Some battery questions

[RossWilliams]

My understanding is that for purposes of this discussion battery cycle is a fixed amount, whether its 100% discharge or some other discharge level. There is no alternative definition used for AGM or other RV batteries.

" The bottom line of this is that the high discharge rate does not use more of the energy stored in the battery, than a lower rate would, for the same amount of watt-hours removed from the battery."

I think Peukert's Law says the capacity is lost, Wikipedia mentions the specific example of electric vehicles. Preservation of energy does not mean it is available as electricity. Almost any battery will recover from heavy use some after sitting, but I don't think that is what the Peurkert's Law is referring to.

But, hey, I am just an RV owner trying to figure out how to use my batteries most efficiently. It appears to me that larger battery arrays, lower draws and shallower draw downs all help to provide the best performance both in terms of immediate capacity and in terms of lifelong power output. It also helps explain the performance problems people have when they try to use air conditioning and other heavy draws on batteries or dry camp on battery power for extended periods without solar or other regular charging.

[booster]

Here is a link to Bogart Engineering's site with their very well written article on why they don't use Peukert in their monitors. It explains their ideas very well, and they mesh with what I have seen. As I mentioned, I have tested it, and the electrical power left in the battery is very readily available and usable as long as the load is smaller. There are many, many, many, sources like Wiki that say the power is lost, but that is not what I have seen, or Bogart says so well.

http://www.bogartengineering.com/wp-...tsComments.pdf

Here is another link to the Lifeline battery technical manual. If you go to the definition section, you will see that the define a cycle as a discharge and charge cycle with no requirement of DOD.

http://lifelinebatteries.com/wp-cont...cal-Manual.pdf

Here is the Lifeline predicted amount of cycles vs depth of discharge. This chart wouldn't exist if the cycle was defined as any group adding up to 100% DOD. If you look at 80% DOD, you would get 550 cycles, so at 20% per your explanation, you should get 2200 cycles. But the chart shows about 2800 cycles implying non linearity. If the addemup theory were accurate, this chart would be a straight line and not logarithmic. While the 50% rule is, IMO, not accurate, neither is the additive DOD theory as deeper discharges do shorten battery life. The shortening is just a whole lot less than the 50% rule would have you believe.




I am certainly not saying that having a big battery bank won't give you a bit longer life (in total watt hours lifetime of the battery) with shallow discharges. Not a huge difference, but some. Where big banks come into their own is if you need to supply large loads, like air conditioning, microwaves, etc, as you need the capacity to limit the voltage drop. Obviously, they also give more reserve capacity which can be nice if you have a compressor frig.



But, not everyone has those big loads. If they don't try to run AC of batteries or a lot of microwave or electric cooktop use, a bank of 1/2 the size will work fine. If it discharges to 80% DOD instead of 40%, there would be no real use penalty other than the small shortening of energy life. (you would buy batteries slightly more often than twice as often-but just 1/2 as many each time. In our case, where we have 4 GC2 Lifelines for 440ah, going with 1/2 as many would save over 130# of weight and a reasonable amount of space, both of which count big in a B. We chose to go big so we didn't have run the engine if the batteries were low and couldn't hold voltage running the microwave, as we are generator free. Also having 5 days worth of reserve power may come in handy, although we have never needed it. The large bank, if discharged to 50% also will accept over 200 amps of charging from out high output charging system, so we can recover a days use much quicker than we could on 220ah of battery which would only take about 100 amps.

[RossWilliams]

" so at 20% per your explanation, you should get 2200 cycles"

Uh no, you misunderstood. Each cycle is a fixed amount. The cycle life is how many full cycles you will get over the life of the battery if you discharge to different levels. If you discharge to 80% you will get fewer total cycles. That chart shows that. An 80% discharge is .8 cycles. A 50% discharge is .5 cycles. But if you discharge to 50% you will have a lot more cycles over the life of the battery, as the chart shows. Its not how many 80% discharges you can get compared to how many 50% discharges, it explicitly defines the Depth of Discharge (DOD) for a cycle as 100%.

Here Lifelines Conclusion:

"To get the best cycle life, the average depth of discharge should be as low as possible. Concorde recommends the average depth of discharge be no greater than 50% of the battery’s 20 hour rating."


