Absorption Time

NorthGuy

I understand that every situation is different and therefore it's impossible to determine absorption time in general terms. That is probably why the absorption duration is missing from most battery guides. However, it's very interesting to know what are real numbers.

How much absorption time do your batteries need to return to full charge (as indicated by SG) after being discharged to about 50% DOD?

stephendv

Re: Absorption Time

Couldn't say for 50% DoD, but for 25% DoD to fully charged takes about 3 hours.

Cariboocoot

Re: Absorption Time

There's more than one factor at work here. First is how big the batteries are in Amp hours. Second is the state of charge. Third and so on are the smaller issues such as temperature, manufacturer's recommendations, et cetera.

You can determine a good length for your own set by using the right equipment and a bit of patience.
Set your charge controller's Absorb time limit to the maximum. Discharge batteries to 50% +/- SOC. Let the charging begin. Observe when Absorb stage starts, and time this until the current flow is down around 2% of the battery capacity (with NO loads drawing). This would be the End Amps. Round the time up to the nearest hour and that would be your maximum Absorb time. Set the End Amps if your charge controller has this function, but be sure to take into account any 'steady' loads such as feeding the inverter.

That way you'll have a maximum time for Absorb and an End Amps setting that will stop Absorb at the maximum practical point. If your charge controller won't do this, get a new charge controller.

Unfortunately you can't just say 'X' Amp hours of battery needs 'Y' hours of Absorb at 'Z'% SOC.

NorthGuy

Re: Absorption Time

Cariboocoot wrote: »

That way you'll have a maximum time for Absorb and an End Amps setting that will stop Absorb at the maximum practical point. If your charge controller won't do this, get a new charge controller.

Unfortunately you can't just say 'X' Amp hours of battery needs 'Y' hours of Absorb at 'Z'% SOC.

I understand that there are many factors at work. What I'm looking for are real-life examples. What absorbtion time woould you need with your batteries to fully charge after discharge to 50% DOD.

Cariboocoot

Re: Absorption Time

NorthGuy wrote: »

I understand that there are many factors at work. What I'm looking for are real-life examples. What absorbtion time woould you need with your batteries to fully charge after discharge to 50% DOD.

That's easy: two hours.
That's 232 Amp hours of 24 Volt battery. The MX60 is set to a max of 4 hours just because and the End Amps is set to 7. Since I do take it down around 50% almost daily, this works.

If this was a bigger bank, it would need more time. That's where you can easily run into the "oops! Out of daylight!" problem.

NorthGuy

Re: Absorption Time

Cariboocoot wrote: »

That's easy: two hours.
That's 232 Amp hours of 24 Volt battery. The MX60 is set to a max of 4 hours just because and the End Amps is set to 7. Since I do take it down around 50% almost daily, this works.

What kind of batteries do you have?

Cariboocoot

Re: Absorption Time

NorthGuy wrote: »

What kind of batteries do you have?

East Penn PS2200
6 Volt 232 Amp hour

Fullpower

Re: Absorption Time

Rolls S1590 2Volt, 1200Amp-Hour
Absorb 4 hours, 2.5 volts per cell.

Cariboocoot

Re: Absorption Time

Fullpower wrote: »

Rolls S1590 2Volt, 1200Amp-Hour
Absorb 4 hours, 2.5 volts per cell.

See, that's not easy to do with solar; get 4 hours of Absorb time in any given day.;)

Vic

Re: Absorption Time

NorthGuy, recall that your battery bank is a nominal 670 Ah 1.260 sg,

For the banks here, from about 60 -65% SOC, Absorption requires about 3.5 - 4 hours on the first day. The Abs time DOES depend on the charge current available during Bulk. A high Bulk current requires a longer Abs time than would be required if the available current in Bulk was lower.

Also, when these bank(s) are cycled deeper, the recharge on the following day is affected -- it requires more recharge energy than one would expect for the amount of discharge on the on the night after the deep discharge/recharge. It appears that the bank was not really fully recharged, even when using EA as an Absorption terminator. In my case the EA value is 18 on one system and 15 on the other -- when monitoring actual battery charge current, this is very close to 1% of actual 20-Hr Capacity.

The Vabs is in almost always 58.3 - 58.5 on these banks. The optimum EA value is affected by the Absorption V (of course).

