Longer absorb charging for 48V systems?

terrynew · January 2011

Hi, I'm the proud but overwhelmed owner of a new off-grid solar house, and of course there are teething problems. May I ask your comments on three of them? One is that a Surrette technical support manager I've phoned recommends a 6.3 hr absorb charge, compared to 3 hrs recommended by the inverter/charger technical support staff and two local solar companies.

I have sixteen S600 batteries, two banks of eight = 48 V and 900 Ah @ C20. The Surrette guy says the ideal absorb charging is 57.6 V at 100 Amps. But because my Magnum 4448 inverter charges at 60 A max (30 A per string), he says it'll take a lot longer than ideal to charge my batteries. Using the Surrette charging equation, the Absorb time should be 0.42 * C20 / I, where C20 is the bank's Ah at the 20 hr rate, I is the current, and 0.42 is 20% / 50% + 5% losses (assuming that over the time of the absorption charge that 50% of the maximum current will be available). This is 0.42 * 900 / 60 = 6.3 hrs for my bank.

Has anyone else ever had such a long absorb time recommended? I wonder if this is just a battery manufacturer's perfect ideal and a generator fuel waste for me.

The time would be much lower with a higher charging rate on the inverter/charger. Do all 48 V chargers have this low charging current and thus long charging time?

Secondly, the Surrette guy also says I can only equalize with the generator, not the panels. That's because my 2.5kW PV array can only output 43 ADC (2560 W / 57.6 V), so charging the batteries by sun will take even longer than charging by generator/inverter (43 A vs 60 A charging). This comes out to 10-11 hrs, which of course is longer than a day (or two, in winter)'s sunshine. He also says this means that almost all of my solar charging cycles won't charge the batteries past 90% full, so I should equalize monthly (at least in winter). Anybody care to support or comment on these recommendations?

And thirdly, the Magnum support tech says I shouldn't pay attention to the inverter/charger's "AC In - VDC" setting, which is currently 45.2 to 56.4 V. He says that's because it's for when there's an alternate charging source beyond the PV array and a generator (e.g., wind or hydro). But I don't see a way to disable this setting, so do I just ignore whatever voltage range it has? Or will it affect my generator charging, for example conflicting with the Auto Gen Start at 45.2 V and Auto Gen End at 59.6 V?

Thanks for your generous offering of time to help out the Clueless like me, and have a great day.
...Terry

mike95490 · January 2011

Re: Longer absorb charging for 48V systems?

Always best to follow the Mfg's recommendations.

While it seems like a long absorb time, it's at a lower voltage than my batteries want for absorb. (59V 3 hrs)

This makes sense, as lower absorb voltage, will need longer time to come to full charge. For a solar system though, it's not workable, as there are not enough hours to get Bulk, Absorb and then finally to float.

You can start with running the genset early AM to complete bulk, and let solar take over the absorb.

Do not let things go to where you are damaging your batteries. Shed some loads, run the genset, but get the batteries to full charge every 3rd day at least.

But, after a short time in absorb, you should see the amps start to drop off, then you can add some loads after noon time.

is 900AH your total capacity @ 48V ? Your solar is a bit short to keep up with that, so adding more PV, and another controller would be a first step.

How about the firmware, do you have the latest versions in your system ? There was some buggy outback firmware that was messing up charge. I'm not an outback owner, so I didn't follow it too much, maybe someone else will sound off.

I have 3KW, 400ah, and it's marginal till the longer days of summer, with no overnight loads (but I pump a lot of water for the farm in summer)

You have twice my battery, and less than my solar, so there are going to be problems. You will be spending $ for diesel fuel, or else buying new batteries.

RCinFLA · January 2011

Re: Longer absorb charging for 48V systems?

Six hours on absorb is going to keep you busy replenishing water in cells.

If you can avoid AC loads for a charging test, take the profile of the current drop off versus time during absorb stage. Once it quits dropping off in current it doesn't need much more time at absorb voltage. The absolute current should be less then about 3% of AH rating of batteries in DC amps.

As batteries age, this time will increase.

blackswan555 · January 2011

Re: Longer absorb charging for 48V systems?

One is that a Surrette technical support manager I've phoned recommends a 6.3 hr absorb charge, compared to 3 hrs recommended by the inverter/charger technical support staff and two local solar companies.