Ross

[Yoshimura]

Originally Posted by booster

If you look at 80% DOD, you would get 550 cycles, so at 20% per your explanation, you should get 2200 cycles. But the chart shows about 2800 cycles implying non linearity.

I think his argument is not that there is no loss of battery life (a linear chart), but that the loss of life is less than one might think.

As per his explanation,

550 * 80% * C = 440 C
2800 * 20% * C = 560 C

So the total amount of charge you could store and restore from the battery during its life would be reduced by "only" 21% if you discharge it at 80% instead of 20%.

As per your sources, battery life would be reduced by over 80% (550 C versus 2800 C) if you discharge it at 80% instead of 20%.

[Yoshimura]

Originally Posted by RossWilliams

Its not how many 80% discharges you can get compared to how many 50% discharges, it explicitly defines the Depth of Discharge (DOD) for a cycle as 100%.

In APPENDIX A, they define a cycle as one sequence of discharge and charge. This seems contradictory, no?

[Yoshimura]

Originally Posted by RossWilliams

Its not how many 80% discharges you can get compared to how many 50% discharges

As checked many scientific papers and it seems that this how the cycles should be interpreted!!!

For exemple, check page 22 of this paper

[RossWilliams]

I have not seen any reference that treats a cycle as a anything other than a fixed depth of discharge, usually 100%. Cycle life however is not fixed, as the chart shows. At an average depth of discharge of 25% you will get roughly 2000 cycles, at 50% you will get 1000 cycles and at 80% you will get roughly 500 cycles.

If you have three battery banks, one of 1200 watts, one of 2400 watts and one of 4800 watts and each one uses 1200 watts of power per day and then recharges. The 4800 watt bank discharges to 25% the 2400 watt to 50% and the 1200 watt to 80%, requiring at least one recharge during the day. Here is what happens over the life of the battery.

4800*2000 = 9600000 watts 8000 days/1200 watts per day
2400*1000 = 2400000 watts 2000 days/1200 watts per day
1200*500 = 600000 watts 500 days/1200 watts per day

If you assume cycle depth is equivalent to the depth of charge then the lifetime watts would look like this:

4800*.25= 1200*2000 =2400000 2000 days at 1200 watts per day.
2400*.50 = 1200*1000 = 1200000 watts 1000 days/1200 watts per day
1200 *.80 = 960 *500 = 480000 watts 400 days/1200 watts per day

In the first example the typical 2400 watt bank will last about 5 to 6 years, in the latter example it will last less than 3 years. If you draw that 2400 watt bank down to 80% each day you will get 1920 watts of power and it will last a little over a year:

2400*.80=1920*500 = 960000 watts 500 days at 1920 watts per day.

I am hoping for actual usage closer to the first example of around 5 years or more. That seems a lot closer to people's actual experience. I don't want to be replacing my batteries every year or two.

[RossWilliams]

"In APPENDIX A, they define a cycle as one sequence of discharge and charge. This seems contradictory, no? "

Not to me. I think if cycle meant how many times you can charge your battery it would say that.

[booster]

Originally Posted by RossWilliams

" so at 20% per your explanation, you should get 2200 cycles"

Uh no, you misunderstood. Each cycle is a fixed amount. The cycle life is how many full cycles you will get over the life of the battery if you discharge to different levels. If you discharge to 80% you will get fewer total cycles. That chart shows that. An 80% discharge is .8 cycles. A 50% discharge is .5 cycles. But if you discharge to 50% you will have a lot more cycles over the life of the battery, as the chart shows. Its not how many 80% discharges you can get compared to how many 50% discharges, it explicitly defines the Depth of Discharge (DOD) for a cycle as 100%.

Here Lifelines Conclusion:

"To get the best cycle life, the average depth of discharge should be as low as possible. Concorde recommends the average depth of discharge be no greater than 50% of the battery’s 20 hour rating."


Ross

Earlier you said this

I think you read wrong. A battery cycle is always a 100% discharge, so 5 80% discharges is the same as 8 50% discharges, both are 4 battery cycles. You get more total power if you get more cycles.

