Optimal Absorption Time,,, Good –Bad- Wasted Effort?

Coach Dad · May 2012

One of the guys on another thread had what seems like a great idea to me but I wanted to start a new thread to discuss it.

Here is the setting: I have an off grid cabin that is typically used Friday evening through Sunday afternoon. It sits idle Monday through Thursday with no electric loads other than the inverter in search mode (<6 watts).

PV System: I’ve got 1410Watts of PV’s, TriStar MPPT-60Amp Charge Controller, 24volt Concorde SunXtender AGM battery bank (305AH). Concord recommends a 2 hour absorption to charge the batteries.

Idea: Instead of doing a full charge with a (normal) 2 hour Absorption every time the sun comes out, I would reduce the Absorption time on the days when the batteries are already fully charged (typically Tuesday through Friday). The goal is to avoid over-charging and increase battery life.

Approach: I would program the Charge Controller to do 30 minute absorptions unless the battery voltage drops to 25.0 volts (75%). When the voltage drops below 25.0V the charge controller will do a 30 minute absorption PLUS an absorption extension of 1hr 30 min for a total of 2 hours. Otherwise it will do a daily absorption of 30 min.

Concerns: While it makes perfect sense to me, I’m concerned that this seems like an unusual approach. That tells me that perhaps this approach is not optimal.
*** I checked with Concorde Battery and they said they are “not familiar with this approach but it sounds reasonable”
*** I checked with Morning Star and they said “this would work”

Questions:
** Is doing 30 min Absorptions Monday through Friday instead of 2 hr. Absorptions a good idea?
** Will it do any good at prolonging the live of the batteries, OR am I wasting my time, OR I’m I reducing the battery life??!!
** Is anyone else doing a similar approach?

Cariboocoot · May 2012

Re: Optimal Absorption Time,,, Good –Bad- Wasted Effort?

Or, Why Outback Charge Controllers Are Really Good.

See, the Absorb Time has two parameters: minimum and maximum. Outback (and MidNite I'm sure) adjusts the Absorb Time within these programmable limits according to how long Bulk charging takes. This means if the batteries aren't drawn down much and charge up quickly (as when the system is not in use) the Absorb time is limited and keeps the water loss to a minimum.

With AGM's it shouldn't be an issue at all, as they are sealed to retain and recombine the hydrogen & oxygen. The only problem you might have there is with too high a Voltage popping the safety valve which might then allow water loss in subsequent charging.

I'm not sure how Morningstar handles Absorb Time. But so long as the batteries when not being used don't require more than the 30 Minutes for Bulk charging this should be fine. Being able to "extend" the Absorb Time if the battery Voltage falls below 75% (after several cloudy days perhaps) sounds reasonable too.

Coach Dad · May 2012

Re: Optimal Absorption Time,,, Good –Bad- Wasted Effort?

thanks Cariboocoot...
Will this extend my battery life? Is it a good thing to do?

I can set the extension voltage to anything I want... so I picket 75%... What would you recommend?

Cariboocoot · May 2012

Re: Optimal Absorption Time,,, Good –Bad- Wasted Effort?

Whether it will have a measurable effect on battery life is something you'll probably never know the answer to, as there are too many things that are involved. Let's say that in theory it should, because the less time in Absorb when not needed means less heating of the plates. This is usually not a problem with AGM's either. I'd say you'd be more likely to see the benefit on flooded cells.

I think 70 to 75 percent Voltage is a good target. That is about the threshold of "low charge state sulphation" (batteries left too long below that SOC sulphate more quickly than those kept up).

Anybody want to try this with some FLA's? We'd have definite answers sooner. The trouble with AGM's is you can't "look inside" to see what's happening (i.e. take SG readings, check water usage). Without actual load testing, you'll never really know until they're dead. If this comes after 8 years I'd call it a success.

Vic · May 2012

Re: Optimal Absorption Time,,, Good –Bad- Wasted Effort?

Coot,

I have never understood the use of Bulk time = Asorb time for my systems. Here, especially after the depths of Winter, the sun rises early, and the PV array gets a bit of sun -- enough to start Bulk, but for a number of hours the sun angle is SSSOOoooo low, and the sun rises behind the PV (we do not track), this means that the Bulk takes hours and hours at a fairly low charge current. The ideal Asorb takes less than one hour. Today Bulk will probably require 4 to 5 hours. The Bulk = Asorb time certainly would work poorly in this situation.

