Depth of Discharge Myth?

[Dockhead]

Originally Posted by ramblinrod

I thought I did. Try reading again.

I did, and I still couldn't gather anything significant of relevance to my question.



Maybe you (or anyone else) could just answer a couple of straightforward, technical questions:


1. True or false: Discharging a FLA battery below 50% regularly causes it to wear out faster, than keeping it above 50% all or most of the time. "Faster" doesn't mean "number of cycles"; it means kWh of energy handled.


2. If the answer to the question above is True, then what is the mechanism, and why is it not reflected in the cycle life charts?



3. If the answer to the first question is False, then why are we advised to avoid discharging our FLA batts below 50%? Is there any good reason for this advice, or not?




I don't want a generalized philosophical treatise; I am trying to understand what is the technical truth behind the conflict between the cycle life charts, and the advice we get on usage patterns.


If you don't know; that's fine. If no one knows, that's fine too. But if someone knows, I'd like to hear about it.

[Dockhead]

Originally Posted by Jammer

What he's saying is that you should buy good, flooded lead acid batteries, use them in whatever way meets your operational needs, and replace them when they start to show signs that they are failing. If you do not make a serious mistake, like forgetting to water them, or allowing them to become completely discharged if you are away from your boat for a period of weeks or months, they will typically last 3-5 years or more although a run of bad luck could lead to an early failure.

That's fine, but everyone knows that already, and it's not responsive to my question. Inquiring minds want to know technical basis!

[skipmac]

While not exactly on point, focused on AGM and gels, and it doesn't get into the whys and wherefores, this short article from Practical Sailor based on tests by Rod Collins (aka Mainesail) reconfirms his recommendation to discharge lead acid batteries to no more than 50% SoC.

https://www.practical-sailor.com/blo..._Grabbag081918

[Dockhead]

Originally Posted by skipmac

While not exactly on point, focused on AGM and gels, and it doesn't get into the whys and wherefores, this short article from Practical Sailor based on tests by Rod Collins (aka Mainesail) reconfirms his recommendation to discharge lead acid batteries to no more than 50% SoC.

https://www.practical-sailor.com/blo..._Grabbag081918

I wish MaineSail would comment.


There is no basis at all stated in that article, for the recommendation not to ever discharge below 50%. In the test, the batteries were discharged only to "just under 50%".



The only thing that this test demonstrates is that regular failure to charge to 100% ruins AGM's quickly. Whether FLA's are ruined as quickly, I don't know. The tested bank lost 30% of its capacity after only 30 partial cycles! Yowza! I did at least that many partial cycles to my own bank just in the month of July!!





My first reaction to this is lithium looks awfully good, after reading this, for anyone who doesn't have solar.

[Jammer]

Originally Posted by Dockhead

OK, but I don't understand the reference to an "inflection point" in the cycle life curve. Which inflection point? It is the absence of any inflection point in the curve which exactly inspired this thread. I don't think I understood this part of your post.

You're correct. I went through for all the grid points in the chart and figured out the lifetime capacity, and it's all about the same.

[john61ct]

It is untrue there is something magical about the 50% number.

It is a continuous curve.

The more often you go "too low" the shorter life in cycles will be.

But it's open to judgment if you have the tools and knowledge to actually be aware of your SoC.

99% of boaters don't, so the simplistic "rule" is needed - for them.

LFP have the same issue, but if it's 17 years rather than 25, and with the up-front risk so high. . .

LFP does abolish the need to get to 100%, which is a completely separate issue with lead but there just as important.

[Jammer]

Originally Posted by Dockhead

1. True or false: Discharging a FLA battery below 50% regularly causes it to wear out faster, than keeping it above 50% all or most of the time. "Faster" doesn't mean "number of cycles"; it means kWh of energy handled.

True but the difference is very slight (10-15%), and is only relevant for batteries that have to be removed from service because their cycle life was exhausted.

2. If the answer to the question above is True, then what is the mechanism, and why is it not reflected in the cycle life charts?

Typical life cycle charts do show a slight difference in lifetime KWH output. Remember that the output voltage drops as the battery becomes further discharged, while DoD is expressed in terms of amp-hours drawn compared to capacity.

3. If the answer to the first question is False, then why are we advised to avoid discharging our FLA batts below 50%? Is there any good reason for this advice, or not?

As noted upthread, this well-meaning advice is based on preventing sulfation. The real problem is leaving the batteries discharged.

