Depth of Discharge Myth?

[Dockhead]

Originally Posted by sailinglegend

.. .

During charging the reverse happens converting the lead sulfate back to lead and sulphuric acid, but if not fully recharged then any lead sulfate crystals not converted back will harden over a relatively short time and will provide a permanent layer of lead sulphate crystals on the lead plates which will reduce the capacity of the battery.

Simple!

Sure, but what does it mean for crystals to "harden"? Not in the literal sense -- Brinell hardness of different lead sulfate crystals is probably all the same. And what is the relationship between DOD, time, and this "hardening" effect? To what extent can it be reversed? If you have no option to do a full 100% finishing charge every cycle, what use regime causes the least harm? Shallower discharges left longer or deeper ones left less long? Followed how often by full finishing charges? Better to have one really good one once in a while with full equalization, or more frequent charges up to almost 100% And in every case, why?


These are the questions we are interested in, and it is not actually so simple. The simple explanation we have all heard long ago.

[ranger58sb]

Originally Posted by Dockhead

* The scientists basically condemn amp-counting as a method of evaluating state of charge, and they explain why -- the relationship between power input into the battery during charging and state of charge achieved is highly non-linear because of different side reactions which occur at different stages, particularly near full charge. Acid concentration (which we measure as specific gravity) and resting voltage have a highly reliable, linear relationship to each other and are far more reliable measures of state of charge.[/SIZE]

Originally Posted by sailinglegend

This is why SG can be used to determine when a battery is fully charged.

Sounds like it would be nifty to have a (electronic?) SG readout for each cell in each bank... especially for those of us with sealed lead-acid batteries of whatever sort.

-Chris

[evm1024]

Originally Posted by Dockhead

Sure, but what does it mean for crystals to "harden"? Not in the literal sense -- Brinell hardness of different lead sulfate crystals is probably all the same. And what is the relationship between DOD, time, and this "hardening" effect? To what extent can it be reversed? If you have no option to do a full 100% finishing charge every cycle, what use regime causes the least harm? Shallower discharges left longer or deeper ones left less long? Followed how often by full finishing charges? Better to have one really good one once in a while with full equalization, or more frequent charges up to almost 100% And in every case, why?


These are the questions we are interested in, and it is not actually so simple. The simple explanation we have all heard long ago.

I would presume that the formation of large (and not very soluble) lead sulfate crystals is the result of constructive metamorphosis.

This metamorphosis from small crystals to larger is driven by increased temps and by low specific gravity of the electrolyte. (again presumed)

Gradients in the electrolyte solution would be a likely driver of constructive metamorphosis (man those chemistry classes were long ago...).

https://pubs.acs.org/doi/pdfplus/10.1021/ja01327a019

[rgleason]

Dockhead, thank you for the research. So I should not just bring my fla batts up to 85% and then hope that solar will bring them up to 100% over 3 days, because during that time, large crystals will form?

Will equalization for 1-2 hours after full charge to 100%C at 16.2v get rid of recent large crystals?

[Maine Sail]

Originally Posted by Dockhead

Sure, but what does it mean for crystals to "harden"? Not in the literal sense -- Brinell hardness of different lead sulfate crystals is probably all the same. And what is the relationship between DOD, time, and this "hardening" effect? To what extent can it be reversed? If you have no option to do a full 100% finishing charge every cycle, what use regime causes the least harm? Shallower discharges left longer or deeper ones left less long? Followed how often by full finishing charges? Better to have one really good one once in a while with full equalization, or more frequent charges up to almost 100% And in every case, why?


These are the questions we are interested in, and it is not actually so simple. The simple explanation we have all heard long ago.

In the PSOC testing for Practical Sailor permanent irrecoverable loss of capacity happened in as little as 30 deep cycles to 11.7V/80% DOD. The only battery to survive that testing, with 100% of its original capacity, was the Firefly.

One brand actually lost approx 1% of Ah capacity for each PSOC cycle. Permanent damage/capacity loss from sulfation happens more rapidly than many folks often assume it does.

The batteries that did the worst in that PSOC testing failed, by industry standards; ≤80% of rated capacity = Failure, were technically dead in about 20 PSOC cycles to 80% DOD.

The Cycle Life to DOD chart from that manufacturer suggests these batteries are good for about 275-300 cycles to 80% DOD yet in a more real world type of cycle test they were scrap lead in 30 cycles. Even premium deep cycle products lost between 7% and 11% in just 30 PSOC cycles to 11.7V.


