Depth of discharge

zoneblue · December 2012

I finally managed to get a datasheet from my battery manufacturer. The cycle life looks fairly typical for an AGM cell. However, it got me thinking about cost per cycle, or cost per lifetime, and i concluded that that ubiquitous cycle life graph doesnt really show the lifetime cost by DOD very well.

If a battery will do 3300 10% cycles but 2600 20% cycles then clearly the 20% cycles have stored more power more cost effectively, no? Yet people say to not cycle deeply. Heres my graphs, what am i missing?

Attachment not found.

Cariboocoot · December 2012

Re: Depth of discharge

I don't think you're missing anything.
But then I tend to recommend 25% DOD for daily use systems, not 10%.
Some people wonder why. :roll:

icarus · December 2012

Re: Depth of discharge

It is a calculated guessing game. Deeper discharges, few total cycles as opposed to a bigger batter bank ( and charging regimen!) with shallower DoD. Tht is why I went with the "cheap" T-105s as opposed tot he more expensive L 16s. My calcs were (simply) if the T105s last 5 years, and the l16s 8, the T105s would be cheaper life cycle cost.

Tht said, it is hard to guess is,, the 105/ are still going strong after 4,, if they now go 8, would the l16s have gone 10? (not to mention the added PV needed to properly charge the large bank.

Tony

BB. · December 2012

Re: Depth of discharge

It appears when playing with the math--A battery bank will last a bit more than ~2x as many cycles with 1/2 the depth of discharge. So--For 2x the cost of batteries, you get 2.x times more life... Plus, if you have 2x more batteries, you may need more solar panels (and charge controllers) if you want to stay 5% to 13% or so range of charging currents.

It is a cost benefit set of questions for you--More batteries, more solar panels, more costs, more capable system, and hopefully, the batteries last 2x longer (less work for you).

On the other hand, more batteries, more costs, more to replace when they go bad, and more money at "risk" if somebody does an "oops" and leaves a water pump turned on over winter, etc. and kills your 2x more expensive battery bank.

Probably no right answer--But if this is a "newer system" and/or you are new to off grid solar power--I would suggest starting with less expensive batteries and a smaller battery bank--Just in case something "goes south" on you--Or if you decide you need a larger system in the "near" future.

-Bill

And remember there are other reasons batteries die--Not just cycle life... There is aging, temprature, how well they are taken care of, etc... So a smaller bank my die from cycle life, and a larger battery bank may die from age...

Ken Marsh · December 2012

Re: Depth of discharge

Good point ZoneB,
I have noticed this too.
There is also the issue of charge efficiency.
The last 10% of the charge and any equalization comes at considerable higher voltage, much of which is not recovered upon discharge.

With Solar, most of us are opportunity charging.
The big question is how deep should we let the discharge go.
It seems to be a good idea to at least occasionally get them up into equalization voltage when you have the sun.
You don't want to have to do equalization with a generator.
But according to the curves it is less costly as cost per KwHr to use deep discharge.

I tried to bring this up with Trojan but could not get anybody that wanted to talk about this.
All my life I have heard about lead acid battery memory.
Now we are being told there is no memory. I am not certain this is completely the case.

You would think that after 100 years of extensive battery use we would know more about them.

niel · December 2012

Re: Depth of discharge

the only batteries i have ever heard with a memory effect are nicads.

as to cycling too deeply, it is below 50% dod we have advised against going beyond. yes, do exorcise your batteries as they must serve a function, but too deeply will destroy them prematurely and too lightly is a waste. there is a happy design area that satisfies life, cost, and usefulness and will most likely be between 50% and 10%. remember that was depth of discharge as that would also refer to 50% and 90% soc which is state of charge (aka how full).

zoneblue · December 2012

Re: Depth of discharge

niel wrote: »

there is a happy design area [..] most likely be between 50% and 10%.

For these batteries (judging solely by their cycle life graph), the reverse /appears/ to be the case. The maximum life should theoretically be achieved by deeper discharges. Instead of 9x 10% daily discharges, a single 90% discharge over 9 days appears to yield 2.6 times better cell life.

However if i was to pick the 'knee' in that curve it would be 50%, at 2.4 times better life than 10%.

niel wrote: »

it is below 50% dod we have advised against going beyond.

Exactly. Im interested in why that is.

Two obvious reasons spring to mind:
- a comfort margin generally for misjudging SOC
- in particular where cells become imbalanced, it maintains a good distance from a single cell polarity reversal (causing instant cell failure)

But here we must also define 'cycle'. Of course we assume the cells are brought back to 100% after each cycle. But how long can a cycle be, ie is 9 days enough to start sulphation? 3 days? Is a cycle a single day?

