Depth of Discharge Myth?

[David B]

Originally Posted by Dockhead

Is it a myth, that it is harmful and unprofitable to discharge your lead acid batteries below 50%?


I'm mulling over these questions again while contemplating switching to lithium. …………………….

For what it is worth to add to the discussion/experience database:

We took delivery of our boat (new) in 2012 - all good (we thought).
Apart from initial testing at hand-over, we never used the generator for the first two years. The various battery banks always being charged by engine or shore-power, and during winter lay-over, plugged in for a day each 4-6 weeks.

On returning to the boat in 2014, and checking with a multimeter, I found all batteries fine except the generator battery - that was down to 10.4V, and no doubt had been for some time during the previous winter at least (the panel only measures engine, service and bow-thruster banks - does anyone know if a VDO panel can be configured for monitoring a 4th bank?).

Investigating further, I find there is no charging facility to the generator battery except when the generator is running, so it has not been charged for two years! (that information was left out of the hand-over …..).

So one would expect heavy sulphation etc at least, with this little FLA generator start battery. None-the-less, I put an intelligent 7-stage charger on it (that I had brought over to do a re-condition and saturation charge on all batteries), and it seemed to come up OK.

The conclusion to this tale is that this little battery (a regular 'cheap and cheerful' off-the-shelf car battery from what I can see) is still going strong after 7 seasons (as are the other batteries), and so far there is no indication I need to replace it. When that time comes, another standard FLA will go in there, but this just adds to the argument that you can heavily discharge these things and get away with it, so more weight to the overall economy of regular 'sealed' (you can still prize the covers off to top up the electrolyte) FLA's. We have ten of them on-board an are very happy with them.

(since then, I have installed a solar system, and a port from the controller is dedicated to the generator battery!)

[kmacdonald]

Originally Posted by john61ct

Given a very high-amp source reasonably high CAR lead like AGM

The charging time difference to 100% between

50% depleted and 80% DoD for the same bank, or

as with your example

is not as great as you'd think.

Getting from 90% to true 100% endAmps can take much longer than from 50% to 90%.

Adding the extra 30% from 20% SoC may only be another 30-45min, compared to 6-7 hours overall.

Yes, I agree.

A 1000 Ah bank takes big chargers and a lot of solar never mind the weight. If you need it fine but if you can get away with a smaller bank it's a LOT less expensive overall.

[ramblinrod]

Originally Posted by kmacdonald

Sorry, but what I said is true. BATTERIES CHARGE MORE EFFICIENTLY BELOW 80% SOC. It's very common basic knowledge.

One must define charge efficiency. If one defines charge efficiency as the ratio of A-hrs in vs ratio of A-hrs stored you are incorrect.

At higher states SOC this efficiency may be 95%, whereas at lower SOC it may be 85% because this charge efficiency is poorer the hotter the battery is.

When the battery is on float, it is running very cool and charge efficiency is high.

When the battery is accepting everything the charge source can give and has for some time, it will be hot and charge efficiency is poor.

Consider an electric charging source where the power consumed to charge is directly related to the amount of charge delivered. Ie shore power, wind, solar, etc.

Conversely, if the charge source energy is supplied by ICE (internal combustion engine) the source may be consuming all kinds of energy, yet delivering very little electrical energy to the battery.

This is not really charge efficiency. This is ICE unused power capacity generated loss. This can be very high if the big diesel is burning 5 gallons per hour and the battery can only accept 2 A-hrs because it is fully charged. But if 2 A-hrs are supplied and 2 A-hrs storage energy gained, charge efficiency is 100% (not really achievable but hopefully you get the drift.)

Yes, this is a very complicated subject, and one cannot apply a little tidbit they learned on the internet about acceptance rated inversely proportional to state of charge, without considering all the other vatiables that come into play.

Charge efficiency proportional to SOC and acceptance rate inversely proportional to SOC are two very, very different things.

[David B]

Originally Posted by a64pilot

People like Dockhead likely ought to cycle from something to about 85% SOC and then turn the generator off and just buy batteries more frequently, probably ought to invest into an auto start generator module. Most people, especially those without Solar are not full time, they cruise when they can, and of course nothing wrong with that, these people will still likely get years out of their banks, assuming that they equalize upon returning to the dock and shore power.
It’s the people that run the generator every day, day after day that seem to go to Solar and just accept the things are ugly and have windage, cause they get tired of that generator and know it only has X number of hours life in it.

