My Trouble With Skip Days, Charging and Battery Life

Started by Matrix, April 04, 2018, 10:03:40 AM

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Vic

Quote from: boB on May 23, 2018, 12:31:43 PM
Quote from: jimbo on May 22, 2018, 07:20:42 PM
I have been told that short discharge means that the FLA battieres doesn't get much time to absorb and thus mix up the electrolyte.

Ahhh !  YES !   This CAN be a problem with using only return amps to stop the absorb cycle.

The batteries must get a good "timed" absorb once in a while...  Like at least once every week or two is what
I hear.

What would be best probably would be another timer that makes sure there is a timed absorb every so often.

When using Skip-Days, probably should use only timed absorb on those days there is an absorb.  i.e. Set the ending
amps to zero and set Skip-Days to just a few days.  Maybe even a bit longer time than usual for absorb ?

These are just suggestions to try of course and don't take over any hard instructions from the particular battery
manufacturer.

boB

Hi boB,

In my opinion,   FLAs  do not so much need a long Timed Absorb,   as much as a sufficiently long Absorb  to  really fully charge the battery AND also allow enough mixing of the electrolyte.

Most "experts"  say that taller batteries need longer Absorbs,  or perhaps Absorbs at a somewhat higher voltage than might otherwise be required for a full-charge on less tall batteries.

If one chooses an Shunt EA value that is fairly far out on the curve of Rate-0f Change in battery charge current,   this long Absorb time will be met,   but still allow some time-variability in Absorb times,   depending on the actual DOD  of the battery.   But,  going out that far on the tail of the battery charge current to allow this long-ish Absorb,   can mean that (as stated above) that other factors can disturb  this finely tuned EA setting;

One of these factors,   is  large Opportunity loads  that vary in intensity or duration from day-to-day.  This can cause a large enough variation in voltage drop in the CC-to battery cables and breakers to cause an early Absorb termination.

AND  another,  is if there is a sufficiently greater discharge of the battery (to a lower  SOC),  from one full-charge day to the next (using Skip Days),   Then there is some risk that the set EA value will not be reached,   and the CC will hang in Absorb,  when a long Absorb timer setting is used.  (this is why folks like me have been asking for a WBSr,  or similar device that will allow MN CCs to know actual battery voltage (measured at battery terminals  --  Sense leads).

Certainly,   using Shunt EA  requires monitoring of actual Absorb times (this is one reason that some of us have been BEGGING  for a stored Actual Absorb Time,  after the CC has gone to Float),  and   monitoring of SGs with a Hydro (or Refractometer).

While not absolutely perfect,  for off-grid FLA batteried systems,   Shunt EA WITH Skip Days works quite well,   and has been  in continuous use with Classics,  here,  since it was available.

Thanks again boB,   and all the MidNite team for all of the great products,   each,  with an array of important,   innovative functions.  There are simply NO other such products in the solar industry,   today.

Thanks for taking the time to still keep us on track on this (and other) Forum/s.

73,   GL,   Vic
Off Grid - Sys 1: 2ea SW+ 5548, Surrette 4KS25 1280 AH, 5.25 KW PV, Classic 150,WB, Beta Barcelona, Beta KID
Sys 2: SW+ 5548s, 4KS25s, 5.88 KW PV, 2 ea. Classic 150, WB, HB CC-needs remote Monitoring/Control, site=remote.
 MN Bkrs/Bxs/Combiners. Thanks MN for Great Products/Svc/Support&This Forum!!

Matrix

I was/am definitely in the "needs higher voltage" camp for my tall L16s. The trouble I was having with EA settings was before I set up my opportunity load AND before I upped my absorb charhing volts. I could probably flirt with EA again now that I have a better charging voltage.
Grid-Assist Off-Grid
REC TwinPeak 2 285W 3S-3P 2565w-STC 1926w-NMOT Array / Classic 150 / Conext SW 4024 Inverter
Trojan L16H-AC 435Ah bank 4S  with 7' of 4/0 cable
24 volt system /  Need 3200Whs Daily
My System Install

boB


The reason that you want to do a timed absorb (however long for the particular FLA battery) is so the batteries don't sulphate.

The reason you don't want to absorb every day (unnecessarily)  is so that you don't prematurely wear the batteries out.

This is my understanding

boB

K7IQ 🌛  He/She/Me

Vic

Quote from: boB on May 23, 2018, 06:16:45 PM

The reason that you want to do a timed absorb (however long for the particular FLA battery) is so the batteries don't sulphate.

The reason you don't want to absorb every day (unnecessarily)  is so that you don't prematurely wear the batteries out.

This is my understanding
boB

Hi boB,

Just for my final beating of this poor horse ...

YES,   an adequate Absorb time and voltage will return sulfates on the battery plates to the electrolyte,  and thus,  increase the electrolyte SG.   This is an important thing for maintaining battery Capacity,   and sulfates that are allowed to remain on the plates  can harden,  and can cause permanent loss of Capacity,  if batteries are not fully charged on a regular basis.   Agree with your statement (previously),   that it IS better to overcharge (longer Abs time,  and/or higher Vabs than really needed for a full-charge) batteries,   than to under-charge them.