But with solar, the cells do tend to cycle daily, the question is more about battery sizing, as @icarus and others have noted.

Cariboocoot · December 2012

Re: Depth of discharge

zoneblue wrote: »

Exactly. Im interested in why that is.

Because when you habitually discharge a battery below 50% the chances of it not recharging at all increase. Forklift batteries are designed to handle repeated 80% DOD.

For these batterys (judging solely by their cycle life graph), the reverse /appears/ to be the case. The maximum life will be achieved by deeper discharges. Instead of 6x 10% daily discharges, a single 60% discharge over 6 days appears to yield 2.5 times better cell life.

You'll probably find that each battery is different, but all will have an 'ideal' balance point between DOD and cost. It will take a lot of number analysis to determine this, and not all companies are forthcoming with cycle data & expected lifespan. That last item is one they particularly like to avoid talking about because a battery can fail unexpectedly.

But here we must define 'cycle'. Of course we assume the cells are brought back to 100% after each cycle. But how long can a cycle be, ie is 6 days enough to start sulfation?

Right; they definitely expect to be recharged fully, quickly, and properly for best lifespan. Sulphation actually begins immediately; it is an expected part of battery function. It is reversible to a degree, as that is part of the recharging process. Over time it becomes less so, and the battery loses capacity. The deeper it is discharged and the longer it stays that way the faster the sulphation degradation will occur. Even three days below 75% SOC will accelerate the damage, although you probably wouldn't notice the effect of one such incident over the life of the battery. Make a habit of it and it will show up sooner rather than later. Six days at 80% SOC probably wouldn't be noticeable with a good weekly recharging. Six days at 70% charge is likely to cut years off what the lifespan could have been.

An unfortunate truth is that battery lifespan is not a fixed amount of time; there are too many variables involved.

Blackcherry04 · December 2012

Re: Depth of discharge

In my terms sulfation takes place on every cycle, a battery becomes " Sulfated " when it the recombination process has not taken place or the sulfur is unremovable . The question I would ask, is 6 Days is long enough for it become hard and crystallized to a point where it cannot be recombined. If your able to bring the SG up to it's base line, I'd say very little has taken place. It would be a easy test to perform.

Cycle life is something I count fairly close on generic GC-2's. On a well maintained set you should get between 1200-1500, 50 % DOD cycles or 5-6 Years. On a $90 battery that's between $.06 and $.08 a cycle. There are plenty of exceptions to the above. I have a harder time with fuel costs to re-charge them them the battery cost per cycle. Once they drop below 75% capacity, they are hardly worth the recharge cycle of $4 - $8 per cycle.

westbranch · December 2012

Re: Depth of discharge

Blackcherry04 wrote: »

... If your able to bring the SG up to it's base line,...

BC04, I agree with your points, problem is he has AGM's so no SG readings...

Zoneblue, what brand AGM's do you have?

zoneblue · December 2012

Re: Depth of discharge

They were imported from one of the biggest chinese manufacturers (here). Its a long story, i got them cheap from a guy who brought in a pallet, and didnt need all of them. The equivalent battery here is AA Champion, which cost about 5K for a 24 set of 400Ah 2v cells. I also priced FLA but to get the cycle life and capacity would have needed to pay around 6K.
Like you guys are saying these are definitely learner cells.

Just hanging out for the classics new battery monitor for better control of absorb. Without that i fear the agms are toastable.

westbranch · December 2012

Re: Depth of discharge

Had a look at their site so I assume you found this, http://www.chinatoyo.com/en/introduce010.html?introID=57238...

Combine that info with this and you may be good to go for fine tuning http://www.cdtechno.com/pdf/ref/41_2128_1199.pdf it gets into some fine points of charging AGMs...

I would not wait for the battery monitor , get your Classic and start charging, fine tune later...

Blackcherry04 · December 2012

Re: Depth of discharge

westbranch wrote: »

BC04, I agree with your points, problem is he has AGM's so no SG readings...

Zoneblue, what brand AGM's do you have?

Opps, I knew it and as I started typing, I just went off on my own. I agree that he needs to follow those Manufacturer Recommendations and start charging as soon as possible. Any delay is to long.

niel · December 2012

Re: Depth of discharge

"For these batteries (judging solely by their cycle life graph), the reverse /appears/ to be the case. The maximum life should theoretically be achieved by deeper discharges. Instead of 9x 10% daily discharges, a single 90% discharge over 9 days appears to yield 2.6 times better cell life.