Your earlier statement says that a cycle is any group of charge and discharge events that ADD UP TO 100%.


If you are reading the chart and then putting the add up to 100% on, at 20% DOD, you would have the 2800 "cycles" divided by 5, but that is not how Lifeline defines a cycle.


You have now added another term of "life cycle" which I assume you are saying is the total number of 100% added up single cycles a battery can produce. This is the same thing as what has been being called the "lifetime total energy storage before failure" or "lifetime battery capacity" and is calculated by taking the chart cycles at any given discharge depth and multiplying it by the amount of AH recovered (80AH for 100AH battery at 80% DOD).


20AH times 2800 cycles = 56,000AH lifetime


80AH times 550 cycles = 44,000AH lifetime


This is the 21% reduction that Yoshimura just mentioned


If you convert that to using cycles that add up to 100% but don't use varied cycle depths (which negates any advantage of doing this) you would get the following.


2800 cycles times .2 = 5600 added up cycles


550 cycles times .8 = 4400 added up cycles


You get the same numbers without the battery factored in so 21% change. So as soon as you start using the new term of "life cycle" and use it at a fixed DOD, you are saying exactly what I had said earlier, just different terms.


This discussion all started with the comments about a "cycle" being the added up, different DOD, individual discharge/charge events and that all "cycles" of that type are equal in the amount of battery degradation no matter what the various DODs are. That all of that type of cycle are equal is not correct, IMO, and that is what Markopolo originally was addressing in his post. The chart shows that, and you just also confirmed it with your own calculations albeit with different terms.


You said

Its not how many 80% discharges you can get compared to how many 50% discharges, it explicitly defines the Depth of Discharge (DOD) for a cycle as 100%.

The part about the number of discharges at any given % isn't important is what we have been saying, it is about how much total energy is returned at those discharge depths.



Please show us where it says anything explicitly about defining a DOD of a cycle as 100%. I don't see that anywhere except for when they do the AH rating at various discharge rates of 5,20, and 100 hours and there you have to discharge 100% to know how much power was in the battery (those ratings are also biased by Peukert which was discussed earlier).



What it all boils down to is that in the RV batter world a "cycle" is nearly universally considered a single discharge/recharge cycle of a any depth, as Yoshimura mentions. That is why the chart is made the way it is.



So bottom line is that, per your own explanation, you get more 100% added up cycles at lower DOD, which is true, but unless the DOD is consistent, you can't know how much it changes, so you are right at the same data a in the chart, but with different scaling. The only way I can think of to have an accurate calculation of life in the real world would be to take every discharge/charge cycle and use the chart to figure out it's effect on lfie. A single 80% discharge would use up 1/550th of the life, a 20% discharge would use 1/2800th of the life. When those numbers add up to 100% that life should be over if the chart is correct.



Of course, in the real world almost all of us are gong to do much more damage to our batteries from dumb mistakes like accidentally killing them dead too many times, so all of this is pretty mute.


One thing of particular interest is that per your quote, Lifeline says that they recommend that the AVERAGE discharge discharge depth should not be more than 50%. This is different than the traditional 50% rule that would make you believe that if you ever went below 50% the sky would fall. It also makes the entire understanding of battery management much easier for the masses, as it is pretty simple to understand. The inference, I think, is that if take the average DOD, you will get the number of cycles that the chart says, which is likely to be fairly close (probably within the 21% calculated at the 80/20 points), but not perfect. We have speculated on the forum in the past about if the average DOD would be the best and easiest to use tool to calculate what battery bank size is best for any given RV, rather than either counting on going all the 80% all the time or never under 50% DOD. My guess is that it is a pretty good indicator as long as you also have enough total capacity to get through a worst case happening.

[Phoebe3]

Originally Posted by booster

... the amount of AH recovered (80AH for 100AH battery at 800% DOD).


.

Believe you meant to say 80% DOD there...

Thanks all for the informative discussion. As a result, I quite worrying about the DOD on my batteries and just use them. If i have to replace at 4 years instead of 5, no great loss.

[markopolo]

I type way too slow to keep up here

Originally Posted by RossWilliams

................. It appears to me that larger battery arrays, lower draws and shallower draw downs all help to provide the best performance both in terms of immediate capacity and in terms of lifelong power output. ...............

