set for bank size, voltage, shunt, Peukert, temp, etcl), and also measured the actual amp draw and charge using an ammeter and the shunt seems to be as accurate as my meter. I think that my batteries, when about 50% discharged, should be reading less than the indicated 24.55 volts and that the Xantrex is incorrect. Usually a battery monitor will read high due to some loads not going through the shunt, but in this case it is reading low (I think).
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Old 17-01-2014, 09:34   #5
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Re: Technical question - bank state-of-charge question

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

Link-series Charging Algorithms -- The "Gotcha" Factor!

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

You might also be interested in the parallel discussion on "Volts Vs. Amp Hrs." http://www.cruisersforum.com/forums/...24#post1442824
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Old 17-01-2014, 09:55   #6
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Re: Technical question - bank state-of-charge question

One of the points that I was making is that the voltage reading on my Link 2000 is incorrect (according to my fluke), I don't think there is any way to calibrate the voltage readout on the monitor (if anybody knows a way, please tell). The link 2000 reads about half a volt low, but not always, someone on the other thread mentioned this problem as well. If you can't trust the voltage on the monitor, it makes determining soc from voltage even more flukey. But you can trust amp hours removed on the monitor which is the reason to use the monitor in the first place. It just seems silly that some of our monitors can't seem to give reliable voltage readings.
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Old 17-01-2014, 10:16   #7
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Re: Technical question - bank state-of-charge question

I've got an accurate measurement from a voltmeter and from the Victron charger plus one from the instrument panel and then the one on the Xantrex and all are relatively close, within 0.1V, of each other. So the voltage reading isn't my problem, it is that I believe the Xantrex is reading more amps removed than I think are actually being removed. Even now, my bank is approaching 100% full (the genset has been running almost 4 hours) but the Xantrex still believes that it is at -106Ah, which would be 25% of the battery capacity. I actually have 3 chargers for the main bank, and I usually only run the Victron (since it can pump in 120Amps) and don't use the built-in 50Amp chargers, but I've now tried using all three separately and independently they all do about the same thing (same voltage/current charge) so I don't think it is my charging system.
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Old 17-01-2014, 10:50   #8
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Re: Technical question - bank state-of-charge question

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Originally Posted by Zanshin View Post
....My battery bank consists of 8 Exide A412/120Ah batteries in a 24V configuration. Because I'm a liveaboard and have things running, I can't let the batteries just rest for several hours.....what should my battery voltage be at 50% DoD?.....
The solution to measure the open circuit battery voltage to determine the SoC is quite simple - just disconnect one wire from one battery pair and leave for as long as you can - at least 30 minutes - and measure the open circuit voltage. If you can, leave for 4 hours and see if the voltage rises any higher. This will definitely give you no load and no charging. The next time you do it use a different pair of batteries.

Doing this often helps to check that all pairs are working the same.
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Old 17-01-2014, 11:04   #9
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Re: Technical question - bank state-of-charge question

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...but the Xantrex still believes that it is at -106Ah, which would be 25% of the battery capacity. I actually have 3 chargers for the main bank, and I usually only run the Victron (since it can pump in 120Amps) and don't use the built-in 50Amp chargers, but I've now tried using all three separately and independently they all do about the same thing (same voltage/current charge) so I don't think it is my charging system.
Have you reset the monitor when full? Have you read my frequently posted "Gotcha" topic?
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Old 17-01-2014, 11:39   #10
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Re: Technical question - bank state-of-charge question

The real problem here is the battery technology. It is not possible to get a definitive sate of charge reading for a sealed battery. Open circuit voltage is a good approximation but just that, an approximation. The only test is a hygrometer reading of acid density which you can't do. They system I work is to use the amp/hr counter as a fuel gauge and recharge at about 25% if charging is available. Recharge until the charge rate drops to about 5% of batt capacity then reset the monitor
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Old 17-01-2014, 11:47   #11
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Re: Technical question - bank state-of-charge question

I posted the same question on another forum and think that I've been given the correct answer to explain my reading discrepancy - the Peukert value. I used a conservative setting of 1.25 while I just checked my manufacturer and they state 1.06, this means that the Xantrex is subtracting more from my bank than I'm actually using, and the Peukert doesn't apply during the charging cycle. This, I believe, accounts for my discrepancy between reported and actual state-of-charge values and the fact that it grows pretty rapidly over time.

Stu - My charger is a Victron Charger/Inverter and the genset behind it is a 9.5Kw one, so it barely hiccups when I turn on even higher loads during charging. The last time a synchronize happened was 2 weeks ago when I was on shore power, but right now I'm biting the bullet and running the genset until the battery bank is fully charged and the Xantrex synchronizes (my setting is 1% current for 2 minutes, BTW) and then I'll see if the new Peukert 1.06 setting will result in a better value in week's time.

Thanks for all the help!
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Old 17-01-2014, 12:10   #12
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Re: Technical question - bank state-of-charge question

Zanshin, that's a good setting. Good luck. Please let us know how it turns out.
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