Fullpower notes his 2V bank of Surrettes use a fairly high Vabs for the noted four hour Absorption, probably in an attempt to get the recharge done as quickly as possible, as he is in AK. Just my observations.

Also, the recharge parameters will change when the bank has been cycled 30 - 50 times, and as the bank ages. BUT, since your batteries are Flooded, it is easy to determine just how the bank is responding to your charging parameters.

Am curious how the bank is doing now, after the initial difficulties.
Opinions, Good Luck, Vic

inetdog

Re: Absorption Time

Vic wrote: »

The Abs time DOES depend on the charge current available during Bulk. A high Bulk current requires a longer Abs time than would be required if the available current in Bulk was lower.

You are figuring that the higher available Bulk current will result (because of wiring and internal battery resistance) in the max Bulk voltage being reached sooner (at a lower SOC) than with a lower current? If so, then monitoring the voltage at the battery to offset wiring voltage drop and maybe raising the Bulk to Absorb transition voltage setting should allow that effect to be tuned out, yes?
Or are you thinking that the higher current Bulk just does not do as effective a job at charging the battery?
Looking at the actual current versus time during Absorb should answer that question. If there is a long period of constant current at the start of Absorb, then you switched out of Bulk too soon.

NorthGuy

Re: Absorption Time

inetdog wrote: »

You are figuring that the higher available Bulk current will result (because of wiring and internal battery resistance) in the max Bulk voltage being reached sooner (at a lower SOC) than with a lower current?

I think it's simpler. Let's say you have two current levels - "High" and "Low". While charging with High current, the batteries will reach the absorption voltage at point A. Then the current starts decreasing and then becomes equal to the "Low" current at point B. If you would charge with the "Low" current from the beginning, the absorption would only start at point B (in terms of SOC), and then would proceed the same as with "High" current. Therefore, absorbtion with "High" current is longer than the absorption with "Low" current by the time that is necessary to move from point A to point B.

inetdog

Re: Absorption Time

NorthGuy wrote: »

I think it's simpler. Let's say you have two current levels - "High" and "Low". While charging with High current, the batteries will reach the absorption voltage at point A. Then the current starts decreasing and then becomes equal to the "Low" current at point B. If you would charge with the "Low" current from the beginning, the absorption would only start at point B (in terms of SOC), and then would proceed the same as with "High" current. Therefore, absorbtion with "High" current is longer than the absorption with "Low" current by the time that is necessary to move from point A to point B.

Got it! The total charge time will decrease because of the greatly reduced time spent in Bulk, but the amount of time spent in Absorb will increase both as an absolute time and even more as a proportion of the time spent in Bulk. That makes the use by some CCs of an algorithm which calculates maximum Absorb time as a multiple of the time spent in Bulk seem misguided. :-)
Even if the maximum Bulk current setting the in the CC is not changed, the available current from the PV panels will change during the day. Definitely food for thought.

NorthGuy

Re: Absorption Time

inetdog wrote: »

Got it! The total charge time will decrease because of the greatly reduced time spent in Bulk, but the amount of time spent in Absorb will increase both as an absolute time and even more as a proportion of the time spent in Bulk. That makes the use by some CCs of an algorithm which calculates maximum Absorb time as a multiple of the time spent in Bulk seem misguided. :-)
Even if the maximum Bulk current setting the in the CC is not changed, the available current from the PV panels will change during the day. Definitely food for thought.

For some reason, which I don't understand, if I had a deeper discharge (and consequently longer bulk given the same current), the current falls slower during the absorption, and the absorption time increases. At least that's what happens with my batteries.

NorthGuy

Re: Absorption Time

I want to thank you everyone who posted.

I started this thread because I wanted to find out if the batteries with thicker plates and longer exoected cycle life (Surrette 5000 at the extreme) would require longer absorptions as batteries with thinner plates and shorter expected cycle life.

I looked at the results here as well as elsewhere on this forum and the Internet and it appears that this is indeed true, but the difference between absorption times is not big - everyone seem to fit between 2 and 4 hours (most between 3 and 4). Even though long absorption times are very bad for the efficiency of the system, especially in the North, this probably shouldn't be a leading factor when selecting the batteries.

BB.

