Author Topic: Request opinions on charging cold AGMs  (Read 241 times)

Powerplay

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Request opinions on charging cold AGMs
« on: December 15, 2016, 01:27:50 PM »
My absorb phase on this 48V AGM bank is terminated by Ending Amps as read by Wbjr.  I'm setting End Amps at .01 X T-comped Capacity.  The bank is 255Ah capacity, so at 25C, End Amps is 2.5, & at 10C it would be set to 2.2, etc.  I've got a spreadsheet that calculates all this and I just plug in the temps being read by the BTS.  At temps 10C and above this seems to work fairly well and absorb phase has been less than 2 1/2 hours usually around 2 hrs.  Now that battery temps have been getting colder than that range, I've noticed the absorb phase can stretch out to around 4-hours and won't stay in absorb any longer if the 4448 inverter is turned on.  Today is the coldest battery yet and began with the battery bank at -7.1C and absorb set point of 62.1V.  The End Amp is 1.8A and it looks like the charging will finish around the 5 hour mark for absorb phase length.  So questions.

Should I be setting the end amps this low on cold days?
Does a 5-hour absorb phase when the bank is below zero Celcius sound out of line?
Is it hard on the bank to have 62.1V set point for this long?

BTW, my controller that owns the 95.5Vmp string gets sole possession of the absorb phase under these these conditions since it has no trouble making the volts even in the cold & ice.

Thank you for the input.

 

 
41 degrees N, 255 Ah 48V AGM, MagnaSine 4448 Inv, 3500W PV, Midnite Classic 150 A (12/6/15), Midnite Classic 150 B (4/14/16), WBjr, BTS, MNPV6 X 2, SPD X 2, Apps: Mini Split AC, Car Charger, Water pumps, Lighting, -> 48V string plan 500W, 94.5VmP, 112.5VoC, -15C - 40C TCVoC -.3%, TCIsC +.04%

Westbranch

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Re: Request opinions on charging cold AGMs
« Reply #1 on: December 15, 2016, 02:41:27 PM »
Powerplay,

Hmmm it seems a bit high.  If they were FLA's there wouldn't be the issue of venting...
What is the unadjusted value, before the BTS reading is used to increase the Voltage?
Do they have a manual for charging?

Have you spoken to the battery rep about a 62.1V Absorb? 
I would follow his recommendation.

BTW I have my max V set to 29.0V just to be safe as per my makers maintenance manual...
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CL150 29032 V.2126-NW2097-GP2133 175A E-Panel WBjr, 3Px4s 140W > 24V 900Ah AGM,
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Powerplay

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Re: Request opinions on charging cold AGMs (48V bank)
« Reply #2 on: December 15, 2016, 03:24:50 PM »
Uncompensated absorb is 58.4V & Float is 54.7V.  These numbers are 2X what I told the manufacturer I would do when the bank was configured as a 510Ah 24V bank, I.E., 29.2 V & 27.4 V.  The manufacturer replied those were perfect settings &  in the middle of the recommended ranges.  Compensation is -5 mv/C/cell and the manufacturer said erm Okay sounds about right, as I recall.  Maybe I should lower the uncompensated number.  Or maybe things just get asymptotic near the lower recommended charging limit (-10C). 

 
« Last Edit: December 15, 2016, 03:27:30 PM by Powerplay »
41 degrees N, 255 Ah 48V AGM, MagnaSine 4448 Inv, 3500W PV, Midnite Classic 150 A (12/6/15), Midnite Classic 150 B (4/14/16), WBjr, BTS, MNPV6 X 2, SPD X 2, Apps: Mini Split AC, Car Charger, Water pumps, Lighting, -> 48V string plan 500W, 94.5VmP, 112.5VoC, -15C - 40C TCVoC -.3%, TCIsC +.04%

Powerplay

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Re: Request opinions on charging cold AGMs
« Reply #3 on: December 15, 2016, 03:43:48 PM »
The bank went over into float (at 58.2V!) about 15min. ago.  It is -5.7C temp and floating at a value just under the non-compensated absorb set point.
41 degrees N, 255 Ah 48V AGM, MagnaSine 4448 Inv, 3500W PV, Midnite Classic 150 A (12/6/15), Midnite Classic 150 B (4/14/16), WBjr, BTS, MNPV6 X 2, SPD X 2, Apps: Mini Split AC, Car Charger, Water pumps, Lighting, -> 48V string plan 500W, 94.5VmP, 112.5VoC, -15C - 40C TCVoC -.3%, TCIsC +.04%

Vic

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Re: Request opinions on charging cold AGMs
« Reply #4 on: December 15, 2016, 07:15:02 PM »
   ...    on this 48V AGM bank is terminated by Ending Amps as read by Wbjr.    So questions.

Should I be setting the end amps this low on cold days?
Does a 5-hour absorb phase when the bank is below zero Celcius sound out of line?
Is it hard on the bank to have 62.1V set point for this long?

    ...   Thank you for the input.

Powerplay,

As you probably know,  some manufacturers of AGMs suggest/require that the Range of temperature compensation be Limited.   Seems that I've seen a lower Limit of 10 C for some AGMs.

Know that you have spoken with the manufacturer,  but  you may want to ask the question about Limiting the range of Temperature Compensation,  if you have not done so already.

Many of the AGMs that I have looked at have had either a --3 or --4 mV/C compensation value,  so yours being --5 mV/C,  seems to make them a bit different that quite a few of the AGMs in the market,  at least in the USA.

FWIW,  Vic
« Last Edit: December 15, 2016, 07:25:45 PM by Vic »
Off Grid - Two systems -- 4 SW+ 5548s, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 3X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  Thanks MN for Great Products/Service/Support!