How do you Count Battery Cycles

[oilman]

I was looking at a new B with a lithium option. The 600 amp Lithionics battery claims a minimum 2000 cycles.

What exactly is a cycle? Every time I stop for 30 to 60 minutes during the day and leave the a.c. running and the battery has to recharge a small amount is that a cycle?

I read claims of how lithium batteries will seemingly last forever but it seems they only assume 1 cycle a day.

They way I figure it on a two week trip I would go through 50 to 75 cycles and several hundred a year. Any Thoughts...

[B-RV]

Lithionics has tested their 600 AH over 4000 cycles, they go from full charge to 5%.
The average RV user should get 15 years out of the Lithionics Lithium. After the 15 years you can send it back for a reman. At this time the charge is $3,500.00 in 10 years most likely 1/2 that.

[eric1514]

A battery cycle is a complete discharge and a complete recharge. It does not need to take place in one event. 5 discharges to 80% and recharges to 100% is 1 cycle. 1 discharge to 50% and 1 recharge to 100% is not a cycle, but 2 of those would be.

[B-RV]

Difference being, the Travato Lithium can only do down to 20%, Lithionics can be safely discharged down to 2% with no concerns or battery harm.

[booster]

Originally Posted by eric1514

A battery cycle is a complete discharge and a complete recharge. It does not need to take place in one event. 5 discharges to 80% and recharges to 100% is 1 cycle. 1 discharge to 50% and 1 recharge to 100% is not a cycle, but 2 of those would be.

That is certainly not the way I think any of us here think of a cycle, nor do most of the battery manufacturers.



If you look at the cycle life vs discharge depth charts the have the curves drawn at specific discharge depths. Marko just post one of the charts in another thread on the forum. The curves plot the full capacity left in the battery vs the number of cycles. Nowhere is there any kind of calculus style integration done to get the number of cycles.





Many of us agree that the total energy in and out is what counts the most, not number of recharges, but that is not the way the "cycles" are determined in any of the life charts I have ever seen.

[eric1514]

Originally Posted by booster

That is certainly not the way I think any of us here think of a cycle, nor do most of the battery manufacturers.

This is a quote from Apple.

"A cycle count is one complete charge and discharge cycle on the battery."

[booster]

Originally Posted by eric1514

This is a quote from Apple.

"A cycle count is one complete charge and discharge cycle on the battery."

Where does it say how deep the discharge is? That can easily be interpreted as the "complete charge and discharge cycle" means that the discharge and recharge of whatever depth has to be completed to count as a cycle. That just means you discharge to 50% and then recharge to 80% it doesn't count as a cycle because the charge wasn't complete, or as is often stated it would be a short charge.


Did you look at the chart?

[eric1514]

Originally Posted by booster

Where does it say how deep the discharge is? That can easily be interpreted as the "complete charge and discharge cycle" means that the discharge and recharge of whatever depth has to be completed to count as a cycle. That just means you discharge to 50% and then recharge to 80% it doesn't count as a cycle because the charge wasn't complete, or as is often stated it would be a short charge.


Did you look at the chart?

If you discharge to 50% and recharge to 80% that constitutes 30% of a cycle, according to Apple.

[booster]

Originally Posted by eric1514

If you discharge to 50% and recharge to 80% that constitutes 30% of a cycle, according to Apple.

That is nuts because nobody would ever be able to figure out how many cycles they had or test specs without an accumulating data logger. Look at any RV battery or charger site and see what they say about cycles and cycle life, and it is not as you say Apple is saying. Apple is probably trying to make sure they can't get held accountable for crappy battery life, as they got in trouble for that before.

[markopolo]

It is a great question.

I suspect Apple's definition is for a particular purpose. It might be warranty or explaining how an app tracks cycles etc.

I think it is too simple to be accurate in the long term as all cycles are not equal (rate, depth, temperature) but it's probably good enough for a iPad or iPhone.

If you look at Trojan LFP battery chart here:



You can clearly see that there are penalties for both rate and depth.

Similarly, this PDF from Lithionics shows that there is a depth of discharge penalty: WhatIsLifeExpectancy.pdf

The rainflow algorithm seems to have been adapted to simulate capacity loss in scientific modelling.

There's an illustration on Page 15 of this paper that shows common aging effects. FULLTEXT01.pdf

A weak point may end up being the BMS in some systems as it too is subject to continual stresses. Failure of the BMS could lead to failure of the battery.

[avanti]

A "battery cycle" is not a physical unit like volts or amps. Rather, it has an "operational definition", I.e., it is defined by how it is measured. So, the question of "how you measure it" is backwards. When you are interpreting a particular statement involving "cycles", you need to first ask how it is measured and make your measurements the same way. Sadly, this information is often hard to come by.