I don't think you'll find many who disagree with that very general statement. The point I was trying to make earlier is that the penalty is not as bad as just looking at cycles makes it appear. Do your own lifetime total kWh output calculations at various DoD levels using published data from lead acid battery manufacturers to see if your understanding of this changes.

Originally Posted by RossWilliams

" ................. An 80% discharge is .8 cycles. A 50% discharge is .5 cycles. ..............

You'll find more disagreement with that statement. It suggests that any penalties (from chemical interactions, degradation, plate corrosion etc.) are consistently applied at all DoD levels and that you can simply add them up. I think that you'd at least first have to calculate and assign a weighting based on DoD and then sum the weighted partial cycles. I suspect that even that is too simple as it ignores the rate of discharge and charge and temperature etc.

It begs the question - Why focus on cycles at this point when a more useful measure is lifetime total kWh expected output?

Originally Posted by RossWilliams

" ................. it explicitly defines the Depth of Discharge (DOD) for a cycle as 100%.................

You have to provide a lead acid battery related source for that .

A more typical definition of a lead acid battery cycle is: a discharge and subsequent charge to its fully charged state. DoD is not mentioned. Battery manufacturers supply DoD info when providing information on expected cycles.

Here's an alternate link to the research paper Yoshimura referenced: http://orbit.dtu.dk/fedora/objects/o...710966/content

Originally Posted by RossWilliams

......I have not seen any reference that treats a cycle as a anything other than a fixed depth of discharge, usually 100%.......... .

Share the academic sources that you have found supporting your argument of a lead acid battery cycle defined as being 100% DoD (and subsequent recharge). That would be useful to have.

[avanti]

If the group will forgive a meta-comment:

I would like to say that I think that this discussion is great, and of the kind that represents this list at its best. It is fact-based, rational, and spirited (even to the point of aggressiveness). But it focuses on the data and reliable sources, and not on personal attacks, snideness, hearsay, or made-up opinions.

This is good. Keep it up.

[Bruceper]

Whoa. I believe this "fact" stuff needs to stop. This will just turn this forum into something useful and we can't be having that.

[booster]

Originally Posted by Phoebe3

Believe you meant to say 80% DOD there...

Thanks all for the informative discussion. As a result, I quite worrying about the DOD on my batteries and just use them. If i have to replace at 4 years instead of 5, no great loss.

Thanks Phoebe, got it fixed within the edit time.

[Yoshimura]

Originally Posted by markopolo

Share the academic sources that you have found supporting your argument of a lead acid battery cycle defined as being 100% DoD (and subsequent recharge). That would be useful to have.

Personally, the only source that I could find that explicitly defined a cycle as 100% DoD (or multiple DoDs that add up to 100%) is from Apple. But by defining a cycle that way, this allows them to not have to provide a "Cycle vs DoD" graph and allow them to simply says that their battery is good for X full cycle no matter how deep you actually discharge your battery each time. Which reinforces our claim that discharging the battery deeply only have a minor impact on the battery life.

[booster]

Originally Posted by Yoshimura

Personally, the only source that I could find that explicitly defined a cycle as 100% DoD (or multiple DoDs that add up to 100%) is from Apple. But by defining a cycle that way, this allows them to not have to provide a "Cycle vs DoD" graph and allow them to simply says that their battery is good for X full cycle no matter how deep you actually discharge your battery each time. Which reinforces our claim that the discharging the battery deeply only have a minor impact on the battery life.

Good information, thanks. I totally agree that the Apple thing may to simplify it all for the masses, similar to the "average" DOD that Lifeline uses. Great data is only great if everyone can figure out how to use it



The other thing that using the totaled up cycle definition, or even simpler total energy stored/delivered is that is should make putting a battery life monitor on any system really easy, as it just turns into an AH totalizer.


There is a very good past discussion on the forum that has lots of charts and graphs from various companies showing some of the variations in how much you really lose with deep discharges. Most are in the 15-20% range, some a bit lower some a bit higher. I think the same type of accuracy or better would be achieved with a totalizer as you the split of DODs would not be known. Phones would likely be on deep discharge end, many RVs would be on low discharge end, I think.



