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Discussion Starter · #1 ·
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/sonnenscheina412120a.pdf
http://www.sv-zanshin.com/manuals/sonnenschein400series.pdf
http://www.sv-zanshin.com/manuals/sonnenschein_gel_handbook_part2.pdf

I'd welcome suggestions as to figuring out what my true battery stage-of-charge is.
 

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Sorry, but with only a 10 minute rest your voltage readings are going to be nothing more than extremely "ballpark." You need to let them rest for a couple of hours at the very least, to get a meaningful reading.
 

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Discussion Starter · #3 ·
Since my rate of discharge is constant at C180 or less overnight and I'm a liveaboard I don't have any choice in letting the batteries rest too long. I am not looking for a 100% answer, but a ballpark figure. The voltage after 10 minutes no longer changed, and the drain was down to .5A (the monitor itself, standby current for watermaker and Victron, SSB 24V-12V regulator (100mA), etc.) which is as good as I'm going to get.
 

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Discussion Starter · #6 ·
ChucklesR & MainSail - I think that might be it. I kept my very conservative Peukert setting at 1.25 and didn't think it would affect the reading, since I mistakenly assumed that the value was identical between charging and discharging. But it is not a symmetrical equation and I've just run the formula and the difference between my 1.25 setting and the manufacturer's (optimistic) value of 1.06 could account for my discrepancies growing over time. Even though I rarely use more than 8 amps (on a 480 Ah bank), that can account for the discrepancies I've been seeing.

I've changed the Xantrex to 1.06 and am charging the battery bank to 100% for a synchronization (I hate doing this on the genset and I've made all the water I can already) and then I'll see if the numbers look better in a couple of days.

Thanks!

p.s. I just remembered that I can turn on the watermaker again and wash the decks with freshwater so that my poor genset doesn't idle itself to death. I think I can find some extra gallon water jugs in the galley storage and fire up the freezer as well.
 

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ChucklesR & MainSail - I think that might be it. I kept my very conservative Peukert setting at 1.25 and didn't think it would affect the reading, since I mistakenly assumed that the value was identical between charging and discharging. But it is not a symmetrical equation and I've just run the formula and the difference between my 1.25 setting and the manufacturer's (optimistic) value of 1.06 could account for my discrepancies growing over time. Even though I rarely use more than 8 amps (on a 480 Ah bank), that can account for the discrepancies I've been seeing.

I've changed the Xantrex to 1.06 and am charging the battery bank to 100% for a synchronization (I hate doing this on the genset and I've made all the water I can already) and then I'll see if the numbers look better in a couple of days.

Thanks!

p.s. I just remembered that I can turn on the watermaker again and wash the decks with freshwater so that my poor genset doesn't idle itself to death. I think I can find some extra gallon water jugs in the galley storage and fire up the freezer as well.
I strongly suspect that your bank is not a 1.06, though a lot of Chinese manufactures come up with some awful fuzzy math on Peukert.... 1.06 is pushing down into LiFePO4 territory.. Even the best performing AGM's such as Odyssey, a 1.112, & Lifeline a 1.125 are higher than your batteries? I don't know of too many AGM's that can outperform Odyssey or Lifeline at high loads...

Keep in mind that you are only "full" when current at ABSORPTION voltage falls to less than 2% of bank capacity. This is still not technically full but full enough for a batt mon reset. ALWAYS manually reset the monitor when the bank is known full.....They key is that you need to see less than 9A flowing onto the bank at 14.4V - 14.6V...

Many are chargers are FAR DUMBER than we give them credit for and simply work on time, regardless of how big or small your bank is. Some Charles chargers literally start a 4 hour clock from the moment they turn on before dropping to float. You can still be in bulk with some Charles chargers when they trip over into float and completely bypass absorption......... I have literally seen cases where a Charles Charger took 4 hours and was still at 14.2V and not even yet at absorption voltage. It hit 4 hours and dropped into float... NOT FULL...... Some chargers can be tricked by shutting it down and restarting the time-clock all over...

Peukert:
As an example if you use 1.25 and did a capacity test on your bank using an 8A load you would get 632Ah's out of the bank before hitting 10.5V..

If you correct that to 1.12 you get 548Ah's at an 8A load...

A Peukert of 1.0 at 8A would yield 480Ah's and a Peukert of 1.0 at 240A would yield 480Ah's but no 1.0 battery exists, Li comes close though....
 

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Discussion Starter · #8 ·
I ran the genset until I was loading 9A (at 24V) in the 480Ah bank so I'm as close to 100% as I am going to get without going to shore power, which I don't really want to do for another 2-3 months.
The Victron charger is pretty smart and doesn't work on time, just on voltage and amperage so I'm fairly confident that I'm doing a good charge (it has a specific setting for my battery's bulk/absorption and trickle voltages. The other 2 chargers on the boat work on a somewhat higher absorption voltage but deliver similar results as well, but I don't usually use them. I have a good constant load with my fridge using most of the power here in the hot subtropics, I occasionally will make an espresso that uses a 50A but only short-term (10-15 seconds) but apart from that my usage rarely will go above a value of 10A, even the autopilot in difficult seas will use 15A or the electric winch will draw more, but only for a couple of seconds at a time.