If one had a PV array in a valley, where there was shading until mid-morning, and then full sun, this approach might work much better.

Here, End Amps on the MX-60 or MN Classic IS nirvana. Just MHO, Vic

Cariboocoot · May 2012

Re: Optimal Absorption Time,,, Good –Bad- Wasted Effort?

Vic;

That's the reason for the Absorb Time maximum.

The only trouble with using End Amps is the unpredictability of loads skewing the number.

This is why we all want that charge controller with the built-in battery monitor! (Are you reading this, boB?

)

Vic · May 2012

Re: Optimal Absorption Time,,, Good –Bad- Wasted Effort?

Yes, EA MAY not be a good idea if the loads are unpredictable, AND out of control. But, even if a refer comes on, the typ 48 V system batt current is about four amps, and it runs only a few mins. This is no big deal in the EA scheme of things. One day the Asorb time gets a bit extended, and the next day it does not. The batts are teriffic charge integrators over an extended period of time. With FLAs, it is still quite easy to find the perfectly acceptable EA value, unless one's loads are LARGE, and out of control -- for ex, a large pump for domestic water COULD upset the balance, but, again, if the run-time of the pump is reasonable, it is still not a biggie when the effects are averaged over days/weeks. And with FLAs, one can actually measure the SOC, what a great concept!

With AGMs, ALL bets are off. Again MHO. Vic

boB · May 2012

Re: Optimal Absorption Time,,, Good –Bad- Wasted Effort?

Cariboocoot wrote: »

Vic;

That's the reason for the Absorb Time maximum.

The only trouble with using End Amps is the unpredictability of loads skewing the number.

This is why we all want that charge controller with the built-in battery monitor! (Are you reading this, boB? )

I sure am !! But I've known this for a LONG time now !!

That's why I originally put the option to pick the source modbus address for the battery monitor.

But, we weren't ready for an external monitor yet so I pulled that menu stuff out and in doing so,
didn't get the EA code going until..... until today after Vic and a couple others on our forum found that
it wasn't working right... OK, wasn't working at ALL because the code wasn't ever quite
implemented ! Oops !! Only the Ending Amps placebo effect code was evidently sort of working !
EA is working now and will be in the next rev in a couple (few?) days.

Battery monitor is very high on my list but there are still a couple of little things to get out
of the way first.

Thanks for all the input !

Now, back to pop some more placebos !
boB

PS... Did Ya know that if you mix IBM with LSD you get a Business Trip ??

SolaRevolution · May 2012

Re: Optimal Absorption Time,,, Good –Bad- Wasted Effort?

boB wrote: »

Oops !! Only the Ending Amps placebo effect code was evidently sort of working !
EA is working now and will be in the next rev in a couple (few?) days.

You mean to say I'm not crazy? I was trying, in vain, to get a system with two FO80s :-) to act in a predictable way.
So, what was stopping the charge? Not just the 90 min Minimum Absorb timer?

Now that we're going to get the real thing (instead of the placibo), how long is it gonna take to come on?
That is, how long does the current have to be below the End Amps before it goes to float?

Alex

BB. · May 2012

Re: Optimal Absorption Time,,, Good –Bad- Wasted Effort?

boB designed the Outback FX family (years ago), and now is at a "newer" company and has designed the Midnite Classic -- Which is the one with the missing end amp code (if I understood everything here).

I don't his post applied to the FX (now FM) family of Outback charge controllers.

-Bill

SolaRevolution · May 2012

Re: Optimal Absorption Time,,, Good –Bad- Wasted Effort?

BB. wrote: »

boB designed the Outback FX family (years ago), and now is at a "newer" company and has designed the Midnite Classic -- Which is the one with the missing end amp code (if I understood everything here).

I don't his post applied to the FX (now FM) family of Outback charge controllers.

Oh. My bad. I lost track of who did what with who.
This industry is so incestuous.:p

I'm still tryin to figgure out why I can't get two FM80 to interact in ways where I can predict the absorb time as it will show in the logs.

Alex

boB · May 2012

Re: Optimal Absorption Time,,, Good –Bad- Wasted Effort?

SolaRevolution wrote: »

You mean to say I'm not crazy? I was trying, in vain, to get a system with two FO80s :-) to act in a predictable way.
So, what was stopping the charge? Not just the 90 min Minimum Absorb timer?

