AGM charge acceptance

[john61ct]

Originally Posted by ranger42c

Just a note that absorption voltage decreases slowly (as A64 notes), not immediately...
-Chris

Do you meant current (amps)?

By definition Absorption is constant voltage

[sailinbob1]

Absolutely

[john61ct]

Originally Posted by ranger42c

Our maker suggests a minimum of .4C, and in our configuration that absorption period (after reaching that, post bulk) could last up to 4 hours...

That .4C will very quickly no longer apply during absorption, lead scales back demand for amps pretty rapidly once CV hits.

But yes, in your case, multiple large banks, will factor into the reduced genny runtime.

[john61ct]

I won't try of course to speak for MS, but "100%" for purpose of the "increasing Amps input shortens engine runtime" discussion is along the lines of AH replaced, compared to total AH required, as revealed by the 20- hour test.

"The actual tested capacity of the group 31 AGM battery was 95.69Ah's, its rated Ah capacity is 105Ah."

Significant AH input happens in the first few hours only, and completely disappears in the last X hours of a proper charging cycle when shooting for maximum cycles of lifetime, given a standard say 40% DoD every cycle

There, the "100%" point you're shooting for is measured either by charge current dropping to say .05C (3A in my 300AH example), or a resting voltage measured accurately at least four hours after charging has stopped.

Obviously no loads involved at all when testing for a given bank's benchmarks.

For big banks, that sort of 100% takes many hours past the definition (I think) MS was using in that article.

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Edit: MS states his testing was done using the first definition above.

Maybe the top-notch quality AGMs he was testing with carry their CAR right into the relatively low-amp charging tail?

Or maybe putting in high amps at higher voltages leads to the current dropping faster, maybe even a bit prematurely when using "resting voltage" to define 100%.

Or maybe requiring so many hours to get there is the result of 90+% in the real world having less than MS's optimal charging gear.

Either way, I will stand by this assertion, unless MS sets me straight:

For most real-world setups, getting from 50% to "longevity full 100%" requires a good long Absorption cycle, certainly much longer than the Bulk stage.

If using fossil fuels only, it certainly would be worth making the large incremental investment in the optimal gear required to get a large bank there in just a couple hours of runtime.

If regular shore power or enough solar is there to finish the job even when overcast, then you can get the longevity at much lower cost.

[Maine Sail]

Originally Posted by john61ct

For most real-world setups, getting from 50% to "longevity full 100%" requires a good long Absorption cycle, certainly much longer than the Bulk stage.

Absolutely and to further complicate matters I was using lab grade equipment that does not do "premature floatulation" like most marine installed equipment does. In order to match the times I did, in going from 50% to 100% SOC you'd need to hold absorption voltage until you attain 0.5% of Ah capacity, in tail current, before dropping to float. Odyssey and Deka both want to see 0.3% for 100% SOC when using return amps at ABV or tail current at ABV as a definition of 100% SOC.

Suffice it to say that when you drop to float too early you add HOURS onto the time it takes to attain 100% SOC... So 5:30 minutes to go from 50% to 100% SOC at .4C with a slightly used Lifeline AGM is going to be closer to 6-9+ hours out in the real world with the way most installed on-board charge equipment works.


Seeing as this was quoted there is a difference between short run time charging & usable stored capacity and jaunts back to 100% SOC.:

1- 50% SOC to 100% SOC at .2C = 5:42 - Exited Bulk Charge at 1:16
The lower the charge rate the longer bulk lasts. That 5:42 minutes to 100% was using lab grade equipment that di not drop to float early.

2- 50% SOC to 100% SOC at .4C = 5:30 - Exited Bulk Charge at 19 Minutes
Just because we doubled the charge rate it do not shorten our time to 100% SOC by all that much. It does however lead to healthier life with AGM batteries to hit them hard.

3- 50% SOC Charged at .2C For Exactly 1 Hour = 71% SOC - Never Exited Bulk
If you're charging at .2C (20% of installed Ah capacity) and start from 50% a 1 hour run time is only getting you to 71% SOC and you never even get to absorption..

4- 50% SOC Charged at .4C For Exactly 1 Hour = 85% SOC - Exited Bulk Charge at 20 Minutes
Here doubling the charge rate to 40% of installed Ah capacity yielded 85% SOC in 1 hour, a gain of 14% over the .2C charge rate.


5- 50% SOC Charged at .2C For Exactly 2 Hours = 87% SOC - Exited Bulk Charge at 1:16
You would need to run the engine for two solid hours to get to 87% so by doubling the charge capacity to .4C, and dropping an hour off, you'd only wind up at 2% less.