Some solar systems can be pretty confronting. Our decision was based on enough to ensure the batteries were looked after during winter lay-over (rather that relying on someone else, or having the power lead disappear the day after you walk away from the boat).

I designed a stainless arch that serves for comms' antennae, plus davits, plus the panels. It's curved to replicate the cockpit arch and we feel enhances the aesthetics. The actual panels are long and narrow so sit within the arch. Only 120W total, but enough to look after the batteries when we are not there.

[james247]

Originally Posted by dkenny64

I think it would be nice to be able to discharge lead batteries to more than 50%..
but so far, I think, no one has mention the voltage curve during the discharge..


lithium is near flat..12v is 12v until its 90% discharge..then its like cliff..


lead the voltage trends down from the start..
you might have 50% left in you battery but its at ?? what voltage 13.5? of 10.5?
make a big difference on how 12v devices function.. I know my VHF start flagging a low voltage at 11V..
is the starter going to turn over? the bilge pumping enough volume?


I'm not for or against either type..there are other differences besides life cycles to consider and why 50% is common rule for lead.


just some thoughts
-dkenny64

This for me is the real issue - voltage. You can't use most of your equipment as the voltage drops below 11. So it is necessary to keep your lead acid system well charged. This tends to defeat the requirement of heavy discharge to "exercise" the chemistry of lead acid batteries.

[Stu Jackson]

Dockhead,


It seems to me that you also needs to understand where folks are coming from in their answers.


kmacdonald is a "small, small, small" skipper. I believe IIRC that he was once "out there" and has returned and gone small.



a64 has just started cruising and understands a LOT about how things work now that he's out and describes things very well. Her has done his homework. His laugh at the solar crowd "I'm filled up by noon" always makes me proud of him.



rod knows his stuff, too.


What all this says to me is this: HOW YOU USE YOUR Boat has more to do with the answer than all the input you can get.


It's basically no different than it has been for decades: Marina hoppers can use a smaller bank because they're plugged in all the time and usually don't have solar. "Local" cruisers like me, two or three nights on the hook, plug in once every few days overnight, can do with a healthy 400 ah house bank for two overnights with a fridge and a 100 ah per day load. Off the beaten track skippers need more capacity and more charging sources and don't depend or can't on shorepower.


What technical information you seek, from my reading of your OP and your responses, is rather simple. If I was researching this I'd find out why and how batteries sulfate or sulphate. That should be easy to find.



From following Maine Sail all these years, it's almost always PSOC. Whether it's between charges, left unattended, or never to 100% is immaterial IF you KNOW what your proposed "mission" may result in given whatever charging sources you have. Otherwise known as "What can you do to get your house bank full after you discharge it?" Or as full as you can make it given the resources you have. Getting back to 85% is better simply because battery acceptance reduces in input from WHATEVER source is available for charging.


This is why a64's repeated notes of using "power" first and then solar makes so much sense from a recharging POV.


I think finding out more about battery chemistry and sulfation is the key to your quest. That material is available.


Ain't quests fun? Living with chemistry...

[Jerry Woodward]

Bravo to Dockhead for questioning the status quo and trying to understand the basis behind loss of battery capacity. Like many, I have read all the usual articles and come away with the thought that sulfation is the root of all evils. But Dockhead is right to question what exactly is sulfation and exactly how is it shortening battery life. So, being a scientist, I sought some publications in respected peer reviewed journals and found this article in the Journal of Power Sources.

I'm sure there are many other articles and that this is by no means the last word, but there are a few take-aways that have not really been discussed in this thread, although Calder and Mainsail do discuss these issues.

1. Sulfation is an overused and often misunderstood word when used to discuss battery failure.

2. Sulfation is actually a good thing: it is the fundamental chemical process by which the Lead Acid battery operates:

During discharge, Lead and lead dioxide on the plates combine with sulfuric acid (H2SO4) to form lead sulfate (PbSO4)
During charging, Lead sulfate is converted back to sulfuric acid and lead or lead oxide.

