My Trouble With Skip Days, Charging and Battery Life

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

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Matrix

Been playing around with the "Days Between Bulk" setting - aka Skip Days.

Here's my trouble ... if it is a problem I'll let you decide.   I am using a Conext SW 4024 Inverter in AC Support mode.  This mode allows for you to be grid connected but only use grid power when the battery falls below a user set threshold.  (I assume not unlike Outback's GridZero idea). 

The Issue with AC Support Mode is Two Fold:

1. AC Support Mode really only works well if your loads are perfectly balanced between L1 and L2,  like a pump or water heater element on 240v.  In normal operation of a Split-Phase system this is tough to do with 120v loads that vary constantly.

2. AC Support Mode requires that it use 2%  from the grid at all times.  This (according to Schneider) is to keep from back feeding the grid with power.   How you define 2% is subjective,  but what it works out to is about 200-250  watts that must be being pulled from the grid at all times.   

But there is a relationship between point 1 and 2.  If the loads are not balance,  say you are using 400 watts on L1 and 50 watts on L2 you will not enter AC Support mode.  You will pull all the loads off the grid and about 25 watts off the battery which occurs almost constantly in AC Support mode. 

I was thinking of getting OutBack's Auto Transformer - the PSX-240.  This might keep all 120 loads perfectly balanced at all times between L1 and L2.   But a pricey "fix" for such little wattage savings from the grid.

Here's my trouble
... I only use about 150-500 watts an hour on and off at night unless the pump comes on (not very often and not very long).  And then when we go to bed that load drops to  between 60 and 150 watts an hour because of the fridge,  so even if the load is balanced, it would not be enough for the SW to come off grid usage very often.  And even then the grid would supply 200-250 watts while the batteries supplied 100-250 watts.

SO ... in AC support mode,  my battery bank only discharges about 5-9% over night. 
- Using the Skip Days setting for 2 days .... over night the battery drops to 93%,  but then during the skip day,  They Float Back to 100% and that is with a nice dump load of 1000 watts to heat water.  So I generate about 4.5 kWh from the array.
- Go into the next night ... same thing.  Loads drop the batteries to about 95%,  The Float their way back to 100% the next skip day
- Of course this means there is decent weather and limited cloud cover.

I am running Skip Days to extend battery life so there are fewer cycles on the batteries.   My goal is to save the batteries for when we really need them ...   BUT ... Is it hard or bad for batteries to go thru these very shallow cycles? 
- My thought is to perhaps do deeper 30-40% DOD once a week or so.
- But unless I get an Auto Transformer (expensive fix if it would even work well at low watts to perfectly balance the 120v loads) ...
- Or better balance my loads (which is kinda tough as they vary so much off and on) ...
- There is no real way get any DOD below 90%ish if  I stay in AC Support Mode.   
- And buying an Auto Transformer might be a waste of money anyway because even if the loads to balance,  I am using so little power that pulling 100-150 watts an hour off the battery (and none at all after we go to bed) may not discharge them very much anyway.


So on the one hand I am trying to extend battery life by Skipping Days,  but on the other hand,  if I am not discharging the batteries below 90% each night,  maybe that is doing more harm than good??   See the Dilemma?   ??? :)

Thanks.
Mike
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

Vic

Hi Mike,   a couple of quick thoughts ...

It is not good to recharge FLA batteries,   frequently,   when their ACTUAL SOC is above about 90%,   you are correct,   IMO.

As stated a number of times previously;  the indicated SOC  from any battery monitoring device  is APPROXIMATE,   NOT absolute.   Even it your battery monitoring device says,   that the batteries have been fully-charged from one day of recharge at Vflt,   this is most likely,  not the case.   And successive days of Floating to "100%"  will mean that this SOC indication becomes less,   and less accurate for each Skipped day.

Will not try to address the way that Schneider inverters behave,   etc.   Thank goodness,   have no experience with the modern SW line.

IMO,   Skipping days is better for the batteries,   than is recharging each day from SOCs that are above 90%.   Fewer cycles,   and less integrated battery heating should result from Skipping days,   as long as the actual SOC  is not depleted below about 65- 70%.

I would shoot for recharging your FLAs about twice per week,  not,   once.