Re: Absorption Time

Here is another take on Sulphation (assuming talking about wheel chair batteries that are around 12 AH to 90 AH--so 200 mA rate of charge is probably around 1% or so for this discussion):

http://batteryuniversity.com/learn/article/sulfation_and_how_to_prevent_it/

Electric wheelchairs have a similar problem in that the users might not charge the battery long enough. An eight-hour charge during the night when the chair is free is not enough. Lead acid must periodically be charged 14–16 hours to attain full saturation. This may be the reason why wheelchair batteries last only two years, whereas golf car batteries deliver twice the service life. Longer leisure time allows golf car batteries to get the fully saturated charge.

Solar cells and wind turbines do not always provide sufficient charge, and lead acid banks succumb to sulfation. This happens in remote parts of the world where villagers draw generous amounts of electricity with insufficient renewable resources to charge the batteries. The result is a short battery life. Only a periodic fully saturated charge could solve the problem, but without an electrical grid at their disposal, this is almost impossible. An alternative is using lithium-ion, a battery that is forgiving to a partial charge, but this would cost about six-times as much as lead acid.
...
There are two types of sulfation: reversible (or soft sulfation), and permanent (or hard sulfation). If a battery is serviced early, reversible sulfation can often be corrected by applying an overcharge to a fully charged battery in the form of a regulated current of about 200mA. The battery terminal voltage is allowed to rise to between 2.50 and 2.66V/cell (15 and 16V on a 12V mono block) for about 24 hours. Increasing the battery temperature to 50–60°C (122–140°F) further helps in dissolving the crystals. Permanent sulfation sets in when the battery has been in a low state-of-charge for weeks or months. At this stage, no form of restoration is possible.

-Bill

NorthGuy

Re: Absorption Time

BB. wrote: »

Here is another take on Sulphation

I think that wheel chair chargers are purposedly undersized to prevent bubbling. People in wheel chairs may have hard time adding water to their batteris. And they don't want their batteries to smell like rotten eggs. Hydrogen may be dangereous too because you cannot provide adequate ventilation. This causes undercharging and helps sulphation.

Solar batteries do not have any of these constrains.

Cariboocoot

Re: Absorption Time

I should think wheelchairs would have sealed AGM batteries anyway (standard flooded cells wouldn't be safe in my opinion). They tend to charge at lower Voltages than FLA's. Whereas they are still lead-acid and still subject to sulphation the effects are not as severe as with flooded cells.

NorthGuy

Re: Absorption Time

Cariboocoot wrote: »

I should think wheelchairs would have sealed AGM batteries anyway (standard flooded cells wouldn't be safe in my opinion). They tend to charge at lower Voltages than FLA's. Whereas they are still lead-acid and still subject to sulphation the effects are not as severe as with flooded cells.

You're probably right. But in this case, you cannot really compare them to golf cart batteries, which are usually FLA.

AGM do not have problems with absorption time because you do not need to steer the electrolyte. I would think that longer absorption times may be even harmful to them.

inetdog

Re: Absorption Time

NorthGuy wrote: »

I would think that longer absorption times may be even harmful to them.

As long as you stay within the oxygen recombination limit of the AGM and do not overheat it, I do not think it matters.

Vic

Re: Absorption Time

inetdog wrote: »

You are figuring that the higher available Bulk current will result (because of wiring and internal battery resistance) in the max Bulk voltage being reached sooner (at a lower SOC) than with a lower current? If so, then monitoring the voltage at the battery to offset wiring voltage drop and maybe raising the Bulk to Absorb transition voltage setting should allow that effect to be tuned out, yes?
Or are you thinking that the higher current Bulk just does not do as effective a job at charging the battery?
Looking at the actual current versus time during Absorb should answer that question. If there is a long period of constant current at the start of Absorb, then you switched out of Bulk too soon.

Hi inetdog,

Well, in the system with the largest PV array, the voltage drop at around 80 A charge current IS about 0.5 V, which has an effect, but should not a large one. The MN CCs on that system do not (at this point) have remote V sense. But this could possibly change when they release the Shunt-based battery current monitor, which will be able to terminate Absorb at a specified battery charge current.

It is my belief after wacthing the two main systems here, that on a deeper discharge, the first recharge, using EA as the Abs termination does not fully recharge the battery. The second day's recharge seems to require more more AH to reach that same EA value. Seems to me that discharging the bank significantly below its normal DOD reqires one or two more days of recharge to the identical EA value to really return the bank to 100% SOC, based on chargeing to the identical EA value. These two systems use two different CCs -- an MX-60 on one, and MN Classics on the other.