3. Shortened battery life is often blamed on the formation of “hard sulfate”, crystals of lead sulfate that cannot be easily dissolved back to sulfuric acid and lead during charging. However, strong data supporting this as a primary cause of shortened battery life from undercharging or over discharging seems to be lacking. According to the article, “this is a failure mode that rarely appears”.

4. Two other related facts of battery chemistry may play a more prominent role in shortening battery life: stratification and low H2SO4 concentration.

Effect of H2SO4 concentration:
As the battery is discharged, the H2SO4 concentration is reduced. No surprise there, that’s why we measure specific gravity as a measure of state of charge. But what I didn’t realize is that the low sulfuric acid concentration speeds up the corrosion and deterioration of the lead plates. The lead and lead dioxide that make up the plates react with each other and corrode over time. This is normally very slow but irreversible and probably one of the main limitations on battery life. However, the rate of this corrosion increases as the sulfuric acid concentration decreases. Therefore, the greater the depth of discharge, the lower the concentration of sulfuric acid, and the greater the rate of corrosion of the lead plates. This is probably the biggest reason why keeping a battery at a low SOC greatly decreases battery life, not sulfation. This also fits with the advice to quickly recharge the battery once it is discharged—you don’t want it sitting very long with low sulfuric acid concentration.

Related to this is stratification: during charging, the H2SO4 produced is more dense than water and settles to the bottom of the battery. Thus, you get a gradient of high concentration to low concentration H2SO4 from the bottom toward the top of the plates. Again, the tops of the plates will corrode much faster in the low H2SO4 concentration, shortening battery life. As Calder and Mainsail point out, this stratification can be avoided two ways: 1) charge at a high enough voltage to create bubbling. This will help mix up the layers and even out the concentration. 2) go sailing while charging or shortly thereafter to mix up the layers.

So, thinking in terms of H2SO4 concentrations and lead plate corrosion, it is probably better to keep the DOD closer to 50% than 80%, but regardless, keep the discharged time to a minimum. Also, charging from 80% DOD would exacerbate the stratification effect making it more important to ensure the electrolyte gets suitably mixed.

I know this isn't the hard data that you are looking for Dockhead, but it is based on some pretty hard facts.

[Stu Jackson]

BINGO!


Thanks Jerry.

[David B]

Excellent stuff Jerry - thank you.

[daletournier]

Regarding sulphation. I purchased my T105's new approx 4 mths ago. There were 100's on pallets sitting in a warehouse, this was the only supplier in the country. The DOD was approximately 50%, not ideal when purchasing new batteries, I believe they'd been sitting for quite sometime. I'm not sure how long it takes a t105 to self discharge to 50%?

I'm not on shore power at all and haven't equalised them, although I could of used the little Honda . We live of them everyday.

The lowest acceptance rate at 14.8v I could achieve was approx 15-17a, my main bank 675ah, therefore end amps approx 2.3% ish of overall ah capacity. I assumed the batteries were permanently damaged a little due to long term sulphation, long term PSOC.

Over the last 4 mths using mainly solar alone (some motoring) I've been holding them in absorption mode at 14.8-14.9v daily (it's always sunny here) , they have recovered 50% of their lost capacity, if my understanding is correct. End amps are now getting as low as 7-7.5a.

Maybe long term 50% Dod and PSOC isn't as damaging? or is more recoverable than I thought, based on what I've read.

[Pizzazz]

The battery discharge question pops up regularly on this discussion board. In my opinion, there are two ways to approach the charging issue both technically correct:

a) FLA batteries, engine/generator charging, 85%-to-15% discharge (giving you 70% usable capacity), try to charge them as soon as possible, try to peak them at 100% charge after a long motoring session or at the marina. Will get 2-3 year life likely.

Wet cells are 2x better for partial discharge than AGMs, so avoid AGMs unless you need them in a vertical position or sth like this. Be prepared to replace wet cells twice as frequently if need be (stick with golf cart batteries). While true than many factors impact battery life, in real life many of these factors cancel out instead of being additive. So, do not worry too much, be happy.

b) Baby your batteries and become a slave to them, discharge to 50% depth only, use only 35% of available capacity, look at your battery monitor 15 times a day, etc. It will be similar cost, double the weight and double the life expectancy. You will feel good about your electrical system.

