Battery DOD definition - Page 1
Forum section: electronics / projects

author: eecook
author_rank: Regular Contributor
timestamp: May 10, 2016, 01:18:45 pm
content: Hi Folks,I have stumbled upon this while trying estimate the EOL for a battery bank in an off-grid solar system. The problem basically is how DOD is defined when one wants to use EOL plots (number of cycles vs DOD). The alternatives are:1) DOD = Energy Consumed/Nominal Storage Capacity2) DOD = Energy Consumed/Actual Storage CapacityUsing 1) or 2) will resut in different approaches to compute EOL. In case 1) you just use the value and go to the plot.In case 2), however, it's trickier. As you cycle the battery, the actual storage capacity drops, which makes the DOD change as you cycle. This means the EOL has to be computed iteratively.So, whats the correct definition? Any thoughts?UPDATE: I am talking about a lead-acid battery.

author: Siwastaja
author_rank: Super Contributor
timestamp: May 10, 2016, 01:29:19 pm
content: So, to rephrase, you want to only partially cycle the battery to increase the lifetime, but choosing #1 would lead you to go nearer to the limits when the capacity fades?IMHO, there is no standard nor consensus for this. You decide what is most optimal for you, or makes sense in your application, and document it well enough so others know which one you chose.#2 would make more sense in most cases from the battery viewpoint (avoiding top and bottom (more importantly top) to increase lifetime), but if your application requires a certain amount of energy storage, you need what you need and go with #1; quite the opposite, as the DCR is increasing too, you actually need to lengthen the cycle (measured in Ah) a little bit to compensate for the voltage drop to get the same energy, so the %SoC cycle length goes up slightly more than linearly.

author: eecook
author_rank: Regular Contributor
timestamp: May 10, 2016, 03:20:30 pm
content: Maybe I wasn't clear enough, so I rephrased my initial post. I basically need to know what's the correct definition in order to use the datasheet curves.

author: Siwastaja
author_rank: Super Contributor
timestamp: May 10, 2016, 03:44:49 pm
content: Quote from: eecook on May 10, 2016, 03:20:30 pmAny example to such a datasheet curve?The cycle life curves almost always use full 100% depth of discharge, fully charging and fully discharging the cells using the voltage limits given. In this sense, it's #2, because if it was #1, the cells would be overdischarged and destroyed by more than 100% DOD.So, DOD is always 100%, doing full cycles, in cycle life evaluations.The exceptions are specific cycle life studies where DOD is less than 100%, but then they specify how they did the test. <100% DOD cycling data rarely appears in datasheets.

author: eecook
author_rank: Regular Contributor
timestamp: May 11, 2016, 12:31:34 pm
content: Figure 1 is for an AGM Lifeline GPL 27T and Figure 2 is for a Sunnyway SWE122500.Figure 1.Figure 2.In figure 1, EOL is defined as 80% of the capacity (for this articular battery). With the DOD you can figure out how many cycles it will last. If DOD uses definition #1 in my original post, then no problem, if it uses definition #2 things get a bit more hairy since the DOD will vary over time due to gradual battery capacity drop (This makes me suspect the right definition is #1).In figure 2, you have a different plot. It shows, at different DODs, how much the capacity will drop as a function of the number of cycles. Again, with definition #1, no problem, with definition #2 it gets complicated to assess the number of cycles till end of life (how ever EOL is defined).Makes sense?

author: Siwastaja
author_rank: Super Contributor
timestamp: May 11, 2016, 01:23:34 pm
content: Ah, you are talking about lead acid! Omitted the most important information in the opening, I assumed li-ion since, IMO, lead acid doesn't make sense anymore in 2016.Strictly speaking, it absolutely cannot be definition #1; otherwise data points near 100% would be impossible (as it would go into overdischarge soon, destroying the battery quickly) and that would be seen as a huge drop in number of cycles near 100%.Only #2 makes really sense, but they do not specify how they do it, I don't know either. One way would be to give a full 100%DOD cycle every ten cycles or so, to update the "actual" capacity for the next 10 cycles.But it may be some #3.

author: eecook
author_rank: Regular Contributor
timestamp: May 11, 2016, 01:32:39 pm
content: Quote from: Siwastaja on May 11, 2016, 01:23:34 pmOups!!!QuoteManagerial desicion. The original Engineering proposal was li-ion.QuoteNot sure I understand what you are saying.QuoteWhat would #3 even look like?

author: Siwastaja
author_rank: Super Contributor
timestamp: May 11, 2016, 02:42:31 pm
content: Quote from: eecook on May 11, 2016, 01:32:39 pmWell, let's assume 10Ah capacity. If DOD% was defined as capacity consumed per nominal (original) capacity, and cycling test was performed using 100% DOD, it would be 10Ah discharge first, but later, when the battery has faded to, say, 9.5Ah remaining capacity, it would still be a 10Ah discharge (per your definition #1), which would destroy the battery, so the battery would be destroyed very very quickly at 100%DOD cycling. Hence, definition #1 cannot be the right definition for DOD in this case, as they indeed show curves going to near 100% DOD.So, depth-of-discharge should be defined as in #2, but it indeed makes it difficult to perform cycle testing at any other DOD% than 100%, as you cannot exactly track the actual capacity. Voltage limits work, but they are not very exact, especially with flat curve battery chemistries (NiXX, lead acid, LFP li-ion).