They are both right,,,,, Some times, Depending on a lot of factors, Some day`s you will only need 3 ish hrs of absorb, some a lot longer, The "rule of thumb" there being, bulk is 80% of the charge, it takes 20% of the time (With correct charge current) Absorb is 20% of the charge but takes 80% of the time, The only real way to determine is through SG`s or 2 nd best, Return amps,
I would recommend you get yourself 2 good hydrometers (you will break 1) to be able to "read" your SG`s & SOC properly to start,

Has anyone else ever had such a long absorb time recommended? I wonder if this is just a battery manufacturer's perfect ideal and a generator fuel waste for me.

In certain cases I recommend all day absorb or setting float v to same as absorb on SOLAR only, At the charge voltage you are using, They should not use much water anyway, but if you can be CERTAIN of the water being checked regularly, I would go up a little on V keeping a close eye on it till you get to a "happy place" (longer absorb also decreases need for EQ, This was recommended to me by a rolls tech)
On your gen you have a few options, It depends on your loads cycle depth and frequency (Please list) but a general outline with a few "If`s buts and "Ands"",
IF your solar is usually reaching say 90%ish SOC MOST days and 100% occasionally AND you are only using gen to "top up" AND you are EQ`ing when needed, I would set gen charge voltage at the top of rolls recommended and use a short absorb time. measure and log SG`s as you go along to determine when EQ is required,
Depending on how often you reach 100%, if you do not, I would EQ every month and treat that as it`s charge to full,
If you often reach a genuine SG confirmed 100% SOC,You will probably EQ about every 3 months or 100 cycles.

Have a good one
Tim

terrynew · January 2011

Re: Longer absorb charging for 48V systems?

Thanks, guys -- great advice. I would prefer 3 absorb hours at 59.2 V (my installer's recommendation) to 6.3 at 57.6 V (Surrette's). So I did so a few days ago, followed by 4 hours equalizing. Afterwards, six of the 48 cells had SG's outside 1250-1280, but I don't trust my NAPA bulb hydrometer one bit. The inner bulb doesn't float vertically, with the side dragging on the outside glass.

Interestingly, not one of the 48 cells needed fluid topping-up after the EQ, despite the high absorb voltage and long EQ session. I guess that means the 59.2 V is fine?

Anyone willing to tackle the 3rd question on my original post, AC In VDC?

Thanks again,
...Terry

Derik · January 2011

Re: Longer absorb charging for 48V systems?

Not sure where your at but I went to a 4 hour absorb time a few weeks ago and my batteries appear to be close to 100% full now for the past few weeks.

Outback had the default at 1 hour, the solar guy said 2 hours and another solar guy same company said 3 hours.. I figured I would go with four since we had only about 5- 6 hours of sun anyway & someone here mentioned "solar only" all day, abosorb times.. and I don't have a generator. Water usage has been little. I have 2,000 watts FM 80 Charge controller and 12 Trojan wired for a 24 volt system. I plan to cut it back to 3 hours soon since we gained about 30 min of sun the past month.

If I were you I would cut the generator out as much as possible, if you factor in fuel costs then so what if you batteries only last 10 years instead of 12.

We are looking at gas over 3.45 here and diesel is about a 3.70 a gallon, Propane is high as well as is natural gas.

Jogotta · July 2016

Hi,

I just stood up a very similar system and am having a horrible time getting it to charge my battery bank. At first I only had 8-6v 225ah batteries .. They said the charge controller and inverter require over 300AH to function so I bought 8 more. But seriously, if it can't charge 8-6v how do they expect it can charge 16-6v?

My 48V setup is :

8 - 285w panels 2,880watts
Magnum PT-100 Charge Controller, Magnum 4440 Pure Sine wave Inverter, Magnum ME-ARC Remote, Magnum Battery Monitor Kit with Shunt.

Have you increased your absorb time? Seems to me it should be able to automatically run absorb based on battery SOC but the BMK never works right.. I'm thinking I have a bad shunt.

Have you resolved your issues? I'm most certainly following this thread!!

So if the charge controller isn't putting out 100A it's not going to ever charge the batteries? I don't have a generator.. I am solely sun and I have never seen the Charge controller put out more than 15 amps for any length of time!

terrynew said:

Hi, I'm the proud but overwhelmed owner of a new off-grid solar house, and of course there are teething problems. May I ask your comments on three of them? One is that a Surrette technical support manager I've phoned recommends a 6.3 hr absorb charge, compared to 3 hrs recommended by the inverter/charger technical support staff and two local solar companies.

I have sixteen S600 batteries, two banks of eight = 48 V and 900 Ah @ C20. The Surrette guy says the ideal absorb charging is 57.6 V at 100 Amps. But because my Magnum 4448 inverter charges at 60 A max (30 A per string), he says it'll take a lot longer than ideal to charge my batteries. Using the Surrette charging equation, the Absorb time should be 0.42 * C20 / I, where C20 is the bank's Ah at the 20 hr rate, I is the current, and 0.42 is 20% / 50% + 5% losses (assuming that over the time of the absorption charge that 50% of the maximum current will be available). This is 0.42 * 900 / 60 = 6.3 hrs for my bank.