Lithiums are definitely better but at 5x cost. You make your choices.

SV Pizzazz

[Dockhead]

Originally Posted by Jerry Woodward

Bravo to Dockhead for questioning the status quo and trying to understand the basis behind loss of battery capacity. Like many, I have read all the usual articles and come away with the thought that sulfation is the root of all evils. But Dockhead is right to question what exactly is sulfation and exactly how is it shortening battery life. So, being a scientist, I sought some publications in respected peer reviewed journals and found this article in the Journal of Power Sources.

I'm sure there are many other articles and that this is by no means the last word, but there are a few take-aways that have not really been discussed in this thread, although Calder and Mainsail do discuss these issues.

1. Sulfation is an overused and often misunderstood word when used to discuss battery failure.

2. Sulfation is actually a good thing: it is the fundamental chemical process by which the Lead Acid battery operates:

During discharge, Lead and lead dioxide on the plates combine with sulfuric acid (H2SO4) to form lead sulfate (PbSO4)
During charging, Lead sulfate is converted back to sulfuric acid and lead or lead oxide.

3. Shortened battery life is often blamed on the formation of “hard sulfate”, crystals of lead sulfate that cannot be easily dissolved back to sulfuric acid and lead during charging. However, strong data supporting this as a primary cause of shortened battery life from undercharging or over discharging seems to be lacking. According to the article, “this is a failure mode that rarely appears”.

4. Two other related facts of battery chemistry may play a more prominent role in shortening battery life: stratification and low H2SO4 concentration.

Effect of H2SO4 concentration:
As the battery is discharged, the H2SO4 concentration is reduced. No surprise there, that’s why we measure specific gravity as a measure of state of charge. But what I didn’t realize is that the low sulfuric acid concentration speeds up the corrosion and deterioration of the lead plates. The lead and lead dioxide that make up the plates react with each other and corrode over time. This is normally very slow but irreversible and probably one of the main limitations on battery life. However, the rate of this corrosion increases as the sulfuric acid concentration decreases. Therefore, the greater the depth of discharge, the lower the concentration of sulfuric acid, and the greater the rate of corrosion of the lead plates. This is probably the biggest reason why keeping a battery at a low SOC greatly decreases battery life, not sulfation. This also fits with the advice to quickly recharge the battery once it is discharged—you don’t want it sitting very long with low sulfuric acid concentration.

Related to this is stratification: during charging, the H2SO4 produced is more dense than water and settles to the bottom of the battery. Thus, you get a gradient of high concentration to low concentration H2SO4 from the bottom toward the top of the plates. Again, the tops of the plates will corrode much faster in the low H2SO4 concentration, shortening battery life. As Calder and Mainsail point out, this stratification can be avoided two ways: 1) charge at a high enough voltage to create bubbling. This will help mix up the layers and even out the concentration. 2) go sailing while charging or shortly thereafter to mix up the layers.

So, thinking in terms of H2SO4 concentrations and lead plate corrosion, it is probably better to keep the DOD closer to 50% than 80%, but regardless, keep the discharged time to a minimum. Also, charging from 80% DOD would exacerbate the stratification effect making it more important to ensure the electrolyte gets suitably mixed.

I know this isn't the hard data that you are looking for Dockhead, but it is based on some pretty hard facts.

I read that same article yesterday. To get the hard stuff (vs. fluff oriented to a wider audience) you have to pay for it. So I spent some money and bought this and a number of others and did some reading.


It is surprising and interesting that a lot of the hard science is being done in China, of all places.



One of the more useful articles I read include:


Yang, J., Hu, C., Wang, H., Yang, K., Liu, J., & Yan, H. (2017). Review on the research of failure modes and mechanism for lead–acid batteries. International Journal of Energy Research, 41(3), 336-352.






I have not found a definitive answer to the original question posed, but I've learned a lot. The article you cited is kind of provocative, calling "sulfation" as we know it a "myth" altogether, not a position which is widely shared.