Some of the above may result from the these banks being cycled between 100% and 92 or 94%-ish SOC almost daily. I have not run experiments to see the result of cycling these banks to 80-ish SOC on each day, and then revert to the noted shallow cycling to see the difference.

I would love to see the Classic CC be able to avoid recharging batteries until the nominal SOC is 80-ish% (hopefully this threshold value would be settable).

NorthGuy,

The approximate Absorb time that I mentioned previously, was just a rough recollection. I do not actually record data, eventhough the Classic presents it on comm ports.

I agree with your observation regarding a deeper discharge results in a surprisingly short (in my opinion) Bulk, and a surprisingly long Absorption, and yet does not really result in a complete recharge on that day, using EA as an Abs terminator. Believe that perhaps the batteries would like a recharge that more closely matches the nature of the discharge to fully recharge the bank in a single day (whatta I know ?!). In the cases of these 'forced' discharges to lower SOC end with a discharge at a current about equal to the 20 Hour rate, after three or four days of no recharge (PV input shut off).

Am sorry that your batteries have been such a problem for you. Hope that Trojan will accept your returning them for a battery better suited to your needs.

Best wishes for Good Luck, Vic

NorthGuy

Re: Absorption Time

Vic wrote: »

I agree with your observation regarding a deeper discharge results in a surprisingly short (in my opinion) Bulk, and a surprisingly long Absorption, and yet does not really result in a complete recharge on that day, using EA as an Abs terminator.

So, you believe that if you discharge deeper than usual, you would take several days of cycling to eliminate the effect. Correct?

Vic

Re: Absorption Time

NorthGuy wrote: »

So, you believe that if you discharge deeper than usual, you would take several days of cycling to eliminate the effect. Correct?

Yes, that is what I believe from observation, but, 1; I have no actual measured DATA, and 2; The first recharge may well get 90% of the job done, the second day may well get 90% of any remaining undercharge charge done, so this seems (if true) would sort itself out and really be in the noise after the second day.

Many do not use their battery banks as I do, so those who DO take data may not have seen such an effect. When time permits, will try to take some actual data, but the data should come from a Battery Monitor type device to separate the charging data from charging + loads. I may just be full of "it"

Have Fun, Vic

NorthGuy

Re: Absorption Time

I had further thoughts about the difference in absorption burden between different batteries.

When the absorption starts, the batteries are at a certain DoD, and this DoD must be charged into the batteries. In addition there should be some extra charge to produce bubbling etc. The sum of this DoD and overhead will determine the duration. Assuming that we charge at C/10, I created a simple table:

Absorption    Bubbling      Amout to       Duration    Absorption
Start SoC     Overhead     Be recharged     At C/10     duration
  % of C        % of C       % C             hours        hours
-------------------------------------------------------------------
   95             5           10              1.0           2
   95            10           15              1.5           3
   90            10           20              2.0           4
   85            10           25              2.5           5
   80            10           30              3.0           6
   80            15           35              3.5           7
   75            10           35              3.5           7
   75            15           40              4.0           8
===================================================================

The "Amount to recharge" column simply sumps up DoD and overhead. The "Duration at C/10" calculates how much time it would take to recharge this amount at constant C/10 current. However, during absorption, the current will taper from C/10 at the beginning to nothing at the end, so the absorption duration will be twice as long, which is in the last column.

Golf cart batteries get absorbed in 2 hours. This can only be if they do not reach absorption voltages until they're 95% charged and overhead is very small.

L-16 or similar batteries can absorb in 4 hours. This means that they're probably at 90% SoC when absorption starts.

Industrial/fork lift batteries absorb in about 6 hours, which mean they already reach absorption at 80% and have 10% overhead.

If the above is true, this dramatically affects the cycling room. If you cycle from 50% to the beginning of absorption most of the days, then with GC batteries you'll get 95-50 = 45% of capacity. With L-16 you get 90-50 = 40% of capacity, and with industrials 80-50 only 30% of capacity. Of course, with industrials you can go to 20% SoC so 80-20 = 60%, which is better than GC, but not dramatically. Looks like smaller batteries give you more bang for the buck.