Now that we're going to get the real thing (instead of the placibo), how long is it gonna take to come on?
That is, how long does the current have to be below the End Amps before it goes to float?

Alex

Around 60 seconds

Is that OK ??

boB

Vic · May 2012

Re: Optimal Absorption Time,,, Good –Bad- Wasted Effort?

Hi boB,

Well, I THOUGHT that I did see EA work on one of my Classics, once. This was when had induced a Re-Bulk IIRC, and stood there watching it go from Asorb to Float. Must have been something else that ended it ... Min was 00:00, and Max was 3:00 hrs. Perhaps the Classic can measure intent. I really wanted the EA function to work that day.

The venerable MX-60 has been a great box for almost a decade. The Classic is even better, and continues to get better yet. But, considering when the MX was designed, it is truly amazing! Still have one working here, and it does a very good job. Thanks boB! Vic

boB · May 2012

Re: Optimal Absorption Time,,, Good –Bad- Wasted Effort?

Vic wrote: »

Hi boB,

Well, I THOUGHT that I did see EA work on one of my Classics, once.......

The Classic Placebo effect is very strong with this one !

Vic wrote: »

.........Thanks boB! Vic

You're very welcome !

boB

petertearai · May 2012

Re: Optimal Absorption Time,,, Good –Bad- Wasted Effort?

As the coot says, that is why the outback charger is good. That is how my system works, during the week the absorb will typically go for 15 minutes or so before going to float. From memory I have the absorb set to 3 hours, and 6 amps end amps.So during the week the end amps takes over and limits the absorb to 15 minutes.

SolaRevolution · May 2012

Re: Optimal Absorption Time,,, Good –Bad- Wasted Effort?

boB wrote: »

Around 60 seconds
Is that OK ??
boB

Sounds good for the vast majority of systems.

A few issues:

If you have multiple charge controllers one can "EA" out before it "should". Even if the CCs are closely calibrated, the different lengths of wire runs can make one "see" a higher battery voltage than another. My friend Wayne has a classic and a Tristar in his offgrid system. He was checking the logs and noticed that the Tristar logs more daily output than the Classic even though the Tristar has a smaller array. I was trying to explain to him that if you look at the way the Tristar uses low voltage thresholds and timers and the Classic works on EA this makes sense and it is not an apples-to-apples comparison. Wayne already knows everything so it is hard to get him to look at the concept. :-)The batteries seem to be doing well anyway so it's not such a big deal in this case.

The owner of the other system I was referring to (with 2 FM80s) was trying to check his logs to see how much absorb and float time his batteries have been getting on a regular basis. He wanted me to try to program them so that one or the other would log at least 2 hrs of absorb time every day.

He has a nightly DOD of about 25-35%. They frequently get between 30-90 minutes at absorb but his batteries seem to be getting somewhat under charged on a regular basis even with the EA off or set at 1% of battery capacity. He was trying to follow the oft told advice of running his generator for an hour in the morning and letting the PV do a finish charge. He would sometimes get "Batt Full" or more often Float unless he did a manual "force bulk". He has started doing monthly EQs instead.

Another issue I've come across is for grid-tie. If there is a hicup in the grid, the CCs will go to float or in the case of multiple CCs, some may go to "Batt Full" in the 5 minutes it takes for the inverters to reconnect. These hicups can happen for various reasons; grid instability, air compressors, air conditioners, garage door openers... The CCs will often be in float for the rest of the day. The system still sells once the inverters reconnect but only about 60-80% of what it does when the CCs are in bulk. Maybe a "GT" setting in the CCs could make the EA timer last for 10 minutes to avoid changing mode over these hicups.?

BTW: I really like Outback CCs. Especially the logs and the "T-bird" chrome. I'm just getting to know the Midnite Classics.
Thanks for your attention,

Alex Aragon

Cariboocoot · May 2012

Re: Optimal Absorption Time,,, Good –Bad- Wasted Effort?

Another nice thing about some charge controllers is that you can connect the communication ports and get them to share data so there's no discrepancy. Outbacks will do this (requires a HUB and a MATE). MidNite Classic will do this. Two different brands of charge controller won't do this. The trick there is to pick a "primary" and "secondary" charge controller and accept that one of them will be operating less than optimally.

SolaRevolution · May 2012

Re: Optimal Absorption Time,,, Good –Bad- Wasted Effort?