One thing which I think I've learned is that loss of capacity due to "sulfation" (whether we want to use the quotes or not) is definitely not a linear function of time vs. depth of discharge (so that theory is out the window). It may be that the initial lead sulfate crystals are formed according to some such principle, but they are not by themselves harmful. They undergo some further transformation which I still haven't found a good explanation for, which makes them resistant to being converted back to sulfuric acid. This is a really important fact -- it means that you don't "use up" the capacity of FLA batts just by discharging them deeply, at least not in any direct or linear way. This is contrary to the simplistic explanation of "sulfation" which many of our techs believe.


This further transformation, it seems, can be interrupted, at least to some extent, with timely overcharging, which we obviously do far too little of, according to everything, now, which I've read. "Timely" doesn't mean by any means every cycle.


So here's a question, which probably no one on earth really knows the answer to:


What if you cycle a FLA bank (a bank of good, well made, traction batteries with heavy plates) from 20% to 80% repeatedly for 4 or 5 days, then fully charge it and burn the hell out of it with several hours of equalization charge, and then repeat. How long will it last? Would an equalization charge that often, cancel the effects of such usage, or partially cancel it, enough to make this a reasonable regime?



Also another fact which I do think is kind of "on the market", but which seems now after reading all this stuff really important and perhaps neglected -- it's not just cycling which degrades lead batteries, it's improper storage. Apparently murders them to charge them incompletely and "float" them before a real 100% charge has been reached. I have always kind of guessed this, so my practice has always been to do a couple of equalization cycles after a period of time off the grid, first thing after hooking up to shore power.





Another fact: We worry about overcharging and positive plate corrosion, but more than one article I read stated that this is rarely observed. So it would be a mistake to be too tender with the positive plates and overcharge (equalize) too little.


And another: The amount of overcharging which we really need to be doing is not a simple process. It involves gassing and heat production and monitoring SG and watering the batts.



And another: We pay far too little attention to specific gravity, which tells us some crucial things about what our batts are doing, which no battery monitor can tell us.







Bah, what a 19th century, Dr. Frankstein-ish science experiment, lead acid batteries are!! And people call lithium batteries a "science experiment"!


Or I guess, stated another way -- lead acid batteries are a real "science experiment", if you want to operate them in an optimum way, and we don't even know all the science. So most people just use them without trying for "optimum" results and toss them out from time to time.

[conachair]

Originally Posted by Dockhead

Another fact: We worry about overcharging and positive plate corrosion, but more than one article I read stated that this is rarely observed. So it would be a mistake to be too tender with the positive plates and overcharge (equalize) too little.

That seems to have been floating ( ) around in off grid circles for a long time, since that Sandia report.

I never bother with going to float on the hook, but 15.3v just seems a bit too far... someone else first

[john61ct]

Yes equalizing is a good way to slow down the harm from PSOC abuse.

Even weekly can be better than monthly for chronic conditions.

Lifeline AGM say 15.5V for 8 hours, but other AGM say not allowed.

Overcharging's causing loss of electrolyte is fatal for AGM, another disadvantage vs FLS, does little harm to them long as you keep the water up

also inability to check specific gravity.

[Dockhead]

Originally Posted by conachair

That seems to have been floating ( ) around in off grid circles for a long time, since that Sandia report.

I never bother with going to float on the hook, but 15.3v just seems a bit too far... someone else first

It seems to be a fact that our batteries fail a lot from "sulfation" and hardly at all from positive plate corrosion. Did you ever hear about anyone's batteries failing from being equalized too much?


The Sandia report of course is talking about L16 batts, which are bit heavier duty than ours. They don't even call it "equalization" -- for them it's a "finishing charge" and should be done for SEVERAL HOURS at least every week.


I probably don't equalize mine enough in any case because I don't gas them enough for them to really need much watering. My Multiplus does an equalization cycle which I think is only about 45 minutes or so. I should probably be doing a few of them at a time, with hydrometer in hand.


I'm doing it as we speak, as I'm off for 900 miles of sailing early tomorrow (last leg back to Cowes), and the weather looks like we'll be able to sail every one of them. I don't like to run my generator in a seaway with the boat heeled, so I do it as little as possible.





Incidentally, I had a "duh" moment last night concerning the disappointing performance of my batteries this summer -- my batts are in the bilge, and I was floating in water which was +1C and +2C most of the summer. I had forgotten all about the effect on capacity of temperature. I probably should have pulled up some floor plates and let them warm up.