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  #1  
Old 01-17-2014
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Battery state-of-charge question - technical

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 01-17-2014
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Re: Battery state-of-charge question - technical

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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Old 01-17-2014
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Re: Battery state-of-charge question - technical

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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Old 01-17-2014
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Re: Battery state-of-charge question - technical

What is the Peukert law offset you have set up for?
It can greatly effect the status of charge.

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Old 01-17-2014
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Re: Battery state-of-charge question - technical

One word Peukert...

I also doubt your charger is getting the bank full as it sounds like a "dumb" timed algorithm...
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Last edited by Maine Sail; 01-17-2014 at 01:20 PM.
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Old 01-17-2014
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Re: Battery state-of-charge question - technical

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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Old 01-17-2014
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Re: Battery state-of-charge question - technical

Quote:
Originally Posted by Zanshin View Post
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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Old 01-17-2014
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Re: Battery state-of-charge question - technical

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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Old 01-17-2014
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Re: Battery state-of-charge question - technical

Quote:
Originally Posted by Zanshin View Post
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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Old 02-04-2014
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Re: Battery state-of-charge question - technical

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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