My Trouble With Skip Days, Charging and Battery Life

[Matrix]

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

yeah I dont know if the AC grid connection is straight thru the inverter or not.  I guess I could go turn off AC Support AND turn off the inverter from within the software,  but have AC grid connected and see if the house goes dark or if it continues to pass power thru.   

But with the SW ... it is NEVER suppose to backfeed the grid as it is not a Hybrid in the since of a Grid-Tie / Off-Grid / Hybrid would be (as I think Outback has).  It only receives grid power to support loads,   It it was back feeding the grid at all ... I would be in trouble bc I don't have approval and an interconnect agreement to be injecting power back into the grid.    Thus,  there is no disconnect transfer in the inverter  shutting it off  if the grid fails like a grid-tie ... it just keeps on "invertering" and running off the batteries like nothing happened.   But there is some kind of auto transfer switch in it ... because when I throw the breaker to turn on AC ... after 10 seconds the lights flash as the grid connection comes one.

I guess I'll power up the grid,  turn off the inverter and see if I go dark.   

[Matrix]

Here is the SW with Grid Connected and AC Support Mode on .... but loads are unbalance,  so it is not drawing anything (but the smallest of constant 25w "signal") from the battery.   All the loads are coming from the grid.

Even at this 370W load level ... if I turn on a 125W light bulb on L2 with the fridge on L1 it will balance the loads enough to start using 225W off the grid (which is the minimum required 2%) and will pull 145W off the battery.   So I know balancing will get me there ... even kinda balanced as described above.

[Matrix]

Here is the SW with AC support mode off ... I have no idea if it is passing AC grid thru,  Inverter is still on and it is passing thru the inverter.

[Matrix]

And here are my master settings for the inverter (see pic)  .... Inverter is on,  but as you can see from above there is nothing coming off the battery. 


Now .... I will turn off the inverter from the controls and see if I am in the dark.    (I will probably have to reset all my clocks.   ) ... here goes ....

[Matrix]

Hmmmm ... Inverter is off ... and I still have light.  So I guess that answers the pass thru question.   The SW continues to pass power right thru.   BUT I know for certain,  if I went out and switched off the 250 amp breaker from the battery to the inverter ... the house would go dark ... even though the AC grid is still live and connected to the inverter.    So there is some kind of disconnect in the SW if it is shut off. 

(I guess I could take it out of Operating mode and put it into stand by ... I think that would put me in the dark)

[mike90045]

You might call Schneider Electric and see if they can help you out.   I'm just asking questions  and guessing at what the answers might be

[Vic]

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).
   ...

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

Hi boB,

I certainly am not a battery expert.

Agree that deeply discharging FLA batteries  is good for them,   on occasion.   Personally,  would not use Skip Days to do this.   Usually do the deeper discharge by taking FLAs from about 80% SOC to about 55 - 60% SOC in one morning,   then,  fully charge them in that same day.   We try to not let FLAs sit very long at a fairly low SOC.   And  avoid taking the main battery banks here,  below 50% SOC   ...  but that is just me.

Agree with you about avoiding fully recharge of FLAs every day,   unless the DOD justifies a full recharge every day.   Personally would not want FLAs to spend very much time below 70 or 75% SOC,   however   ...   all depends IMO.

In a simple off-grid system,   we try to set Vflt to a voltage that just barely keeps a fully-charged battery,   fully-charged.   At least for systems with appreciable battery Capacity.

Regarding using EA to end Absorb with FLAs,   this is all that I have ever used.   First,  it was CC EA with the venerable MX-60.   Worked fine on systems with loads that were reasonably well-controlled near the end of Absorb (ie not a household full of teen-agers,  ones with large water pumping demands,   etc).  Even without Shunt EA,   FLAs allow one to actually measure SGs to see how well the battery is being charged,   and changes can be made to EA settings that ensure fully battery charge.

Using Shunt EA (and that is all that we have ever used,   since it first became available on the Classics),   is even easier than CC EA was,   on FLAs.

One can choose an EA value that is far out enough on the EA curve to ensure that one gets enough Absorb time to allow full-charge.