But,  one of the important functions of using Return,  or Ending Amps,  is to terminate Absorb when that current reaches a point where  the change in this current,  essentially becomes zero,   even when using Skip Days.   Using Skip Days,  might make finding an optimum EA setting  a bit more difficult,  vs,  not skipping days.

With varying DOD,   from one day,  to the next,   is the main reason that Shunt EA is used on the battery banks here.   Deeper DODs require longer Absorbs,   so  EA is perfect for this situation  verses using only a Timed Absorb.

There is no reason that using well-studied EA value would result in insufficient-long Absorb charge stage.   Depending on DOD on our batteries,  some Absorbs require less than two hours,   and,   some around four hours ...   all depends.

We measure SGs frequently (at least on the Pilot Cells),   and Skip Day Shunt EA  does a wonderful job fully-charging the batteries,  while not needing to resort to a Timed Absorb.   With that variable DOD on these batteries,  one would need to set an Absorb Time,  that was always (or almost always) too long.

After using only CC EA,  or Shunt EA for the 12.75 years of service of each of the two battery banks here,   these batteries are doing fine.   Have needed to increase Vabs somewhat to reach full-charge,  starting about 2.5 years ago,  as the batteries age.  This increase in Vabs,   required raising the EA value a bit (as one would expect).

I DO agree with your statement that deep-cycle batteries DO need to be cycled fairly deeply.   But,  we do not do this too frequently.   The Surrette Battery Bulletin #614  used to council,  that for their customers who are having problems maintaining Capacity,   or  those  with Charge Acceptance issues,  should consider cycling the battery bank down to about 1.83 Vpc (about 11 V, per 12 V equivalent),   and then fully recharge them (without delay).   This is just too extreme for me  --  the deepest that we cycle the banks here,  is to about 50% SOC,  and recharge fully,  immediately.

And,   I would fully charge,  daily,  if the batteries were being cycled to/below about 80% SOC,  or lower.

One other good reason for using Skip Days on off-grid FLAs,  is,  to help reduce battery heating,  plus reduction on positive plate erosion,  plus a couple of other things.

As an aside,   have wondered,  if some of the extremely high Vabs requirements for a number of the Trojan batter  models,  is   that,  perhaps they have some Calcium in the plates (many of these batteries,  seem to need a Temperature Compensation value of -3mV/Cell/C  --  a common value for lead-calcium batteries, vs -5mV for lead Antimony batts).   Lead-calcium batteries Gas less vigorously,   are used in Low Maintenance/Maintenance Free batteries,   and this is probably the main reason for the use of lead-calcium batteries (reduced water electrolysis),  IMO.

Give me Lead-Antimony batteries,   please.

All FWIW,   just opinions,  but,   based on experience,   Vic
Off Grid - Sys 1: 2ea SW+ 5548, Surrette 4KS25 1280 AH, 5.25 KW PV, Classic 150,WB, Beta Barcelona, Beta KID
Sys 2: SW+ 5548s, 4KS25s, 5.88 KW PV, 2 ea. Classic 150, WB, HB CC-needs remote Monitoring/Control, site=remote.
 MN Bkrs/Bxs/Combiners. Thanks MN for Great Products/Svc/Support&This Forum!!

Matrix

1.83 VPC?  :o

Isnt that like lower than 10% SOC?   I'd be afraid to do that even once. 
Grid-Assist Off-Grid
REC TwinPeak 2 285W 3S-3P 2565w-STC 1926w-NMOT Array / Classic 150 / Conext SW 4024 Inverter
Trojan L16H-AC 435Ah bank 4S  with 7' of 4/0 cable
24 volt system /  Need 3200Whs Daily
My System Install

Matrix

#35
QuoteAs an aside,   have wondered,  if some of the extremely high Vabs requirements for a number of the Trojan batter  models,  is   that,  perhaps they have some Calcium in the plates (many of these batteries,  seem to need a Temperature Compensation value of -3mV/Cell/C  --  a common value for lead-calcium batteries, vs -5mV for lead Antimony batts).   Lead-calcium batteries Gas less vigorously,   are used in Low Maintenance/Maintenance Free batteries,   and this is probably the main reason for the use of lead-calcium batteries (reduced water electrolysis),  IMO.

Might be a good catch:

Look at #4 under BATTERY SELECTION on this page.  Seems Trojan does not admit using Lead-Calcium,  but as my experience has shown, at "standard" absorb voltages (and per Trojan spec), charging at 2.46Vpc does NOT get the batteries to full charge.  The EA is reached to a flat line,  but the SG is off by about 10 points (or about 94% SOC).  Cant really find anything one way or the other in reference to the use of lead-calcium.

https://www.trojanbattery.com/tech-support/faq/

Grid-Assist Off-Grid
REC TwinPeak 2 285W 3S-3P 2565w-STC 1926w-NMOT Array / Classic 150 / Conext SW 4024 Inverter
Trojan L16H-AC 435Ah bank 4S  with 7' of 4/0 cable
24 volt system /  Need 3200Whs Daily
My System Install