6- 50% SOC Charged at .4C For Exactly 2 Hours = 96% SOC - Exited Bulk Charge at 20 Minutes
This shows the last 4%, back to 100% SOC, took approx 3.5 hours compared to the first two hours which replaced 46% of the used energy. Attaining 96% SOC at a .4C charge rate in two hours is pretty remarkable for lead acid batteries.

[john61ct]

Thanks so much for clarifying

Originally Posted by Maine Sail

Attaining 96% SOC at a .4C charge rate in two hours is pretty remarkable for lead acid

Just talking about longevity, not reducing charge time, so assume on shore power.

Say a not-high-end but true deep-cycle AGM is drawing down to 80% but never lower, hence rated at 1000 cycles.

But on recharge, is only getting to that 96% every cycle, rather than full 100%.

Do you think that failing will hurt longevity as much as a lower DoD discharge level? I know hard to quantify. . .


Also, now back to fast charging issues, if a manufacturer states maximum say .5C charge rate, is exceeding that by a lot going to affect longevity only, or more serious possibilities?

http://www.sailnet.com/forums/showthread.php?p=3850393

Let me know if cross-posting is verboten here.


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Finally, is top-notch FLA more or less susceptible to these longevity abuses than AGM, or is that effect different for each issue?

[ranger58sb]

Originally Posted by john61ct

Do you meant current (amps)?

By definition Absorption is constant voltage

Erp. Yep, that's what I meant. Sorry for error, and thanks for correction. Guess my fangers got a way from my while I was typing...

-Chris

[Paul L]

My AGMs (Rolls) accept a charge that is equal to the depth of discharge. If they are down a 150 ah then they accept 150a charge rate. As they charge and are down 50 ah they accept 50a. As they near full and are down say 15ah they accept 15a. Doen to about 3 or 4 ah. Pretty linear in practice.
Not sure how this compares to a FLA batt.

[StuM]

Originally Posted by john61ct

Also, now back to fast charging issues, if a manufacturer states maximum say .5C charge rate, is exceeding that by a lot going to affect longevity only, or more serious possibilities?

The problems with exceeding maximum recommended charge rate are overheating and out-gassing. Both of which can cause further problems.

[smac999]

Charge acceptance is one issue. Charge efficiently is another. Agms are also more Efficent.

Ie if you put a 20a charge on a same size and sane soc agm battery and flooded battery. Each accepting 20a, and left it on for an hour. . After an hour The agm will Have have a higher soc . Because Neither has gained 20ah in that one hour. The flooded say will gain16 and the agm gained 18. Because some of that 20a went to heat. Not stored charge. With the same amps going into the battery the agm is charging faster.

[darylat8750]

Originally Posted by Paul L

My AGMs (Rolls) accept a charge that is equal to the depth of discharge. If they are down a 150 ah then they accept 150a charge rate. As they charge and are down 50 ah they accept 50a. As they near full and are down say 15ah they accept 15a. Doen to about 3 or 4 ah. Pretty linear in practice.
Not sure how this compares to a FLA batt.

How big is your bank?

[Paul L]

Originally Posted by darylat8750

How big is your bank?

Around 600ah

[sailorboy1]

Guess I'll just stick with my cheap FLA batteries and suffer having to check the water every 1.5-2 months (might start going longer as they hardly take any)

[ranger58sb]

Originally Posted by sailorboy1

Guess I'll just stick with my cheap FLA batteries and suffer having to check the water every 1.5-2 months (might start going longer as they hardly take any)

Seems OK. Maybe add a watering system if the task is onerous?

Have to say, I don't particularly obsess much about our batteries. Installed in segments: replaced one large-ish bank in early 2006, the other large-ish bank and the genset starter in mid-2009... and they all just work. And that's with several years of underwhelming charging capability.

When we're at anchor, and unless we are running AC all day long (usually only July and August)... we run the genset 2 hours/morning and 2 hours/evening -- coinciding with our cooking schedule -- and that does some charging and makes hot water at the same time. 4 hours genset/day is trivial, for us. Don't need no steenkin' battery management system, seldom have to check bank voltage

I have indeed mistreated that oldest bank a couple times, usually when trolling on one engine (the other engine) while the full electronics suite is active. Not to worry, genset/charger and parallel switch helped solve the engine restart, recovery from there worked OK.

Our batteries are VRLA maintenance free etc etc etc, primarily because access isn't great... but FLAs with a watering system could have solved much of that...

-Chris