[john61ct]

Originally Posted by Dockhead

True or false: Discharging a FLA battery below 50% regularly causes it to wear out faster, than keeping it above 50% all or most of the time.

Yes. But the same is true for any pair of SoC #s. 80% is shorter life than 90%, 20% is shorter than 30%.

But lifetime in all tech discussions **is** # of cycles.

Damage from low SoC is a factor of how long the bank sits there.

The faster it gets back to Full the better.

Increasing cycles from daily to 2 days does much more damage.

Those talking kWh of energy handled want to carry less lead, spend a smaller dollar amount more frequently.

Which is fine as long as they realize their $/AH/yr cost is marginally higher.

If a bank can last 8 years averaging DoD 40-50%

and **you** are OK replacing every 3-4 years going to 80%, go for it.

Just do it with a cheaper more robust FLA bank.

And those numbers are guesses.

There is no precise formula for How Much higher your costs will be, there are too many variables, and no one with the resources cares enough to try quantifying it.

[ramblinrod]

Originally Posted by Dockhead

I did, and I still couldn't gather anything significant of relevance to my question.

So I guess you missed all post content about how lab cycles don't truly represent real life cycles? Read again.

Maybe you (or anyone else) could just answer a couple of straightforward, technical questions:

I recommended you refer to Nigel Calders book. It is called "BoatOwner's Mechanical and Electrical Manual.

Did you?

If not, please explain why I should waste my time posting information that I have provided reference to?

1. True or false: Discharging a FLA battery below 50% regularly causes it to wear out faster, than keeping it above 50% all or most of the time. "Faster" doesn't mean "number of cycles"; it means kWh of energy handled.

Generally true, according to Mr. Calder.

2. If the answer to the question above is True, then what is the mechanism, and why is it not reflected in the cycle life charts?

You posted just one life cycle chart.

Maybe that does not represent all batteries in general.

Maybe it's a lie.

I have seen others with a much steeper curve and more pronounced knee, where the DOD clearly affects the number of lab life cycles (which again, are pretty much meaningless in the real world, other than maybe for comparison of one battery model to another, for the same manufacturer.

3. If the answer to the first question is False, then why are we advised to avoid discharging our FLA batts below 50%? Is there any good reason for this advice, or not?

The answer to the first question is generally true.

The total life A-hrs or more importantly W-hrs of usable capacity will be less if one regularly discharges below ~50%.

Remember that A-hrs of capacity at lower SOC do not represent the same energy as A-hrs of capacity at 100% SOC.

For example, for a 100 A-hr battery, the 100% to 50% SOC (at rest) battery voltage may be average 12.5 Vdc, so the energy per cycle is in the neighbourhood of 625 W-hrs.

For the 50% to 0% SOC region, the average battery voltage may be 11.8, so the energy per cycle is 590 W-hrs.

The deeper the discharge, the lower the W-hrs / Amp-hr of capacity.

When one also consider charge efficiency, the deeper discharge A-hrs having less energy, are more energy intensive to recharge. (Not to be confused with more rapid charge rate due to higher acceptance rate.)

I don't want a generalized philosophical treatise; I am trying to understand what is the technical truth behind the conflict between the cycle life charts, and the advice we get on usage patterns.

It is not my (or anyone else's) responsibility to educate you on demand.

Most people would and should be appreciative when someone tries to help, rather than whine it does not answer their question per their standards (when in fact it did, to a reasonable degree).

I say again, it is very questionable how well lab life cycle charts reflect real world conditions.

1. Real life battery compartments are hot with poor ventilation and heat dissipation so the greater the depth of discharge, the higher the acceptance rate, the more heat generated for a longer period which hurts the battery instant capacity and long term life cycles.

Lab cycles may be performed in a circulating water bath to help dissipate / stabilize heat that will help maintain instant capacity and avoid sulfation.

2. Real life battery cycles have variable periods where the battery is held in a partial state of charge. The deeper the discharge and the longer it is held there the more the battery sulfates and is hurt.
For lab cycle testing, as soon as the battery hits the target SOC it is immediately back on the charger. This is easier on the battery than real world.

3. The deeper the discharge, and higher the acceptance rate, the more frequently the battery requires watering, and the risk that lack of adequate water hurts the battery.

4. In a lab, the battery is always returned to 100% SOC each cycle.

In real life this is not always the case.

The deeper the discharge, the more likely the battery will be left in a lower PSOC, longer, diminishing real world life cycles.

I hope this explanation meets with your approval so you will not be so rude any more.