Does this make any sense?

vtmaps

Re: Absorption Time

NorthGuy wrote: »

I had further thoughts about the difference in absorption burden between different batteries.
<snip>
Does this make any sense?

I think you need to take into consideration stratification. Stratification occurs while charging a battery, at least until gassing begins to reverse it. Stratification ALSO occurs during discharge.

Thus, cycling a battery without achieving 100% recharge 'ratchets up' the stratification with each (partial) cycle. When the batteries are stratified, the lower plates see a higher concentration of electrolyte than the upper plates. This means that the upper plates and the lower plates have a different electrochemical voltage, and since they are connected in parallel, there will be internal currents flowing between the upper and lower plates.

The reason it takes longer (or more daily charge cycles) to absorb a deeply discharged battery may have something to do with the degree of stratification.

GC batteries, being shorter, have less issues with stratification. This may account for some of the efficiency in your analysis. Batteries with electrolyte recirculation systems are known to charge more efficiently.

--vtMaps

NorthGuy

Re: Absorption Time

vtmaps wrote: »

Thus, cycling a battery without achieving 100% recharge 'ratchets up' the stratification with each (partial) cycle. When the batteries are stratified, the lower plates see a higher concentration of electrolyte than the upper plates. This means that the upper plates and the lower plates have a different electrochemical voltage, and since they are connected in parallel, there will be internal currents flowing between the upper and lower plates.

Yes, I can see that with my batteries. The first charge after full absorption, the absorption voltages get reached at 85% SoC, but, after a week of cycling, this happens much earlier, sometimes almost at 70% SoC. It probably would get worse if I continued, but I do the absorption and remove the stratification.

vtmaps wrote: »

The reason it takes longer (or more daily charge cycles) to absorb a deeply discharged battery may have something to do with the degree of stratification.

GC batteries, being shorter, have less issues with stratification. This may account for some of the efficiency in your analysis.

Then the small batteries not only should have better characteristics at the beginning of the cycling, but also the amount by which they 'ratchet up' should be less. So, they may be able to go longer without absorption, which makes them even more suitable for RE applications.

vtmaps wrote: »

Batteries with electrolyte recirculation systems are known to charge more efficiently.

I guess, with electrolyte recirculation, you would get all the benefits of heavy batteries without needs for horrible absorptions. For some reason, batteries with electrolyte recicrulaton are extremely rare.

vtmaps

Re: Absorption Time

NorthGuy wrote: »

Then the small batteries not only should have better characteristics at the beginning of the cycling, but also the amount by which they 'ratchet up' should be less. So, they may be able to go longer without absorption, which makes them even more suitable for RE applications.

In theory, as I understand it, yes. BUT the facts appear to be that tall batteries last longer than the short batteries. That is probably because of other factors in the construction of tall vs short batteries.

I think that cheap, short batteries are a very reasonable choice in small RE systems where the needs can be met by a single string of these batteries.

--vtMaps

NorthGuy

Re: Absorption Time

vtmaps wrote: »

In theory, as I understand it, yes. BUT the facts appear to be that tall batteries last longer than the short batteries. That is probably because of other factors in the construction of tall vs short batteries.

Long life is a different story. They may not last long, but be more suitable for operations. Sort of like tires on race cars.

Also, if you re-calculte to dollars per year, this may not be the case.

vtmaps wrote: »

I think that cheap, short batteries are a very reasonable choice in small RE systems where the needs can be met by a single string of these batteries.

Yes, This is definitely a problem. I would need 4 or 5 strings of GC. If anything, this is 120 caps to water. I would be fine with one string of 2V L-16. Hopefully, by the time I need the next set, there will be better technologies.

Surfpath

Re: Absorption Time

NorthGuy wrote: »

How much absorption time do your batteries need to return to full charge (as indicated by SG) after being discharged to about 50% DOD?

Hi Northguy,
Sorry late to the conversation. I skimmed the rest of the posts (about how stratification affects different kinds of batteries). I found your chart regarding efficiency of charge/absorb time based on form factor (I think I got that right), interesting.

I cycle my "thicker plated" L16 RE Trojans down to about 65-50% each morning. My max absorb time is set as 6 hrs, but it's really controlled by End Amps via the FNDC monitor (it's not perfect - the tricky FNDC had to be jury rigged a little).