Cariboocoot wrote: »

Another nice thing about some charge controllers is that you can connect the communication ports and get them to share data so there's no discrepancy. Outbacks will do this (requires a HUB and a MATE). MidNite Classic will do this. Two different brands of charge controller won't do this. The trick there is to pick a "primary" and "secondary" charge controller and accept that one of them will be operating less than optimally.

Will the networked CCs actually "share data"? As far as I'm aware, the only data the Outbacks share is the RTS for Temp Comp, IF there is an inverter in the system, IF the RTS is plugged into the master inverter. The CCs will still operate individually for charging mode, no?

Are Midnites any different?
I believe that networked XWs can be in different modes too.

Alex

boB · May 2012

Re: Optimal Absorption Time,,, Good –Bad- Wasted Effort?

SolaRevolution wrote: »

Will the networked CCs actually "share data"? As far as I'm aware, the only data the Outbacks share is the RTS for Temp Comp, IF there is an inverter in the system, IF the RTS is plugged into the master inverter. The CCs will still operate individually for charging mode, no?

Are Midnites any different?
I believe that networked XWs can be in different modes too.

Alex

I'm working on the CC to CC network coordination now.

Classic to "other" CC systems' possibilities are also in the works here at MidNite !

boB

Derik · May 2012

Re: Optimal Absorption Time,,, Good –Bad- Wasted Effort?

I would simply do what the manufacture recommends and leave it at that. I have a weekend cabin but don't adjust the charge controller when I am there and then readjust it when I leave. Too much fiddling. I like the idea of learning what works for my system and then leaving it alone. For me it's 5 hours absorb in the winters and 3.5 or so in the summers. I have L-16's so I can check them with a hydrometer.

Coach Dad · May 2012

Re: Optimal Absorption Time,,, Good –Bad- Wasted Effort?

Derik wrote: »

I would simply do what the manufacture recommends and leave it at that. I have a weekend cabin but don't adjust the charge controller when I am there and then readjust it when I leave. Too much fiddling. I like the idea of learning what works for my system and then leaving it alone. For me it's 5 hours absorb in the winters and 3.5 or so in the summers. I have L-16's so I can check them with a hydrometer.

Thanks Derik... I've asked the manufacture about this approach and their answer was "we are not familiar with this approach, but it sounds reasonable."
The Charge Controller software has the capability to automatically do this, so why not use it IF it will increase battery life??

BTY: I've asked the battery manufacture if it will actually increase battery life. I'm waiting for their reply.

stephendv · May 2012

Re: Optimal Absorption Time,,, Good –Bad- Wasted Effort?

Coach Dad, I think you're on the right track by making those changes. And from what (little) I understand about the causes of failure for lead acid batteries, the changes you propose to make should lengthen battery life.

Coach Dad · May 2012

Re: Optimal Absorption Time,,, Good –Bad- Wasted Effort?

If anybody is interested... I just got word from the battery manufacturer regarding this question...

They say: Engineering believes that 30 minutes instead of 2 hours at Vabs should benefit the life of the battery since the battery is not being used Monday thru Friday.

I'm going to make the change to my settings next weekend when I'm up there...

SandyP · May 2012

Re: Optimal Absorption Time,,, Good –Bad- Wasted Effort?

Thanks Coach Dad,
I have wanted to get confirmation of this for quite a while. I've set my Outback FM-60 to stop absorb after reaching ~1% of total battery bank Ah and with a similar usage (weekend) as yourself have seen only ~30 mins per day when no usage on weekdays.
Regards, SandyP

oliline · December 2017

One question I always had and never got to understand..
Let's say the Abs to Flo Current is set to 2% of the capacity of the battery (at C20 ok?) to prevent overcharge. Let's say "the loads exceed that current and do not allow the absorption to terminate".
Why would it be the problem?? Can the absorption not be continued next day?
Why do we really need to limit absorption after a specific time?

Estragon · December 2017

Charging with solar, we probably could get away with no absorb timeout, since the sun reliably sets every night in most places, ending absorb regardless. It just means some needless gassing etc.

mike95490 · December 2017

I've also heard set Absorb to the same time as it usually takes Bulk to run.

Estragon · December 2017

I've heard that too, but my bulk can be anywhere from ~1/2hr to as long as there's light. Not sure what normal or "usually" would be for me. Absorb usually takes 2-3hrs to 1.5% of C though, so I guess that would be it.

Optimal Absorption Time,,, Good –Bad- Wasted Effort?

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