On the other hand,   there are two slight things about even CC EA,   that can trip up things;

One,  is,   that the ideal Shunt EA setting really somewhat depends upon just how deeply discharged the FLAs were discharged in the previous discharge cycle.   A deep discharge seems to require a higher EA setting.    So,   there is a chance that the EA value for a customary fairly light discharge will never be reached,   and the Classic (or other good CC),  will hang in Absorb until the Absorb timer,   times-out.

The other thing,   is,   for CCs which do NOT have battery voltage Sense leads,   large opportunity loads,   with attendant voltage drops can cause early termination to Absorb. [because this voltage drop decreases the Vabs that the battery sees, causing the measured battery charge current to be reduced.  This can cause an early Abs termination].  Even a few tens of millivolts of drop in the CC output breaker,   cables to the battery and in the RFI suppression chokes can cause an EA value that allows for a full battery charge under light CC output loading,   to be reached considerably more quickly.   This results in the battery not having its desirable Absorb that lasts some hours,   and not receiving a full-charge IMO.

Regarding EA settings suggested by many FLA battery manufacturers,   most of them have charge settings (including those for EA),   based on a discharge down to the recommended minimum SOC.   For EA,  this is usually in the range 1.5 - 3% of 20 hour Capacity.   For FLAs that are not cycled that deeply on each cycle,   these EA values are far too high to allow an adequately long Absorb stage.   This does result in an inadequate recharge,   again IMO.

So,   in summary,   for the past 12+ years,  we have used EA on the main banks here (ie,   each battery bank is in its 13th year).

We began using the WbJr when it first became available,   using Shunt EA on Classics.

When Skip Days first became available,  we began using Skip Days with Shunt EA.  One system usually uses 3 days Skipped,   but sometimes this system is set to only Skip 2 days,   depending upon A/C loads in the Summer.   The other almost  identical system has fewer Opportunity loads,   and it usually Skips 3 days in Summer,   and 4 in Winter,   WX depending.

So,   am no expert.   Just have been running three battery banks,   two of which have run EA,   essentially since inception,   with no observed issues.   Some days,  with light discharges,  Abs time is about one hour.   Other days,  Abs is about four or so hours.  These battery banks show full SGs on full-charge days using Shunt EA AND Skip days.

All of the above are simply my observations and opinions,   obviously.

Thanks boB,  for all of your fine work (and the work of the entire MN Team),   over the decades,   producing some very fine RE products,   at a number of different companies.   You and the entire team,   really do Swing For the Stars!

The MX-60  is still a great CC,   particularly given,   that it was probably designed 16 or 17 years ago.   Many of them are still going strong,  after all of these years.   GREAT WORK !!  73,   Thanks,   Vic

[Matrix]

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).

Not sure i understand this... Seems like it says on the one hand you have to discharge FLAs to keep the capacity hi,  But if you dont it does not mater ?? 

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 ?

I mean it will be that for about 3-5 hours until we go to bed.  But in all reality,  thru the waking hours of evening, it is really more like about 250 Watts  per hour for 3-5 hours.  It just varies from 150-450 off and on inside of any given hour depending on what we are doing.  Then From 11:30pm (latest) on it is about 150 Watts or less for the rest of the night until charging starts again the next morning.  I usually see a balance about 8am when there is enough light to make the 150 watts needed and the loads start to be supplied fully by the solar.   Max thru the night Watt hours is around 2000 - 2400 Watt Hours.

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.

That is both legs,  and I should have written 2 amps AC NOT 2%.   But in practice it works out to be actually only about 240-300 watts total.  When the loads are less than that the SW draws all current from the grid and nothing from the battery except 25 watts (all the time)

The SW does not sell back to the grid at all.  It is not designed to,  but I understand what you are getting at.

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.

That is my feeling too.

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 ?

Lowering float voltage has not worked out so well yet.   I am still tinkering with it,  but as of yet,  I discharge down to 93% and thru the day I float back up to 100% even when i set it at 26.3v (for a 24v battery bank) on a fully sunny day.    If I go much lower,  the CC stops supplying (its like it limits) enough current to power the loads and they system will drop below the Waste Not Hi parameters, even though there is plenty of solar available to power the loads.    Like the demand will be 1200 watts but if i set it to 26.3v the CC will only allow for 800,  but if I set it to 27v,  it powers all loads just fine. 