Once the sulfate hardens I know of no means to bring it back to active material. Picture it like dead skin cells, no amount of moisturizer is going to make the cells alive again..



Following the PSOC testing those quickly sulfated batteries (30 cycles) were put on "desulfators" from numerous manufacturers and a couple were tried with various other recovery protocols.. Not a single battery recovered any of its lost Ah capacity nor did they improve on cranking performance, internal impedance etc.. The damage done in just 30 deep cycles was permanent.

[john61ct]

Originally Posted by Dockhead

The scientists basically condemn amp-counting as a method of evaluating state of charge, and they explain why -- the relationship between power input into the battery during charging and state of charge achieved is highly non-linear because of different side reactions which occur at different stages, particularly near full charge.

Everyone knowledgeable says SoC meters are not reliable for controlling charging, in other words on the way up.

Determining 100% Full for the purpose of stopping charging is done measuring trailing amps.

Which then gives you a reset point to calibrate the coulomb counter.

Which is then "good enough" on the way **down** to ensure you know where you are at on DoD% to prevent going lower than you want to, balancing bank longevity vs how critical your loads are.

True resting specific gravity and voltage readings, isolating the bank for 48-72 hours is more of a "science lab" scenario.

Especially since their relationship to SoC changes as SoH does.

A SmartGauge, perhaps combined with a good AH-counting SoC meter - kept calibrated - is as good as it gets out there on the water IRL.

[Dockhead]

Originally Posted by Maine Sail

In the PSOC testing for Practical Sailor permanent irrecoverable loss of capacity happened in as little as 30 deep cycles to 11.7V/80% DOD. The only battery to survive that testing, with 100% of its original capacity, was the Firefly.

One brand actually lost approx 1% of Ah capacity for each PSOC cycle. Permanent damage/capacity loss from sulfation happens more rapidly than many folks often assume it does.

The batteries that did the worst in that PSOC testing failed, by industry standards; ≤80% of rated capacity = Failure, were technically dead in about 20 PSOC cycles to 80% DOD.

The Cycle Life to DOD chart from that manufacturer suggests these batteries are good for about 275-300 cycles to 80% DOD yet in a more real world type of cycle test they were scrap lead in 30 cycles. Even premium deep cycle products lost between 7% and 11% in just 30 PSOC cycles to 11.7V.


Once the sulfate hardens I know of know means to bring it back to active material. Following the PSOC testing those quickly sulfated batteries were put on "desulfators" from numerous manufacturers and a couple were tried with various other recovery protocols.. Not a single battery recovered any of its lost Ah capacity nor did they improve on cranking performance internal impedance etc.. The damage done in just 30 deep cycles was permanent..

Where can we read that article? Those discharges were followed by charging to 100%?

On any case, I guess that's pretty convincing - so NOT a myth.

I would still like to have a deeper understanding of Why.

[john61ct]

Originally Posted by ranger42c

Sounds like it would be nifty to have a (electronic?) SG readout for each cell in each bank... especially for those of us with sealed lead-acid batteries of whatever sort.

-Chris

Specific gravity indicates SoC accurately only after the bank has been isolated "resting" for some time, as much as 72 hours.

Just like voltage, not an accurate indicator while in use.

Unless your amps in / out patterns are consistent and predictable, so you build a custom chart for that bank in each scenario.

Assuming you have a trusted SoC gauge to use.

But then the correspondences change as the bank ages.

[john61ct]

Originally Posted by Dockhead

Those discharges were followed by charging to 100%?

No, that extremely quick damage was PSOC abuse - not getting to 100% Full - combined with too-deep discharge abuse.

And I thought the "myth" myth was put to bed a very long time ago.

Those believing in the "50% black-and-white rule" fairy tale have been educated that reality is actually nuanced, but the fact it is a useful guideline is not still challenged afaict.

Yes of course, the physics / chemistry "Why rabbit hole" remains for those interested.

[Maine Sail]

Originally Posted by Dockhead

Where can we read that article? Those discharges were followed by charging to 100%?

On any case, I guess that's pretty convincing - so NOT a myth.

I would still like to have a deeper understanding of Why.

Practical Sailor May 2015 and August 2015

These were not to 100% SOC it was a PSOC test with high acceptance AGM batteries recharged for 1 hour at .46C.

Many boaters, despite thinking they do, don't get to a true 100% SOC each day even with alternative energy. It is more of a real-estate issue with mono-hulls than it is with Cats..