I've re-checked and the Sonnenschein / Exide give 1.06 for their AGM batteries. I'm not too sanguine about that optimistic value, but the 1.25 would seem to have been very incorrect. I'll update this thread in a week after several recharge cycles and we'll see if changing the Peukert value made my Xantrex state-of-charge reading more reliable.
 

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I ran the genset until I was loading 9A (at 24V) in the 480Ah bank so I'm as close to 100% as I am going to get without going to shore power, which I don't really want to do for another 2-3 months.
The Victron charger is pretty smart and doesn't work on time, just on voltage and amperage so I'm fairly confident that I'm doing a good charge (it has a specific setting for my battery's bulk/absorption and trickle voltages. The other 2 chargers on the boat work on a somewhat higher absorption voltage but deliver similar results as well, but I don't usually use them. I have a good constant load with my fridge using most of the power here in the hot subtropics, I occasionally will make an espresso that uses a 50A but only short-term (10-15 seconds) but apart from that my usage rarely will go above a value of 10A, even the autopilot in difficult seas will use 15A or the electric winch will draw more, but only for a couple of seconds at a time.

I've re-checked and the Sonnenschein / Exide give 1.06 for their AGM batteries. I'm not too sanguine about that optimistic value, but the 1.25 would seem to have been very incorrect. I'll update this thread in a week after several recharge cycles and we'll see if changing the Peukert value made my Xantrex state-of-charge reading more reliable.
Zanshin,

Sorry I should have converted all that to 24V numbers. You should see less than 9A at 28.8V and then you are full.
 

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Discussion Starter · #10 ·
Update - I re-synchronized the battery bank after 11 days and the Xantrex was at -76Ah when it synchronized (the charge went to below the setting of 2% capacity for 2 minutes). I am not sure if the changes to the Peukert setting made it better, or if it was the shorter period between synchronizations this time.
 

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Update - I re-synchronized the battery bank after 11 days and the Xantrex was at -76Ah when it synchronized (the charge went to below the setting of 2% capacity for 2 minutes). I am not sure if the changes to the Peukert setting made it better, or if it was the shorter period between synchronizations this time.
I DO NOT use auto synch and do not recommend it unless your only charge sources are alternator or shore charger. Even then many chargers go into float prematurely and trick them. With solar or wind I suggest programing the monitor so it never automatically syncs and instead synch it manually...

Battery monitors DO NOT re-set on Ah's they reset on current, voltage and time at current/voltage. They are EASILY tricked into re-setting prematurely. This is a CHRONIC problem with all Ah counting battery monitors that very few are aware of...

I have manually reset for the better part of 20+ years and honestly feel it is the ONLY way to keep an Ah counting battery monitor accurate.
 

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I DO NOT use auto synch and do not recommend it unless your only charge sources are alternator or shore charger. Even then many chargers go into float prematurely and trick them. With solar or wind I suggest programing the monitor so it never automatically syncs and instead synch it manually...

Battery monitors DO NOT re-set on Ah's they reset on current, voltage and time at current/voltage. They are EASILY tricked into re-setting prematurely. This is a CHRONIC problem with all Ah counting battery monitors that very few are aware of...

I have manually reset for the better part of 20+ years and honestly feel it is the ONLY way to keep an Ah counting battery monitor accurate.
Like Maine Sail said (paraphrasing) the real problem is that multiple charging sources are using different chargers that all want to do things their own way, and most of them get confused by the battery bank readings because other chargers can raise the voltage and "trick" a charger into thinking the state of charge is different than it really is. Even the current sensing "smart" chargers get confused when other chargers get involved and start dumping their own current into a bank.

Maybe some day venders will come together and create a standard to communicate this information among chargers so that they all know what is going on. I don't think we should hold our breath waiting for that to happen.
 

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Like Maine Sail said (paraphrasing) the real problem is that multiple charging sources are using different chargers that all want to do things their own way, and most of them get confused by the battery bank readings because other chargers can raise the voltage and "trick" a charger into thinking the state of charge is different than it really is. Even the current sensing "smart" chargers get confused when other chargers get involved and start dumping their own current into a bank.

Maybe some day venders will come together and create a standard to communicate this information among chargers so that they all know what is going on. I don't think we should hold our breath waiting for that to happen.
It has nothing to do with confused chargers.... It has to do with sources such as wind or solar tricking the battery monitor itself into thinking the bank is full by replicating "full" parameters. This can happen early or late in the day, with cloud cover or in light winds.

Let's say the BM resets at less than 2% accepted current and 13.4V for XX time. The batts are not yet full but the sun gets clouded/occluded and the panel can only supply less than 2% current and can't get the bank above 13.4V or is holding it at 13.4V for XX period of time.

This simply tricks the monitor into thinking "full" parameters have been met by replicating full parameters when really we only had some clouds move in and trick the BM into thinking it is full.

Chargers simply regulate voltage and their current supply is either above their ON voltage set point which simply turns them OFF, or they are below the voltage set point which turns current supply ON.....