If it doesn't, well, you can lump it.

[wsmurdoch]

Originally Posted by Paul L

Looking at the original chart, if you use the batteries daily you are looking at the difference between 4 years and 2.7 years in the two scenarios you proposed. 2 1/2 years is just too short to be practical on a cruising boat.

Let's say you have a single 100Ah battery and you draw out 10A more each day than you put back in.

If you recharge at 50% state of charge, you start the engine and recharge every five days. The chart in the original posting says you will get 1650 cycles out of your battery. That will be 8250 days before your battery fails.

If you recharge at 80% state of charge, you start the engine and recharge every eight days. The chart in the original posting says you will get 1050 cycles out of your battery. That will be 8400 days before you battery fails.

To me, with the given data, the battery lasts longer and the engine runs fewer times (although for longer). What is not to like?

Or, did I miss something?

Bill

[ramblinrod]

In addendum, if one could trust published lab cycle charts to represent real world life expectancy (again they can't) when one is considering design DOD, for maximum kW-hr per battery, they should pick the point on the knee of the curve (kW-hr / SOC) that gives the greatest total W-hrs of capacity.

But if one factors battery replacement frequency into the equation, they may wish to increase the bank for less design DOD, so they have longer periods between battery replacement.

If one factors in weight and space, they may wish to have a smaller bank and higher DOD.

Just like the entire boat, the battery bank size and chemistry is a plethora of compromises.

(Hopefully acceptable to most people, "it's complicated".)

So the general rule for FLA, design for 50% to 80% when recharging by ICE, or 50% to 100% when recharging includes wind and/or solar (at proper capacities) is valid.

Lastly, these rules of thumb are not a precise limit to be adhered, just like the "best before date" on food. There is not some magical event where at specific point in time (or discharge) the item is perfect and just a fraction beyond it is spoiled.

[cal40john]

Perhaps it is dependent on manufacturer or even which deep cycle battery in a manufacturer's line.

U.S. battery seems to only make (sell?) deep cycle batteries.
U.S. Battery | Leader in Deep Cycle Batteries | EV | Golf Car | Utility Vehicle | NEV Deep Cycle Batteries | U.S. Battery

This link came up on an independent search so I don't know which of their batteries it applies to:
http://usbattery.com/wp-content/uplo...life-cycle.pdf
Computing total amp-hr from above chart (same chart as post #74) -
( I chose 100 amp-hr as capacity of battery)
% discharge/ total amp-hr over life
5 75000
10 70000
20 66000
30 61500
40 59000
50 57500
60 57000
70 54600
80 54000
90 53100

22% drop in lifetime energy output from 10% to 80% DOD
18% drop from 20% to 80%

There is only a 2% drop from 50% to 80%, so you might as well use them to 80% if you were planning on going to 50%.

Then you have this article that states it is using data from Trojan (not stating which deep cycle product) that shows a max energy over lifetime at 40% DOD -
http://www.pacificseabreeze.com/tech...ng/1batcrg.htm

100 amp-hr battery
DOD/ cycles/ amp-hr over lifetime
10% 6,200 62,000
20% 5,200 104,000
30% 4,400 132,000
40% 3,700 148,000
50% 2,900 145,000
60% 2,400 144,000
70% 2,000 140,000
80% 1,700 136,000



I don't think that this Maine Sail article answers your question -
What Is A "Deep Cycle" Battery? Photo Gallery by Compass Marine How To at pbase.com

[sailorboy1]

Here’s a thought:

Maybe, just maybe now, pick up the phone and call Trojan and ask them. Now that’s pretty old school, so if you can’t do that you could go to their website and post a technical question to them.

[thinwater]

A question and a thought:


a. Then what chart should we believe? I would think someone would have gone to the trouble of testing in what they consider a real world manner.
b. A few of us cruise long term. Most of us--98% at least--cruise for a few weeks at a time at most, perhaps anchoring 30 nights a year at most (some nights will be in marinas). Over 10 years we will rack up a whopping 300 deep cycles. Clearly, the batteries are not dying from cycles, they are dying from corrosion and perhaps insufficient recharge or loosing water. Perhaps they go flat at some point for a reason unrelated to overnight usage (power failure at the marina or charger failure). This has NOTHING to do with cruising practice, it is an accident more likely to happen to boats that are NOT used continuously. But unless we take the batteries below 30% while cruising with some frequency, the batteries are going to die a natural death long before cycling is even a factor. This is no doubt true of most recharageable devices in light duty service. Not relevant to the full-time cruiser, but relevant to most of us.