So the lower you go in float voltage,  the more the current seems to be limited not just for charging but also for loads even if the power is available .

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.

As to end amps,  me and end amps never did get along.  So many factors in charging its hard to keep up with it all,  but the short version.  I have to keep a higher than "normal" absorb voltage set (like 31.3v) or I never reach full charge even though I have tapered down to 1% of the battery amp hour.  If I use 30.5v for absorb,  I will get to 1% of Ah, but the batteries are not charged fully.  If I turn off end amp and use time,  I drop to below .5% battery Ah and still not charged and it just drowns on until the sun goes down.  If it is a really good day,  the batteries charge,  but it takes all day.  So the Volts go up ... up to 31.3v + for absorb,  and the batteries reach fully charge at about 1% battery Ah or in my cast about 4.3 amps.

Why ??  I do not know.  Has driven my crazy since installation,  and I have given up and just live with it.    I can never get a firm answer as to if the higher absorb volts harms the batteries ... not even from Trojan. (they actually said no it does not)

[Matrix]

Regarding EA settings suggested by many FLA battery manufacturers,   most of them have charge settings (including those for EA),   based on a discharge down to the recommended minimum SOC.   For EA,  this is usually in the range 1.5 - 3% of 20 hour Capacity.   For FLAs that are not cycled that deeply on each cycle,   these EA values are far too high to allow an adequately long Absorb stage.   This does result in an inadequate recharge,   again IMO.

Thanks Vic,  That about sums up what I see and really sheds some light on things from your vast experience.

[jimbo]

Can I ask what battery life people are getting that advocate regular deep discharges and skipping days of charge?

When i moved into my house the previous owners had the generator come on at 80%SOC but in reality it was a lot sooner due to big loads like the drier forcing it to start.  When i moved in I lowered the value to 70% however for most of the year our SOC rarely gets under 90% at night and batteries are fully charged by midday.  A few times in winter the batteries will see 80% but again big loads normally kick the generator in (taken care of by older sMA inverter) before that.

My batteries (1,600 amp, 24v raylites) are now 12 years old and not showing any signs of wear. They get an equalise charge every 2 months or so but charged every day. 

[Vic]

Can I ask what battery life people are getting that advocate regular deep discharges and skipping days of charge?   ,,,

Just my opinions,   but,  to me,  the main use for Skip Day charging,  is for batteries that do NOT get sufficient discharge,  on most days (and therefore are NOT seeing deep discharges regularly,  without manual intervention).    The banks,   here that are on Skip Days,  usually would not be discharged to less than 90% SOC on many days.   Generally,  it is best to discharge FLAs that are in regular cyclic (off grid) use below 90% SOC  before recharge.   This is the reason that we are using Skip Days.

The main banks here are Surrettes,   and each in their 13th year of off-grid service.

An additional benefit of not recharging FLAs that have been lightly-cycled,  is that there will be less heating of the battery bank,  due to fewer Absorb cycles per given time.

Skipping days,   might mean,   that these FLAs might need a bit more EQing.

FWIW,   Vic

[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).

Not sure i understand this... Seems like it says on the one hand you have to discharge FLAs to keep the capacity hi,  But if you dont it does not mater ?? 

Right, sorta.... As I remember so see my statement of possible insanity below...

If you don't exercise the batteries to some depth of discharge all of the time, then they won't have that much depth of discharge
available until you work them down to that DOD level.  This is what I heard and this MAY have just been some kind of
dream or nightmare, but if it was, then Steve Higgens from Rolls/Surrette battery was in that dream.

  I have NOT seen this behavior  myself !

boB

[jimbo]

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.

[boB]

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

[Matrix]

boB,

What would be the logic in not using time to end absorb all the time?   As opposed to say, end amps?  If time is the better method, why use end amp at all?

I started at first, for the first  6 or so weeks after commissioning my batterues trying to use end amp and get that setting right.... But found after 2 months I was using no water at all, even though my end amp curve was flat at 1-1.5% of total bank  amp hour.  I found my batteries were very under charged. I was not using enough voltage.