All  IMO,   FWIW,   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

Thanks Vic ... that is my goal,  and yes I am aware of the vagueness of the % monitor.  Kind of just a guage.  Guess I need to go and check at the end of the day if indeed the batteries did return to fully charged from their 90%ish state,  but I have every reason to believe they did based on Ah's returned and the actual charging current that seem to be being sued even in float.   The current even in float is at times 3-13 amps.  So they are charging all be it at 27 volts.

The issue seems to be,  as long as I stay connected to the grid for support of the battery bank,  I cannot seem to drift down to a lower SOC than 90% thru the night.  And thus thru the next day ... even in float,  the batteries appear to be (though as noted untested) but appear to be return to fully charged.

I guess I could set the float voltage to 25 or something,  then they really would struggle to keep up and probably more just sit there while any loads where being supplied by the Array if there is sun light. 

Maybe reducing the float voltage would help during skip days to allow a 2-3 day period to actually discharge the bank down to about 75%
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

As to the SW and me ... it is definitely a love - hate relationship.  The inverter has several really nice features on paper that seem marginally thought thru by the engineers at best and so the implementation and actual function of those features is Meh at best.
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

Is there a way to set the CC during skip days so that not charging occurs,  not even float,  but loads are supplied by the solar array?   I was able to slow down charging by setting the float voltage to 25.8v
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

Resthome

Quote from: Matrix on April 04, 2018, 02:00:02 PM
Is there a way to set the CC during skip days so that not charging occurs,  not even float,  but loads are supplied by the solar array?   I was able to slow down charging by setting the float voltage to 25.8v

No the skip day will go to Float
John

10 x Kyocera KC140, Classic 150 w/WBJr, Link10 Battery Monitor, 850 AH @ 12v Solar One 2v cells, Xantrex PROwatt SW2000
Off Grid on Houseboat Lake Don Pedro, CA

Vic

Quote from: Matrix on April 04, 2018, 02:00:02 PM
Is there a way to set the CC during skip days so that not charging occurs,  not even float,  but loads are supplied by the solar array?   I was able to slow down charging by setting the float voltage to 25.8v

Mike,

I have also set the Vflt to a low voltage,   in the early days of experimenting with Skip Days  --  about 25 V equivalent,   but then set it back to the normal Vflt.

Have always tried to keep Vflt as low as possible,   as,  even in Float,   there is some positive plate erosion,  particularly if much time is spent there.

As John mentioned,  a Skip Day starts in Float MPPT,   and will sit in Float as long as there is enough power to maintain Vflt (as you know).

I am happy that MidNite has added this great function,  for those of us who have a bit too much battery capacity vs loads.

Your system has a lot of things going on,   which probably make it difficult to stay on top of managing your battery bank.

I have little respect for Schneider as a company,   as it seems that their Engineers are not permitted to Swing For The Stars.  When something kinna starts working,   they ship it.   And their bug fixes often seem to be that they come out with a new model of that product,   that kinna fixed one small thing,   but may well have other bugs/flaws    ...  just my opinion.

Good luck with your system.     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

Quote from: Vic on April 04, 2018, 04:20:06 PM
I have little respect for Schneider as a company,   as it seems that their Engineers are not permitted to Swing For The Stars.  When something kinna starts working,   they ship it.   And their bug fixes often seem to be that they come out with a new model of that product,   that kinna fixed one small thing,   but may well have other bugs/flaws    ...  just my opinion.

Good luck with your system.     Vic

Well Said,  I am right there with you.  But I am kinda locked in now as I also have an E-Panel specifically made for the SW that ties it all together.  The cost of changing how would not just be an inverter, but also an E-Panel and a lot more. 

So I love to hate it.   ;D
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

mike90045

Beware of trying to balance the Grid with your small autotransformer.  If the feed from the grid is not balanced, then your transformer will be overloaded.