However if i was to pick the 'knee' in that curve it would be 50%, at 2.4 times better life than 10%."

i have not heard of any lead acid type battery with those specs. can you post exactly what was said or link to it?

inetdog · December 2012

Re: Depth of discharge

niel wrote: »

"For these batteries (judging solely by their cycle life graph), the reverse /appears/ to be the case. The maximum life should theoretically be achieved by deeper discharges. Instead of 9x 10% daily discharges, a single 90% discharge over 9 days appears to yield 2.6 times better cell life.

Unfortunately, a 90% discharge over 9 days is not an option for the off-grid user!

A factor which I do not recall seeing discussed in this respect is that it is difficult to regularly hit 80% or 90% discharge, unless you are willing and able to use a generator when you get one or more days of poor sun.

If you aim for 80% DOD during the course of one typical day, you will have no reserve for a single day without sun. If you aim for a reserve for two cloudy days leading to 90% discharge (you need some reserve!) then you will only be getting a 30% DOD on good days.

To the extent that you are willing to run a generator to recharge the bank whenever you get even one bad day, then you could aim directly for 90% DOD.

But with FLA batteries, you have one more constraint, which is charging rate. You cannot plan on more than a C/8 charging rate, and even with a minimum of 4 hours of insolation, you will still only be able to recharge the bank from a 50% level (and power opportunity loads during the day to reduce overnight drain if you have enough panels.)
If you let the battery bank get lower, you will have to run the generator every day.

It is only with AGMs, as in this case, that you can even consider this high a DOD on a routine basis.

One more problem: The manufacturer's End of Life criterion is often 80% of initial capacity. If you design for 90% DOD, you will have to replace your bank well before that point, while with a more generously sized bank, you may actually be able to make good use of the bank past the 80% capacity mark. At the very least you need to confirm what standard for EOL is used by this manufacturer.

And if you design for 80% DOD, then as the bank ages, then with the same load, your load will slowly increase to be 90% and more, causing the aging to go even faster. (Computer simulation to model this effect anyone?)

Remember, your goal is not just to get the maximum number of Kwh out of the batteries, it is to get them to deliver the greatest number of days of useful life per $ of bank cost, under whatever weather conditions and use profiles you plan.

zoneblue · December 2012

Re: Depth of discharge

niel wrote: »

i have not heard of any lead acid type battery with those specs. can you post exactly what was said or link to it?

The left most graph in my OP is the graph supplied by the cell manufacturer. I multiplied the number of cycles by the Wh discharged in each DOD cycle to derive the right hand graph. It was that that lead me to the tentative conclusions noted.

I'm not seriously proposing discharging them to 90%, just trying to understand how to manage them. Folks here have mentioned some very good real world points that help understand this. I foresee that for the majority of the year the batterys will return to 100% SOC each day.

PNjunction · December 2012

Re: Depth of discharge

I'm not sure you'd be able to go beyond 60% DOD even if you wanted to - in my case equipment starts shutting down around 60% DOD due to the low voltage. Non voltage critical things like led-lighting may well take it further perhaps, but this would be by accident in my situation.

niel · December 2012

Re: Depth of discharge

if you wish to try to gain a smidgeon of power by stressing your batteries down to 80% or 90% dod then be my guest. the graph is not necessarily realistic to me as the overstressing will kill the batteries much sooner than the indicated graph shows. it may not happen at 60% or even 70% dod depending on the batteries and their age and do know deep dod discharges will age the batteries quickly. the best overall area i believe is around 30% to 40% dod and could vary for different batteries to some degree acknowledging some have best operating points outside my specified range area. you will note the graph does not indicate anything beyond 90% dod. care to speculate why? to me 80% is even too darn far unless an emergency circumstance let alone doing this on a normal day to day basis. the name of the game is overall performance, reliability, cost, and yes, even watt hours, but not just watt hours.

i know you think you have an argument here with this and you are free to disagree with me if you like as they are your batteries being bought with your $. does the manufacturer guarantee the facts in that graph and as such guarantee you can take your batteries down to 90% consistently in order to gain a miniscule amount of watt hours at that end of the graph? it is a high risk and with diminishing returns the farther you go. let us know how it works out for you if you go that route.

zoneblue · December 2012

Re: Depth of discharge

Hi Niel. Im not arguing one way or other on this, just presenting the data as it was reported to me in a form that suggests something about DOD for those particular cells.

niel wrote: »

the graph is not necessarily realistic to me as the overstressing will kill the batteries much sooner than the indicated graph shows....the best overall area i believe is around 30% to 40% dod

Id be silly to ignore the voice of experience on any of this, so i am interested in the reasons for the position you hold, which, lets face it, is the industry standard view, so there must be something behind it.

you will note the graph does not indicate anything beyond 90% dod. care to speculate why?