Has anyone else ever had such a long absorb time recommended? I wonder if this is just a battery manufacturer's perfect ideal and a generator fuel waste for me.

The time would be much lower with a higher charging rate on the inverter/charger. Do all 48 V chargers have this low charging current and thus long charging time?

Secondly, the Surrette guy also says I can only equalize with the generator, not the panels. That's because my 2.5kW PV array can only output 43 ADC (2560 W / 57.6 V), so charging the batteries by sun will take even longer than charging by generator/inverter (43 A vs 60 A charging). This comes out to 10-11 hrs, which of course is longer than a day (or two, in winter)'s sunshine. He also says this means that almost all of my solar charging cycles won't charge the batteries past 90% full, so I should equalize monthly (at least in winter). Anybody care to support or comment on these recommendations?

And thirdly, the Magnum support tech says I shouldn't pay attention to the inverter/charger's "AC In - VDC" setting, which is currently 45.2 to 56.4 V. He says that's because it's for when there's an alternate charging source beyond the PV array and a generator (e.g., wind or hydro). But I don't see a way to disable this setting, so do I just ignore whatever voltage range it has? Or will it affect my generator charging, for example conflicting with the Auto Gen Start at 45.2 V and Auto Gen End at 59.6 V?

Thanks for your generous offering of time to help out the Clueless like me, and have a great day.
...Terry

Jogotta · July 2016

Now that I've read some of the comments here.. I'm going to up my absorb to 4 hours a day ...

Vic · July 2016

Hi Jogotta,

Think that there is a Typo in the PV power that you have available.

If so, 285 X 8 = 2280 Watts STC. you could expect about 30 Amps available at maximum sun on a good day (2280 X .75 = 1710 (approximate watts), at 57 V, this is about 30 Amps. This is probably even a bit more than you would want for charging your 225 Ah battery.

Assume that the 285 W PVs are wired as four strings of two PVs ... What is the Vmp spec on those PVs (or mfg and model number).

See now that those PVs are LGs, and are 60 cell units. Will try to be a bit gentle ... those PVs will not provide a high enough voltage, with two per string to allow your Charge Controller to charge the batteries fully. This is probably why you are not seeing as much current as expected from your CC.

More later, THANKS!! Vic

vtmaps · July 2016

Jagotta,
You have enough watts to charge your 450 ah 48 volt bank, but as Vic points out, they are probably not configured properly. Either you have four strings of 2 panels per string, or two strings of 4 panels per string.

If you have 4 strings of 2 panels per string, the string voltage (Vmp) is about 60 volts, and that is too low for a 48 volt system. You should have a string length of 3 which will have a string voltage (Vmp) of 90 volts. Can you get another panel? Three strings of 3 panels per string would be ideal.

If you have 2 strings of 4 panels per string, the string voltage (Vmp) would be 120 volts. That could work, but is not as efficient as a 90 volt Vmp would be. Also, 4 panels in series would have a Voc of 160 volts, and in cold weather might be too high for that controller.

--vtMaps

westbranch · July 2016

A couple of additional comments:
Did the Surrette rep mention a break-in period? There is about a 50 cycle period, to about 50% SoC on a portion of those cycles... for most Surrette cells.
As you have 2 strings to start you could disconnect one string and charge the other fully for a few days, then do the other, then back to the 2 strings knowing you started with both up-to-snuff...
Record all the SG data for each cell, especially at the beginning, then you know where you came from
Here is a good tool, http://www.midnitesolar.com/productPhoto.php?product_ID=505&productCatName=Battery Accessories&productCat_ID=39&sortOrder=1&act=pc
Don't forget to rinse the hydrometer several times after measuring each cell, in distilled water.
As said already, you need more PV in SERIES configuration

hth

Vic · July 2016

... from another Discussion,

" ... Here is my setup:
2300W Solar Panels running into Magnum PT-100 Charge controller to feed 8 6v Trojan T-105 (225AH) batteries ... "

The apparent low PV input voltage to the CC should be the first item to resolve.

Many of these items have been covered ... many folks are trying to help ...
http://forum.solar-electric.com/discussion/350909/mspae4440-magnum-48v-inverter-faults-under-small-load/p1

FWIW, otra vez, Vic

Longer absorb charging for 48V systems?

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