We ran some testing here back in June of 2017, our longest days here in Maine, and in the 12 days of testing, mimicking a sailboat boat solar array, not once did any of the deep cycle flooded lead acid batteries reach 100% SOC. I will eventually publish this on the site but it is quite a way down the list.

The problem we had was there were just not enough solar hours in the day for the long tail of acceptance. The other issue we had was that nearly every solar controller dropped to float far too prematurely to even make it possible to get to 100% during the sun up hours. This type of daily behavior will lead to what is called PSOC walk down, decreasing usable capacity the longer it goes on. I would eventually like to test that impact but the testing for this is very, very long and involved. Even after programming float to the same as absorption, or .1V lower, we still could not get to a true 100% SOC, over 12 different days of testing during sun up hours.

This testing was run because about nine times out of ten when a customer tells me they get back to 100% SOC by noon with solar I can simply fire up the engine and show them that what they believe to be true is quite often not the case. Usually the problem is either the controller prematurely dropping to float, the Ah counter is off by a fairly large error or there is simply not enough hours to compete with the acceptance tail off.

Practical Sailor May 2015
Practical Sailor August 2015

[Paul L]

Maine Sail
Why is it so hard find and use a decently programmable solar controller? I have two older Bluesky controllers that allow you to set the drop out of absorption as a percentage of overall capacity(along with a max time). I set mine so it goes to float at an excessively long time frame and at aprox. 5amps into a 600 amp AGM bank. With 500 watts of solar on a mono it gets to float on most sunny days. If I have a concern I can bulk charge with a genset for an hour early in the morning.

[Jammer]

Originally Posted by poiu

There is an assumption in many of the postings here that all lead acid batteries behave the same way. They don't.


There is one type of lead acid battery that will perform much better than the usual Trojan type and that is a traction or fork lift truck battery. They are often made of 2v cells and are made for very deep discharges. A fork lift truck will work for hours non stop, discharging its battery to 70% or more usually 80%, then will roll the battery out, slot another in and carry on working, often round the clock and with several battery changes per day. A battery will last 1500 deep cycles, so incomparably better than a typical boat battery.


If you want the longest life and probably the best value for money and you don't want lithium then get traction batteries.

Forklift batteries have more antimony in the plates that most other battery types, which improves the cycle life but increases the self-discharge rate and increases the rate of electrolysis, leading to more hydrogen generation and a need for more frequent addition of water. They also have thicker plates, which makes them more durable but reduces capacity.


They're great for forklifts.

[john61ct]

Originally Posted by Paul L

Maine Sail
Why is it so hard find and use a decently programmable solar controller?

It is not at all hard, for those willing to spend over $100.

Of course you need both the gear and the knowledge to adjust away from the defaults to suit your situation.

Even a $1000 SC will not "work right" 99% of the time if just left at its defaults.

[newhaul]

John you are very correct . But it doesn't require over a hundred for an adjustable controller.
Mine cost me $25.00 a p30l 30 amp PWM controller with all of the adjustability that is required includes ah in from the array as well as ah out to systems run thru its load out ( also handles 30 amp out) but i did check my setpoints with my fluke to be sure it was right.
Even with the high dollar mppt units I would recommend checking setpoints with a good multimeter.

[a64pilot]

On the I’m fully recharged thing, I just did a little experiment of my own and was going to post a thread on it, but maybe this is the place to post it instead.
Day 1 bank was fully recharged, I kicked the shore power chargers off right after Sun up and ran off of Solar all the day and night, early the first morning both my AH counter battery monitor and my Smart Gauge were reading 80 and 81% SOC, by 12:30 or so my AH counter battery monitor was reading 100% SOC and 100% of the AH that had been removed had been replaced and my Solar Controller had dropped to float.
I was fully charged by a little after Noon.
By the next morning my AH battery monitor was reading I think 80% SOC and my Smart Gauge was reading 73% SOC.
I let the Solar recharge again until it dropped into float and my AH counter monitor again showed 100% SOC. I’m fully recharged again just after Noon, that 1KW of Solar is great.
I then turned on my shorepower chargers and it took almost 5 additional hours until at absorption voltage that my acceptance rate had dropped to .5% of rated capacity. It took according the the AH counter an additional 30 AH to get there. I doubt of course that even half of that 30 AH went into the bank, I believe as you approach fully charged, it gets real inefficient and the last 2 hours of so were well less than 10 amp charge.