The original curve continues to 0 and 100% in the trend direction, but 0% DOD clearly reduces the functionality of the cell to 0. 100% nominally to several hundred cycles, but at some point all you are going to do is reverse the low cells and kill the bank.

in order to gain a miniscule amount of watt hours

But thats my point the data says its not minuscule, its an order of magnitude. The difference between 10% cycles and 50% is 2.4 times longer life expectancy. According to the data. I dont know if it really holds in the real world, or why, or why not.

niel · December 2012

Re: Depth of discharge

my comments are in bold.

zoneblue wrote: »

Hi Niel. Im not arguing one way or other on this, just presenting the data as it was reported to me in a form that suggests something about DOD for those particular cells.

question them on the data.

Id be silly to ignore the voice of experience on any of this, so i am interested in the reasons for the position you hold, which, lets face it, is the industry standard view, so there must be something behind it.

i believe they are guesstimating to a point and do not go to 80% and 90% dod. the possibility for failure gets amplified if more than 1 battery is involved too and most of us use more than 1 battery.

The original curve continues to 0 and 100% in the trend direction, but 0% DOD clearly reduces the functionality of the cell to 0. 100% nominally to several hundred cycles, but at some point all you are going to do is reverse the low cells and kill the bank.

i am referring to the 90% to 100% dod area and it isn't a cliff one drops off of at the 90% point they indicate on the graph. ie, 90% dod = good and 91% dod = bad. rather than arguing the point on the validity of their graphs with me then you should be confronting them on it and see if under those conditions that they will still honor a battery warranty. i'll grant you we push some leeway so that one does not encounter the possibility of a detrimental point to destroy the batteries needlessly, but it isn't without reason. as i said before, feel free to push the envelope on your batteries if you wish. you will notice that the watt hour gains are far diminished when talking of 80% and 90% dod. this is not the same gains one sees going from 10% dod to 20% dod and nobody here in this thread is telling you to only discharge that much. the knee of the gains realized is about 50% dod on that graph if you care to argue gain points as beyond that the gains are diminished and not worth it imho.

But thats my point the data says its not minuscule, its an order of magnitude. The difference between 10% cycles and 50% is 2.4 times longer life expectancy. According to the data. I dont know if it really holds in the real world, or why, or why not.

as i said we aren't telling people to go 10% to 20% dod as it will be better with a deeper discharge, but i disagree that going 80% - 90% is realistic to do based on graphs they provided. 50% is a good safe limitation and in a dire circumstance going further can be done if the voltage doesn't dive too far for proper operations of the equipment connected to it, but 80% - 90% is very risky on any lead acid battery and you can pose your arguments to whoever came up with that graph should your batteries fail prior to the indicated points on the graphs. now would i condemn somebody's action in going to 60% dod? no. 70% dod? yes, as you are pushing to the borderline the envelop for reliable battery operation.

zoneblue · December 2012

Re: Depth of discharge

Thanks Neil, i think we agree

inetdog · December 2012

Re: Depth of discharge

niel wrote: »

as i said we aren't telling people to go 10% to 20% dod as it will be better with a deeper discharge, but i disagree that going 80% - 90% is realistic to do based on graphs they provided. 50% is a good safe limitation and in a dire circumstance going further can be done if the voltage doesn't dive too far for proper operations of the equipment connected to it, but 80% - 90% is very risky on any lead acid battery and you can pose your arguments to whoever came up with that graph should your batteries fail prior to the indicated points on the graphs. now would i condemn somebody's action in going to 60% dod? no. 70% dod? yes, as you are pushing to the borderline the envelop for reliable battery operation.

Well said. There are just too many different reasons why routinely going to 70% or more is untenable. Among the ones mentioned so far by various responders:

1. What do you do when you are at 70% DOD and the next day is cloud covered?
2. What will the battery manufacturer do about warranty replacement issues?
3. Are the loads predictable enough to risk planning for 70%? It is hard to incrementally add capacity later. (Although if this use plan ends up with yearly battery replacement anyway, that would certainly make changing the bank size easier!)
4. How do you charge the batteries up again with PV if you increase their life by letting them go for several days without a recharge?
5. What happens as the batteries lose capacity and your load is now 80% of their current capacity not 70%, etc.

zoneblue · December 2012

Re: Depth of discharge

All good points inetdog. Thanks .

Depth of discharge

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