If you are looking for more efficient than the PSX-240, here's a source for expensive toroidal versions
https://toroid.com/Products/Auto-Transformers
http://tinyurl.com/LMR-Solar

Classic 200| 2Kw PV, 160Voc | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph )| Listeroid 6/1, st5 gen head | XW6048 inverter/chgr | midnight ePanel & 4 SPDs | 48V, 800A NiFe battery bank | MS-TS-MPPT60 w/3Kw PV

Matrix

Quote from: mike90045 on April 05, 2018, 02:10:39 AM
Beware of trying to balance the Grid with your small autotransformer.  If the feed from the grid is not balanced, then your transformer will be overloaded.

If you are looking for more efficient than the PSX-240, here's a source for expensive toroidal versions
https://toroid.com/Products/Auto-Transformers
Oh Mike now you've cooked my brain.   I was thinking I was balancing the loads only coming off the inverter,  but you are correct,  these same loads are powered by the grid at least in a limited way.   

My thought was the Grid power is going into the inverter same as the battery power and the all power is passing thru the inverter,  but I think you are correct,  grid power by-passes the inverter within the SW and is "blended" with inverter AC power.   But you are saying that if the limited support grid power is not balanced it will over load the transformer?  In what way am I looking for it to be balanced?  Volts?    My grid power is coming strait off my L1 and L2 from the Meter thru the main panel and to a 2 pole breaker and then to the E-Panel breaker to the Inverter.

Seems the AG Grid power voltage between L1 and L2 has about a 2-4 volt difference.   Like L1 is at 121v and L2 is 123v but it swings.    Other than that,  I would not be exceeding total load wattage of the inverter (3400W) for my loads regardless of the source, either battery or grid.
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

This is an an interesting discussion...

As for how far to let the batteries discharge over a couple of days, I am not sure so I'll let that one go for now
except to say that I have heard (from "experts")  that you want to discharge FLA (unsealed) to keep their
discharge capacity high and if you do NOT do this, that there is NO permanent damage (memory).

Matrix, when you say,
"I only use about 150-500 watts an hour on and off at night unless the pump comes on"

do you mean 150 to 500 "Watt-Hours" per night of energy ?

The 2% minimum from grid requirement, I assume, is per leg.  Just a guess.  But this is so that they
have time to react and stop supporting quickly when the consumption goes below 2% on either leg so
that they don't sell back on either leg for any length of time.  I really do not know how all residential
power meters work when they phone home that you've been selling back power. 

I would also assume that these power meters would have their own safety margin in them so that even
when you do sell back power above some threshold for some minimum amount of time, that it would
only then register that activity.  After all, even when NOT selling back energy, some current can be
sent back to the grid every cycle when low power factor loads are connected.  It's just not in-phase with
the voltage so it's not real power, just VARs.

The idea for Skip days and battery life is to just keep that higher voltage absorb cycle from happening
all the time... BUT you want it to happen once in a while.  Maybe not even every other day but every
few days ?   Maybe every situation is different ?  I don't really know what is best.  Just kind of a feeling.

I like the idea of lowering the float voltage but not too low.  I would adjust that voltage to the minimum
SOC% battery voltage that you want the batteries to discharge to I guess.    50% maybe ?

I also question the use of ending-amps to stop an absorption charge, especially when using skip-days.
You  really do want a full 2 or 3 hour (or ?) higher voltage absorb charge cycle once in a while to help
keep the sulfation away.

boB

K7IQ 🌛  He/She/Me

mike90045

Not knowing how your inverter is wired internally with the transfer switches, but my XW has 2 sets of switches, one for Generator Input, and one for Grid Input.

  I know for sure the generator input switches to the inverter Load, and when the inverter is in Gen Support mode, I can make the generator howl by uneven loading of the L1 + L2  **

I think that in Grid Support, you also have Grid and Loads connected.     So, do you have your own transformer on a pole in the yard, or do you share a transformer with a couple neighbors ?  If just your own, you are in pretty good shape, but if you share a pole pig with several neighbors, your L1+2 balancing would be doing the whole neighborhood  !!

**
A firmware upgrade from Xantrex added a mode GenSupport+ which activates the internal transformer in the inverter as a balancing transformer.
http://tinyurl.com/LMR-Solar

Classic 200| 2Kw PV, 160Voc | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph )| Listeroid 6/1, st5 gen head | XW6048 inverter/chgr | midnight ePanel & 4 SPDs | 48V, 800A NiFe battery bank | MS-TS-MPPT60 w/3Kw PV

Matrix

boB,  so much to comment back that I will have to wait until later this evening when things slow down. 
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

#13
Quote from: mike90045 on April 05, 2018, 04:19:38 PM
Not knowing how your inverter is wired internally with the transfer switches, but my XW has 2 sets of switches, one for Generator Input, and one for Grid Input.