The more I think about it, I really wish our battery monitor would keep data on average DOD, as that would be very interesting information. Of course in a van that is driven, charging cycles can happen many times a day, or hardly ever, based on use patterns.

[Yoshimura]

I was checking Apple battery warranties and surprisingly they don't warranty all their devices for the same number of complete charge cycles. For example, the iPhone battery is warranted for 500 cycle but the iPad is warranted for 1000 cycles. One could argue that this reflect the fact that iPhone battery are usually more deeply discharged but if you check the Apple Watch (which is probably the most deeply discharged device on a regular basis) they are good for 1000 cycles.

iPhone: 500 cycles
Apple Watch: 1000 cycles
iPad: 1000 cycles
iPod: 400 cycles
MacBook: 1000 cycles

https://www.apple.com/batteries/service-and-recycling/

[booster]

For those tech geeks with nothing better to do, here is a link to the second of two good, and long, discussions on the 50% rule and other related stuff.


http://www.classbforum.com/forums/f2...imit-5595.html


The first discussion is linked in one of the early posts, so you can see that one also.


There is ton of information in these discussions, but it is also from some time ago, so there may be things that contradict what we have thought about since then, so don't be surprised at that. This was almost considered blasphemy at the time, as the 50% rule is so ingrained in RV battery world.

[booster]

Originally Posted by Yoshimura

I was checking Apple battery warranties and surprisingly they don't warranty all their devices for the same number of complete charge cycles. For example, the iPhone battery is warranted for 500 cycle but the iPad is warranted for 1000 cycles. One could argue that this reflect the fact that iPhone battery are usually more deeply discharged but if you check the Apple Watch (which is probably the most deeply discharged device on a regular basis) they are good for 1000 cycles.

iPhone: 500 cycles
Apple Watch: 1000 cycles
iPad: 1000 cycles
iPod: 400 cycles
MacBook: 1000 cycles

https://www.apple.com/batteries/service-and-recycling/

That is very interesting, and I think your are likely correct on the DOD being an influence there.



What immediately jumped out at me (non Apple user so never seen this before) is how low the number of cycles is, even considering they are the cumulative 100% defined ones.


We constantly here about lithium batteries lasting a very long time, with RV version touting things like 3000 cycles at 100% DOD. These Apple numbers would indicate very much less than that, even with the very good charging control the likely have.


It appears that for many people and iphone battery would only last a year or so? I know people that need to charge several times a day because they have gone dead, even on newer phones, so possible I guess.

[RossWilliams]

"You have now added another term of "life cycle"

No I didn't add the term, the term "life cycles" is in the title of the graph you posted.

"Please show us where it says anything explicitly about defining a DOD of a cycle as 100%"

It is stated clearly at the bottom of the graph you posted. "DOD=100%"

"20AH times 2800 cycles = 56,000AH lifetime


80AH times 550 cycles = 44,000AH lifetime"

2800 cycles on 100ah battery will give you 280,000 Ah lifetime. But, in fact, battery AH ratings are based on how much power is provided when drawn down over a 20 hour period. If you draw it down faster or slower than that the amount of power provided will change.

"This is different than the traditional 50% rule that would make you believe that if you ever went below 50% the sky would fall."

I have not seen that going below 50% would damage the battery. I have seen that going below 20% for most AGM batteries can do permanent damage. Every description of the "50% rule" I have seen describes it as a suggested balance point between battery life and power needs.

What the graph shows is that a DOD of 80% average instead of a 50% average will cut the amount of power you get over the life of your battery by half. That assumes you monitor power very finely and never actually drop below 80%. In real life, no one could sustain an average DOD of 80% without damaging an AGM battery.

From the lifeline link you provided:

"In general, batteries should be sized such that the rated capacity is at least twice the capacity required by the load. For example, if 100 Ah is required on average select at least a 200Ah battery. This approach will limit the average depth of discharge to 50% and will dramatically extend the life of the battery (see chart of Cycle Life versus Depth of Discharge in Appendix C)"

Please note the part that says "will dramatically extend the life of the battery" refers you to the graph you posted.