So, in reality the CC typically ends absorb after 2.5 and 5.5 hrs, depending on how much Kwh is used the night before. However, as previously discussed, I believe I experience what I call "diminishing returns" over time.

Specifically, I believe my end of charge SG's diminish over time relative to the time of absorb.

This is what I am referring to (these numbers are fake, and the time frame between the first entry and the last is probably 2 weeks, but it should model what's happening). It would be fun to go back in the CC and study the real data, but not enough time):

Evening 1 (Eq performed that day. SOC 100% as measured by hydrometer)
Morning 1 (SOC 65%)
Day 1 (3 hrs absorb)

Evening 2 (SOC 95%)
Morning 2 (SOC 60%)
Day 1 (4 Hrs Absorb)

Evening 3 (90% SOC)
Morning 3 (55% SOC)
Day 3 (4.5 hrs Absorb)

Evening 4 (85% SOC)
Morning 4 (50% SOC)
Day 4 (5 hrs Absorb)

Evening 5 (80% SOC)
Morning 5 (50% SOC)

Even though Trojan says equalize when SG's vary .30 or more across any cell* I equalize my bank about every 12 days (about when end of charge SOC is 80% or lower the evening before) for about 2-3 hrs or perform a similar length Mini "EQ" (more like a longer Absorb) when ending -chemical- SOC is 80%. This EQ or "EQ" is about every other week. It brings everything up to or near 100% SOC.

Do you see what I mean about diminishing returns? I have learn't some time ago that this is all probably completely normal.

It's part of Flooded Lead Acid Battery's "Mistique" (spelling intended).:roll:


*From the Trojan RE Tech Support (http://www.trojanbatteryre.com/Tech_Support/Tech_Support.html?tab=1)
Equalizing is also required if the specific gravity value of any individual cell varies 30 points or more. In addition, reduced performance can also be an indicator that equalizing is necessary. Equalization should be performed when individual battery voltages in a battery pack range greater than 0.15 volts for 6-volt batteries or 0.30 volts for 12-volt batteries.

NorthGuy

Re: Absorption Time

Surfpath wrote: »

Do you see what I mean about diminishing returns? I have learn't some time ago that this is all probably completely normal.

I have similar experiences. In winter, I get very little solar, so I have to use a generator often. I only run it until the voltage increases to 58V, then I stop it. You can look at this as a regular charge if you will. Here's the data from my last charge cycle:

Nov 14. Fully charged
Nov 17. Charged to -120AH (82% SoC)
Nov 19. Charged to -159AH (76% SoC)
Nov 20. Charged to -175AH (74% SoC)
Nov 21. Charged to -188AH (72% SoC)
Nov 22. Charged to -195AH (71% SoC)
Nov 23. Fully charged

On full charges, I run absorption at 64V to the point where I belive SG is full. Takes 5-7 hours of absorption. On Nov 23 it took 6 hours 40 minutes. You can call it equalization.

Looks similar to what you're doing

Surfpath

Re: Absorption Time

OK, double checked the CC to make sure that these 'fake' figures are representative. Actually, they not exactly representative. Absorb length does drop along with Evening SOC (sunshine being constant).

For example:
Evening 1 (Eq performed that day. SOC 100% as measured by hydrometer)
Morning 1 (SOC 65%)
Day 1 (3 hrs absorb)

Evening 2 (SOC 95%)
Morning 2 (SOC 60%)
Day 1 (4 Hrs Absorb)

Evening 3 (90% SOC)
Morning 3 (55% SOC)
Day 3 (4 hrs Absorb)

Evening 4 (85% SOC)
Morning 4 (50% SOC)
Day 4 (3.5 hrs Absorb)

Evening 5 (80% SOC)
Morning 5 (50% SOC)
Day 5 (3 hours of absorb)
OK 2 weeks of this-it's time to perform an EQ.

But this is almost counter intuitive. Absorb time doesn't compensate for battery SOC dropping (even if end amps is being used)??

I guess the new Whiz bang battery monitor addresses any of these issues for Midnite users. But I bet you a simple bit of programming to the Outback end amps feature could allow a user to program an automatic gradual decrease(?) in the end amps cut off figure.

ie. Start off with 2% of Bank AH...Then on day 4 lower to 1.75% Bank AH. Then day 8...lower to 1.50% Bank AH....etc.
Over a 2-3 week period.