There are a lot of ways to measure stuff like this, and no one of them is "right" for all purposes. So, this discussion is unlikely to converge.

[booster]

Originally Posted by markopolo

It is a great question.

I suspect Apple's definition is for a particular purpose. It might be warranty or explaining how an app tracks cycles etc.

I think it is too simple to be accurate in the long term as all cycles are not equal (rate, depth, temperature) but it's probably good enough for a iPad or iPhone.

If you look at Trojan LFP battery chart here:



You can clearly see that there are penalties for both rate and depth.

Similarly, this PDF from Lithionics shows that there is a depth of discharge penalty: Attachment 6683

The rainflow algorithm seems to have been adapted to simulate capacity loss in scientific modelling.

There's an illustration on Page 15 of this paper that shows common aging effects. Attachment 6682

A weak point may end up being the BMS in some systems as it too is subject to continual stresses. Failure of the BMS could lead to failure of the battery.

That is a very interesting chart for LFP batteries, as we haven't seen many actual charts on them in the past, I only remember one. Interesting that it seems to show at least as much, likely more, shortening of life for deeper discharges than AGM batteries do, which we had hints of from the other chart we saw, but it was unconfirmed. Trojan tends to put out good data, so believable, I think.



Apple battery life came up early on in our first discussions of the 50% rule being very misinterpreted and the possibility of using total energy in and out instead. IIRC, Apple (I think it was either Jobs or Cook) had said that depth of discharge didn't matter at all and that the total energy in and out would always be the same for total battery life no matter how deep or shallow you went. We were a bit surprised by that statement, as the appearance seemed to be similar to AGM batteries where the lifetime energy storage does get less with deeper discharges, but not nearly as much as the 50% rule would claim.



I think that Marko's comment about Apple speaking as they do for a specific, probably business not science, reason is the most likely reason. It is probably much easier for the masses to understand and worry less about how to preserve their batteries, too.


Avanti is right, they can define what a cycle is for their products however they want, even if it doesn't truly fit the science perfectly, their choice. The vehicle, marine, RV, solar, stationery power, etc manufacturers do seem to agree on the more traditional definition we are all used to, so that consistency is a good thing for us.

[michaelingp]

Originally Posted by booster

That is certainly not the way I think any of us here think of a cycle, nor do most of the battery manufacturers.

Since you seem to be speaking for every member of this forum, please explain in layman's terms how "any of us" think of a cycle. The chart you posted does not define a cycle, it just says that the deeper the discharge, the fewer "cycles" expected. If "cycle" is a meaningless term, please let us know that.

[markopolo]

It's not just the deeper the discharge, the fewer "cycles" expected. It's also the rate of discharge and the temperature at which it occurs and probably other variables.

Is the term cycle meaningless. Yes and No!

Lifetime expected kWh at defined depth and rate would be more useful to me.

[booster]

Originally Posted by michaelingp

Since you seem to be speaking for every member of this forum, please explain in layman's terms how "any of us" think of a cycle. The chart you posted does not define a cycle, it just says that the deeper the discharge, the fewer "cycles" expected. If "cycle" is a meaningless term, please let us know that.

Perhaps I should have said "most" of us, or "some" of us-my apologies.


The chart the Marko put up, if I am interpreting it and what he said correctly, was primarily to show that depth of discharge does affect how the life of a battery, even lithium like the Apple ones, combining short discharges to 100% is different than on discharge to 100%.


The one I posted earlier clearly shows that their life data to 60% remaining capacity, is directly tied to depth of discharge as that is what the curves are. This in itself shows that the Apple definition of a cycle, that does not include any depth of discharge data at all except end points is very different, and contrary to the how the life charts of other batteries and manufacturers are constructed, so cannot be compared accurately.


AFAIK, every manufacturer of the lead acid batteries (and probably lithium but not 100% certain) defines a "cycle" as a discharge and then a recharge to full for the life charts. It goes hand in hand with the charts for a charging cycle which always ends at full after starting from whatever depth of discharge they have. I think all them also have statements about not getting to full charge when recharging, claiming significant battery life reductions, often also calling them cycles, as in short cycles. This would imply that in reality they are saying any discharge/recharge of any depth of discharge to any recovery would technically be a cycle, so again semantics. I don't ever recall seeing any information or chart that showed the actual affect of not getting full is, but it is often mentioned as significant. This intangible is likely why they use 100% recharge for lead acid batteries, as there is no real data available. Lithium doesn't need to be fully charged to prevent damage, so is different yet, so hard to tell what would work best to define their life, but I do hope they all do it the same way.



The point of questioning the post about how Apple defines a cycle was not to start a big argument over semantics, and as Avanti said everyone can define whatever they want so that part would never converge to consensus. The purpose was because the Apple definition was given in response to a post asking for the the definition of a cycle, which on this forum would likely not be for an Iphone, but would be for RV batteries. Without some discussion, which has been good IMO, the information could easily be misinterpreted by the OP or anyone searching this in the future, and wrongly applied to RV battery specs.