No magic, no trickery, between charge sources they simply operate on voltage. The source with the highest voltage setting, that can maintain the acceptance current needed to hold that voltage, wins out and other sources drop out as they are not needed, acceptance wise..... If a charge source can't maintain the current to hold the voltage other sources will simply come back on-line. None of this occurs until the system is into the acceptance voltage range....
 

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Discussion Starter · #14 ·
Main Sail - I've mentioned this earlier in the thread. I have 3 distinct, separate chargers for my main bank which I can (and do) manually select. In this case the main charger [Victron] went into float mode and was charging under 9A for a good 10 minutes (my battery bank has 480Ah, so this is under 2% of capacity). Then I turned off the charger, and started up the other charger [Cristec] with the same Bulk/Absorption/Float settings as the main charger. It soon went into float mode with 8A and stayed that way for 10 minutes as well. At this point I checked the Xantrex reading which was -76Ah, then I set the Xantrex to auto-synchronize on 2% and 2 minutes and it then synchronized and read full.

I believe that this hasn't anything to do with false synchronize on the Xantrex, the batteries are indeed almost full, at least as full as I'm going to get while cruising unless I have to motor for extended periods.

The Xantrex either thinks I am taking more out of the bank than I really am, or it isn't seeing all the electricity going in (I have no solar or wind, just the chargers or alternator (I haven't run the main engine since last synchronize so rule that out)). Spot checks with an ammeter on both charge and load concur with the values on the Xantrex. I could understand it if the Xantrex read the other way, meaning that I have some load that isn't going through the shunt.
Right now I think it might be a firmware problem in the Xantrex, perhaps because I'm in 24V mode rather than the more common 12V. I've got a microchip breadboarded together(Atmel ATMega 328 and an Arduino board) programmed to keep a history of voltage and amps but am awaiting delivery of the Hall-Effect ammeter component since I don't want to put in another shunt.
 

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The 2% is not measured at float voltage it is measured at acceptance voltage or 28.8V... Not a huge deal, you are still pretty full, but the acceptance should be less than 2% of C at 28.8V before the bank and the monitor can be considered full.

Which Xantrex do you have? Many of the older Links. Link 10, Link 1000, Link 2000 etc. are now suffering chronic drift issues, I have a whole box of dead ones......... One of them added up -760Ah's use on a start battery over 7 days.....

If your Arduino can use a 50mV shunt then no need for a second shunt, just use the one you have. It is a simple no current mV reading and you can put more than one ammeter on a shunt provided it is calibrated for 50mV...

I've not found any hall effect sensors for DC that are really accurate at low current. Please let me know how yours works at sub 1A because I am always on the lookout......
 

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It has nothing to do with confused chargers.... It has to do with sources such as wind or solar tricking the battery monitor itself into thinking the bank is full by replicating "full" parameters. This can happen early or late in the day, with cloud cover or in light winds.

Let's say the BM resets at less than 2% accepted current and 13.4V for XX time. The batts are not yet full but the sun gets clouded/occluded and the panel can only supply less than 2% current and can't get the bank above 13.4V or is holding it at 13.4V for XX period of time.

This simply tricks the monitor into thinking "full" parameters have been met by replicating full parameters when really we only had some clouds move in and trick the BM into thinking it is full.

Chargers simply regulate voltage and their current supply is either above their ON voltage set point which simply turns them OFF, or they are below the voltage set point which turns current supply ON.....

No magic, no trickery, between charge sources they simply operate on voltage. The source with the highest voltage setting, that can maintain the acceptance current needed to hold that voltage, wins out and other sources drop out as they are not needed, acceptance wise..... If a charge source can't maintain the current to hold the voltage other sources will simply come back on-line. None of this occurs until the system is into the acceptance voltage range....
The difference between what I said and what you just wrote is so subtle I'm surprised you made an issue of it.

My solar charger gets totally confused by both my wind generator and my shore power charger because of the voltage set points (I would say "like you said", but I don't feel like another lecture lol). The voltage sense circuit is essentially a dumb battery monitor, but the solar charger I use is much smarter than that, which causes problems because whenever the wind blows it forces the voltage above a certain point and can reset the solar charger into an undesirable cycle. The charger may reduce its power output when the wind is blowing and then when the wind stops suddenly drop voltage and leave one of its acceptance charge states and move back into bulk.
 

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I have my BM auto sync voltage set at 14.4V and 2% charge and the BM to not go into float till 14.6V. It worked fine in auto all last year on solar while out on a mooring with the frig on all the time.
 

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One word Peukert...

I also doubt your charger is getting the bank full as it sounds like a "dumb" timed algorithm...
Bingo and we have a winner. Switch the charger off for 2 minutes and then on again and it will probably start in bulk mode again
 

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Bingo and we have a winner. Switch the charger off for 2 minutes and then on again and it will probably start in bulk mode again
I do that with my shore charger when it gets in a strange state, I installed a switch so I could disconnect its sense circuit from the battery bank and totally reset it because it was causing problems.
 
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