[Dockhead]

Originally Posted by ramblinrod

So I guess you missed all post content about how lab cycles don't truly represent real life cycles? Read again.



I recommended you refer to Nigel Calders book. It is called "BoatOwner's Mechanical and Electrical Manual.

Did you?

If not, please explain why I should waste my time posting information that I have provided reference to?

Generally true, according to Mr. Calder.

You posted just one life cycle chart.

Maybe that does not represent all batteries in general.

Maybe it's a lie.

I have seen others with a much steeper curve and more pronounced knee, where the DOD clearly affects the number of lab life cycles (which again, are pretty much meaningless in the real world, other than maybe for comparison of one battery model to another, for the same manufacturer.



The answer to the first question is generally true.

The total life A-hrs or more importantly W-hrs of usable capacity will be less if one regularly discharges below ~50%.

Remember that A-hrs of capacity at lower SOC do not represent the same energy as A-hrs of capacity at 100% SOC.

For example, for a 100 A-hr battery, the 100% to 50% SOC (at rest) battery voltage may be average 12.5 Vdc, so the energy per cycle is in the neighbourhood of 625 W-hrs.

For the 50% to 0% SOC region, the average battery voltage may be 11.8, so the energy per cycle is 590 W-hrs.

The deeper the discharge, the lower the W-hrs / Amp-hr of capacity.

When one also consider charge efficiency, the deeper discharge A-hrs having less energy, are more energy intensive to recharge. (Not to be confused with more rapid charge rate due to higher acceptance rate.)



It is not my (or anyone else's) responsibility to educate you on demand.

Most people would and should be appreciative when someone tries to help, rather than whine it does not answer their question per their standards (when in fact it did, to a reasonable degree).

I say again, it is very questionable how well lab life cycle charts reflect real world conditions.

1. Real life battery compartments are hot with poor ventilation and heat dissipation so the greater the depth of discharge, the higher the acceptance rate, the more heat generated for a longer period which hurts the battery instant capacity and long term life cycles.

Lab cycles may be performed in a circulating water bath to help dissipate / stabilize heat that will help maintain instant capacity and avoid sulfation.

2. Real life battery cycles have variable periods where the battery is held in a partial state of charge. The deeper the discharge and the longer it is held there the more the battery sulfates and is hurt.
For lab cycle testing, as soon as the battery hits the target SOC it is immediately back on the charger. This is easier on the battery than real world.

3. The deeper the discharge, and higher the acceptance rate, the more frequently the battery requires watering, and the risk that lack of adequate water hurts the battery.

4. In a lab, the battery is always returned to 100% SOC each cycle.

In real life this is not always the case.

The deeper the discharge, the more likely the battery will be left in a lower PSOC, longer, diminishing real world life cycles.

I hope this explanation meets with your approval so you will not be so rude any more.

If it doesn't, well, you can lump it.

No one is demanding anything, from anyone. If you would like to participate in the discussion, you're welcome. If it is tedious or below your level or if on the contrary you don't know the answers -- then that's fine too; there are plenty of other threads here. Most people on CF with special knowledge in a given area, enjoy sharing that knowledge -- I know I do. If this thread doesn't give you the opportunity for that enjoyment -- then why are you here? And if you do want to participate in the discussion, then surely a clarification of exactly what question is being asked, is useful?




Calder doesn't say anything useful on the subject. He refers to a cycle life table very much like the one I posted (Figure 1-12, page 11) which doesn't support his assertion. He says if you discharge below 50%, you drastically shorten the life, but then refers to a table to back that up, which shows a shallow curve, practically a straight line, going out from the inflection point, which is far up the curve at 10%-20% DOD. So if that table is a true reflection of reality, then on the contrary there is no reason not to discharge to 10% or 20%, rather than recharging at 50%.




So the question is still not answered in any thorough technical way -- why the conflict between the advice, and the data?




I really suspect that the sad truth is that the tables do not actually reflect real life, and that the mechanism is something like this:

Originally Posted by ramblinrod

"Real life battery cycles have variable periods where the battery is held in a partial state of charge. The deeper the discharge and the longer it is held there the more the battery sulfates and is hurt.



For lab cycle testing, as soon as the battery hits the target SOC it is immediately back on the charger. This is easier on the battery than real world."

That sounds right to me, and I had the same thought, but it's just an assertion. Is there really no data, no testing, anywhere on this?