daily absorption time for backup batteries?

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daily absorption time for backup batteries?

Postby Megunticook on Fri Jan 11, 2019 6:23 am

In situations where the battery bank is strictly for the occasional power outage, and therefore stays full most of the time, what's the proper amount of absorption each day?

While checking s.g. last month I noticed some drift downward on a few of the cells (range was from 1.285 -1.255 at 0 degrees C). I figured it was time to equalize, so I contacted a Rolls battery tech., and he thought that the reason for the drift was my daily absorption time was too short (30 minutes). He recommended doing 3 hours, then checking s.g. in a month, and possibly reducing the time based on results.

That seems like an awful long absorption phase for batteries which essentially start the day full.

What are others with similar battery use doing for daily absorption?
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Re: daily absorption time for backup batteries?

Postby JRHill on Fri Jan 11, 2019 12:13 pm

When the days are long enough to support the absorb time, I go for 4.5 hrs at 58.8v. For my relatively light load to the batteries this fully charges them fully every day and negates the need run an EQ cycle. The need to water up the batteries isn't really that much more often. During the winter season when charging needs to be supplemented via the genset (mid Oct through early March) I drop down to 2 hours of absorb. But during this time I still run the full 4.5 hr absorb at least once per week. I learned an expensive lesson with my first battery bank (5 years and they had to go) by being too conservative with the absorb time.

Another lesson learned is to do a deep, heavy draw down and full recharge cycle on a monthly basis. I wish I could also throw more charging amps at the bank with the same interval but am limited to the 20 amps from the FX. Oh well...

One thing I want to bring to your attention is that a bank used just for backup does not get the deep, heavy draw down and the corresponding strong recharge to maintain the bank's health unless it is done purposely. It is my belief that you could be reducing the life of the bank by just topping them off daily, esp. with a short absorb. That may be what you are seeing in the SGs or at least this may be a contributor.

I am not grid tied so I have no experience with the set-up so as to keep the absorb up while still selling back the greatest amount of power. That would seem to be quite a hairball.

Full disclosure: I only verified the above suggestions to be the true best practices for my system. I certainly didn't dream the process up as all the info is in the many threads in this forum related to charging parameters. Thanks to all of you folks who have helped me to not bring another early demise to a new bank of batteries - which are 3 years old and looking really good. At least I think so 'cause I really don't take SGs that often cause I can tell you what they will be beforehand.
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Re: daily absorption time for backup batteries?

Postby Megunticook on Fri Jan 11, 2019 12:57 pm

Thanks for sharing your experience. Using the formula provided by my battery manufacturer, Rolls, my absorb time would be 4.2 hours. But isn't that after a drawdown? My understanding is absorption is the part of the recharge cycle where you need to raise the voltage to "push in" the last 20% of amp hours capacity. But in my case the battery is only dishcarged a very small amount overnight (maybe 1-2% of its capacity), so I would think that after a very short period of absorption it would already have accepted as much as it could. Again, according to Rolls, after the current going into the batteries in Absorption drops below about 8 amps, the batteries are effectively full. In a typical morning absorption for me, the current is less than that.

So basically I'm holding the batteries at a higher voltage but they're not able to accept any more charge. That seems like something to avoid. Better to drop down to float voltage after they're fully topped off, right?

But something was definitely allowing those s.g.'s to drift a bit.

I've been doing deliberate discharges every other month or so. The Rolls tech. said that it was good to "exercise" these batteries regularly. He also confirmed that if you manage charging properly there's never a need to EQ as you said.

Theoretically I could "sell" power during absorption, but I"d have to do that by setting the float to absorption voltage, because the system won't offset excess power when in absorption mode. Then I'd have to manually reset the float voltage after the absorption time is up so it doesn't sit at that higher voltage all day long.

I'm going to try 2 hours of absorption a day and see how that works. The majority of the kWh I sell are generated in the 3-4 hours around solar noon, so I'm probably not losing too much value with a 2-hour absorption window early-mid morning.

We'll see...if the s.g.'s don't drift after a few weeks of this routine, I might cut back to 1 1/2 hours and see how that does.
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Re: daily absorption time for backup batteries?

Postby Kent Osterberg on Fri Jan 11, 2019 5:21 pm

It is very likely that the reason you are seeing the spread in the specific gravity values is that the battery electrolyte is stratified. A short absorption time on batteries that aren't discharged much will leave the batteries with lots of time just sitting. They need to bubble long enough to keep them stirred up. I'd increase the absorption time to 2 hours and then recheck in a month. Don't hesitate to go with a longer absorption time. Occasional deep discharges is a good idea too.
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Re: daily absorption time for backup batteries?