  I know for sure the generator input switches to the inverter Load, and when the inverter is in Gen Support mode, I can make the generator howl by uneven loading of the L1 + L2  **

I think that in Grid Support, you also have Grid and Loads connected.     So, do you have your own transformer on a pole in the yard, or do you share a transformer with a couple neighbors ?  If just your own, you are in pretty good shape, but if you share a pole pig with several neighbors, your L1+2 balancing would be doing the whole neighborhood  !!

**
A firmware upgrade from Xantrex added a mode GenSupport+ which activates the internal transformer in the inverter as a balancing transformer.
Easier to respond to

the SW has one AC L1 / L2 Neutral in and one L1 / L2 Neutral out.   The user decides if that in is a Gen or AC Grid inside the SW.

All of my loads that come off the inverter are the only loads I would be trying to balancing.   Not the loads of the whole house.   The Auto Transformer would be put AFTER the inverter,  not on the L1 and L2 coming in from the pole.    It would be the L1 and L2 coming off the inverter.    Not sure if that would matter based on what you  re saying.

We share a pole transformer with one house next door.   

But I am only trying to balance the loads coming off the inverter L1 / L2 and inverter neutral. 

Outbacks Auto Transformer is used for Step up and Step down, but also for balancing the loads coming off of 2 Outback 120v inverters that are stacked and being used for 240v split-phase service.

My thought was that the SW would be similar to 2 inverters in one box.  Because, even though the inverter is rated for 3400 watts continuous,  the user is not supposed to use more than 1700 watts continuous per leg.

AC support mode in the SW does not work well unless the loads are in perfect balance


So my thought was to use the Auto Transformer from Outback similar to the way they describe using it with 2 120v inverters to balance loads between L1 and L2. It would look similar to this .... but with one Split-Phase L1 / L2 inverter and not 2 120v inverters.   Perhaps it would not work at all as I anticipate.  But I have been told it will but "ButchDeal" (another forum who first suggested it to me) and I also called Schneider who thought it was a great idea for AC support mode and that they had never thought of it.    Which if Schneider knew (which they do know - they dont state it in the manual,  but trust me ... they know) that AC support mode required such concise balance  ... why didn't they build one in to the SW from the start????

I guess for the same reason they did not program little reasonable ideas into the SW like end amps for charging.  Their Absorb time feature is useless.  No mater where the user sets Battery Amp Hour (the program aspect that is supposed to control end amp if not time),  Absorb ends at 7.8 amps,  or 2% of my Battery Ah ... or about 30-45 minutes into Absorb.    And my battery is only at 90% SG when their charger stops.  And there is nothing I can do about it ... except increase charge volts to near EQ volts if I want the SW to fully charge the batteries.
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

mike90045

Quote from: Matrix on April 05, 2018, 06:39:16 PM
.....

the SW has one AC L1 / L2 Neutral in and one L1 / L2 Neutral out.   The user decides if that in is a Gen or AC Grid inside the SW.

All of my loads that come off the inverter are the only loads I would be trying to balancing.   Not the loads of the whole house.   The Auto Transformer would be put AFTER the inverter,  not on the L1 and L2 coming in from the pole.  .........

Right, but if there is a straight wire thru the inverter Loads to the Grid, your transformer is looking at the Grid too, not just your local loads.
If the inverter is powering your loads, AND backfeeding the grid at the same time, I think you have a straight through connection via transfer relay that disconnects the Grid if it fails.
  I don't know if the outback transformer has been tested in a Load Shave configuration or only in Off Grid.   

Anyone else ???
http://tinyurl.com/LMR-Solar

Classic 200| 2Kw PV, 160Voc | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph )| Listeroid 6/1, st5 gen head | XW6048 inverter/chgr | midnight ePanel & 4 SPDs | 48V, 800A NiFe battery bank | MS-TS-MPPT60 w/3Kw PV