If there are any other relevant definitions of cycles or ways that manufacturers are defining the life of their RV batteries, especially lithium which aren't damaged by not fully recharging but still seem to use the same full recharge in their cycle lift calculations, it would be great to have them posted for all to see and learn about so nobody gets confused reading the spec sheets or manufacturers claims.


IMO, number of cycles is a relatively inaccurate way to rate battery life, as was mentioned earlier, because of all the life charts chose to make the charts based on fixed depth of discharge, which doesn't really happen in the RV world very often, and full recharges, so counting cycles won't really tell you how far gone your batteries are with much accuracy. Total energy in and out over the life of the battery really isn't a lot better because it also does not take into account the varying depth of discharges. The Apple definition is basically using total energy in and out as the adding together discharges until the get to 100% normalizes the cycles to constant energy making the number of cycles they give really a total energy number, which coincides with always the same total energy over the life of the battery which Apple had claimed and was mentioned earlier.


So, I would say cycles are not very accurate, but for now are about all we have to look at battery specs to compare different batteries, as long as the manufacturers keep agreeing on how to show them and the customers understand what the specs mean. The big problem with cycles would come if the manufacturers all started to define cycles differently (probably for competitive reasons) as then we would be looking at apples and oranges and not know what was better. IMO consistency and understanding are much bigger factors in most stuff like this, as that can cover any differences in semantics and such. As can be seen from this thread, blanket definitions without all the details, can be interpreted in completely different ways.

[Frankshay]

I can tell you in the EV world that a cycle is typically defined as full charge to full discharge. Most if not all lithium house batteries I've seen are lithium iron phosphate variants; if any case you want to avoid the extremes i.e. the "knees" in the curve. Thundersky used to publish curves that showed cycle life getting (for example) >2000 cycles at 80% depth of discharge and > 3000 at 70% DOD.

I'm not familiar with Lithionics but if they're claiming you can go down to 2% SOC they must use an internal fuse mechanism or some way of disconnecting the load. That's pretty close to reversing the cell.

[booster]

Originally Posted by Frankshay

I can tell you in the EV world that a cycle is typically defined as full charge to full discharge. Most if not all lithium house batteries I've seen are lithium iron phosphate variants; if any case you want to avoid the extremes i.e. the "knees" in the curve. Thundersky used to publish curves that showed cycle life getting (for example) >2000 cycles at 80% depth of discharge and > 3000 at 70% DOD.

I'm not familiar with Lithionics but if they're claiming you can go down to 2% SOC they must use an internal fuse mechanism or some way of disconnecting the load. That's pretty close to reversing the cell.

What you and I are saying is not different at all, I think. If the EV people or Thundersky define and publish a spec the say XXXX cycles at YY% discharge it is a comparable item and is accurate. Some RV battery companies, usually in boasting claims will compare a 50% or 80% discharge cycle life to the competition. What the companies you are referring to don't say, is that the you get XXXX cycles at full discharge and a full discharge can be any number of lesser discharges and recharges added to gether to count as one cycle. That is what I think is of issue in the Apple comment. No data we have seen would indicate that adding up multiple lesser discharges and recharges would equal a full discharge in cycle life. If they did, the cycles vs discharge % would be a straight line.


If one really wanted to be accurate, you would have to take each discharge and recharge "event" and go to the chart and find what % of life that one cycle would be at the discharge % point on the chart and keep a running tally of those results, I think. How accurate it would actually be is anyone's guess, as I don't know that anyone has ever done that, or would want to.


In the real world, especially with lead acid batteries, a crummy charger or an operator error are much more likely to shorten the life of the battery than depth of discharge. Of course this is only true if you look at total energy in and out, not just number of cycles at a given DOD. At this point, we also don't know if the lithium setups will consistently make it to the claimed number of cycles, however they are defined, as they haven't been out in the real world. Camping is going to stress them quite a bit, with high and low temps, vibrations, sitting around, etc so who knows.



The Apple calculation is really and energy in energy out calculation, but from all we have seen there is a difference based on discharge depth, it just isn't as huge as the old lead acid battery 50% rule followers claim it is, with the total energy difference in the 10-20% range not the 50% claimed by the rule.

[Frankshay]

Agreed.

In my experience (assuming good quality cells) the greatest risk is cells drifting apart which can be caused by a lousy BMS. I've lost cells in a (converted) EV because of BMS boards loading/discharging cells differently. Good quality cells don't seem to have any self-discharge so as long as you keep an eye on them and don't over or under-charge they should last for many years.

This, of course, opens the door for the "top balance"/"bottom balance" debate, lol.