Postby SandyP on Sat Jan 12, 2019 3:26 am

Megunticook wrote:.../.. Again, according to Rolls, after the current going into the batteries in Absorption drops below about 8 amps, the batteries are effectively full. In a typical morning absorption for me, the current is less than that.
../...


From forum feedback I was advised that even 1 hour is too short and that really the absorb phase should continue until the charging amps drops to 1% of the C20 battery capacity. I have lowered that even lower for my bank and, like you with low discharges, even then may only see 1/5 hours of absorb but with GEL batteries I do not need to stir up the electrolyte etc.
So see what time the absorb phase takes to get the charging amps down to 4 amps for your battery bank.
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Re: daily absorption time for backup batteries?

Postby blackswan555 on Sat Jan 12, 2019 4:48 am

In situations where the battery bank is strictly for the occasional power outage, and therefore stays full most of the time, what's the proper amount of absorption each day?

None, If it is a pure grid tie system with occasional emergency back up it should stay in float 24,7 untill it has been "used" then bulk & absorb back to full,
General rule of thumb on this is you should run an EQ about every 6 months or when cells have drifted by about 30 points from each other,
It is a very good idea when you do your EQ, to drain the batteries down to about 40 to 50 SOC with a fairly hard load, then charge to full ( Make them work as hard as you can, Combats Sulphation)

End Amps, Be careful using this setting as a charge termination indicator, You can watch "the number" see what it is doing for a reasonable indication (when it stays stable for about half hour, batteries are charged) But dialing it in as a setting can cause problems, It varies due to load and other influences,

Tim
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Re: daily absorption time for backup batteries?

Postby Megunticook on Sat Jan 12, 2019 7:25 am

blackswan555 wrote:None, If it is a pure grid tie system with occasional emergency back up it should stay in float 24,7 untill it has been "used" then bulk & absorb back to full

That was my intuitive sense initially, but Rolls is telling me different. Although mine is not on float overnight, just during the day when the PV is generating.

blackswan555 wrote:General rule of thumb on this is you should run an EQ about every 6 months or when cells have drifted by about 30 points from each other

My battery bank is about 14 months old now, I EQ'd in July and again just last week. Didn't discharge the batteries before EQ, I can see where that might really "stir things up" with the electrolyte and clean off the plates, as long as you don't push too much current in too fast and heat things up.

I've switched to a 2 hours absorb each morning and will do a 70% discharge/recharge every other month and see how that works. But my gut is telling me there's really no need to do much absorption when the batteries are 98-99% full each morning.
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Re: daily absorption time for backup batteries?

Postby blackswan555 on Sat Jan 12, 2019 7:59 am

Although mine is not on float overnight, just during the day when the PV is generating.
It would need to be on float 24hrs,

I've switched to a 2 hours absorb each morning
Watch your SGs near end, if they stop rising, bats are charged
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Re: daily absorption time for backup batteries?

Postby Mike Curran on Sat Jan 12, 2019 8:42 am

My system operates much the same as yours, only used for backup power and SOC stays around 99-100%. I have my "charged time" on the FNDC set for 5 minutes. I haven't done a full set of SG's in awhile but under the above regimen, whenever I do a full set all my SG's are in line.

Right now it's cloudy here but my charge controllers (FM80 & MX60) are in bulk at 29.3 volts, 15°C (about 28.9V TC), and charge current to my battery is only 15.4 amps.
Screenshot_20190112-103846.jpg
I don't expect current will increase much if/when voltage reaches the absorb setpoint (29.4 TC) so I'm already at a point below the 2-3% of C20 recommendation for my battery. So basically, I'm with you and Tim on this, my experience says no need to do a prolonged absorb for batteries that meet charge parameters daily and are kept at float almost all the time. (UNLESS someone thinks the low charge current I'm seeing is due to sulfated batteries, which I doubt.)

Fyi, I do a monthly cycle of my battery down to 70% SOC. Still seems like my old batteries have a lot of life left in them.

FWIW - Mike
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Re: daily absorption time for backup batteries?

Postby Megunticook on Sat Jan 12, 2019 8:58 am

I suppose it's possible the Rolls tech. is giving me bad advice. I wonder if the "drift" I was seeing in s.g.'s was because I have my float voltage too low. I'm using their specs. (54v), but perhaps I should try raising that a little bit.

The Rolls guy actually said I should do a three hour absorption daily to start out with. With nearly-full batteries, that just doesn't make any sense to me, but I hesitate to go against what the manufacturer recommends. But maybe this particular fellow just doesn't have it right, or is misunderstanding my situation.

I guess I have to decide for myself what works and doesn't work for me, based on my own data and observations.
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