author: eecook
author_rank: Regular Contributor
timestamp: May 11, 2016, 09:11:15 pm
content: Quote from: Siwastaja on May 11, 2016, 02:42:31 pmOh! I see your point. But perhaps the way the curves were crafted was to assess the cycle life as a function of the "initial DOD", in which case definition #1 would be applicable, and easy to use! Makes sense?

author: Siwastaja
author_rank: Super Contributor
timestamp: May 11, 2016, 09:32:51 pm
content: Quote from: eecook on May 11, 2016, 09:11:15 pmDoesn't make sense to me.Depth of Discharge is something that is relevant on, well, every discharge. 100% DOD means you discharge fully until the minimum cutoff condition.For example, if we look at the first chart, the battery reaches the end-of-life after about 370 full cycles, discharged as specified in operating limitsevery time. This is simple.The same battery seems to get 1000 cycles whenevery cycleis only half of the length of what would be a full cycle. How they actually manage to test and measure it seems unspecified - how do they actually know they are discharging exactly half of the capacity, without knowing the actual capacity? And how do they decide the end-of-life (capacity drop to 80%)? My bet is that they do run full cycles every 10 or 20 cycle. It's often done like that.In any case, they have tested a bunch of batteries, cycled at different depths, one doing full cycles, one doing 50% cycles, etc., then interpolated the curve.The second chart is more realistic as it shows they have done three discrete tests.Does this help? I have hard time understanding where the problem lies.

author: eecook
author_rank: Regular Contributor
timestamp: May 12, 2016, 12:56:56 pm
content: Quote from: Siwastaja on May 11, 2016, 09:32:51 pmI think it actually makes sense. Imagine the following, and let's assume we define EOL as reaching 80% of the capacity (Trojan also does this btw). You have a 100Ah battery, and set up a load that will deplete 10Ah, then recharge, and the cycle keeps going (Initial DOD=10%). Now imagine you leave that to run and every couple of say 10 cycles, you leave the electrolyte to reast (it's lead acid) and measure the SOC. When you reach SOC=80%, then the battery has reached EOL, and you now the number of cycles it took. Now you have a pair (num of cycle, Initial DOD) data (using definition #1).If you do this, you'll get a curve like Figure 1, and will only be usable when you use the initial DOD, because that's the way the plot was crafted.This would make a helpful and easy to use plot.QuoteThe problem's how to use the curves. Considering manufacturer's data is meant to make your life simple, I think using the initial DOD actually makes sense. I would have to contact the manufacturer on this one.

author: Siwastaja
author_rank: Super Contributor
timestamp: May 12, 2016, 01:57:33 pm
content: Quote from: eecook on May 12, 2016, 12:56:56 pmWhich is #1 by your definition. Yes, it indeed is plausible when testing at DOD=10%, but imagine doing the same test for DOD=100% or even DOD=90%. Then, as I described above, the test would destroy the battery in a few dozen cycles. So #1 can only work sanelyat allwhen DOD% < EOL Capacity%, and even then, the actual (chemical, battery viewpoint) DOD will increase during the cycling. When doing DOD=10%, this effect is small and #1 and #2 are almost the same, but at higher DOD% it makes a huge difference, and the curve should actually stop (or really plummet) at DOD=80% if this was their methodology.It's possible that the actual methodology is more complex than being either #1 or #2. For example, they use the original cycle length throughout the test, or stop discharging when the low limit is reached, whichever occurs first during each cycle. But it wouldn't make a lot of sense to me.Maybe they just simply discharge to a certain voltage which corresponds to a certain DOD; this is imprecise, but provides implementation for #2 very easily. Or they may be able to reliably measure SoC in some chemical way, and adjust the stopping voltage during the cycling. It doesn't matter; what I'm saying is that the methodology must be #2 and it cannot be #1 to produce such a curve (first image, at least).QuoteThey don't specify how. Probably voltage or electrolyte gravimetric density or something like that. Yes, it works.On li-ion, I always prefer to do one full cycle every 10 or 20 cycles to get an excellent indication of the actual capacity, and hence, a reliable and precise number for capacity drop. This is a very common way to do it in academic world on li-ion. But lead-acid is not of huge interest to me so I can't really say.QuoteThis is actually kind of high EOL for lead acid! I have seen 60% and even 50% before. If they really hold up to those cycle number promises, they seem rather good.QuoteGood luck trying to find a representative who even understands the problem, let alone know the answer - battery specifications and customer communication are typically hugely poor.

author: Siwastaja
author_rank: Super Contributor
timestamp: May 12, 2016, 02:15:59 pm
content: Quote from: blueskull on May 12, 2016, 01:45:41 pmIf you compare power density or power/($*lifetime), which are the only sensible metrics here, li-ion probably clearly exceeds lead acid at -40C, too. Sure, very low temperatures may close the huge gap between li-ion and LA somewhat, but li-ion is still better.Both perform poorly in very cold temperatures with increasing DCR, and in both cases, designer needs to account for this rise.QuoteWorks in old cars, as expected. Today, with start-stop engines, much more on-board electronics and low-power hybrid EV functionality, the game has changed. Keeping the battery at exactly 100% SoC all the time doesn't work anymore. Modern cars have had ridiculous lifetimes on lead acid batteries (something like 1 year) in the worst case, and li-ion upgrading has been a matter of discussion for years. I don't know if any production car does it yet, but li-ion is clearly better in almost every regard in a modern car. Smaller size and lower weight also matters in the car industry, even if it's a rather small optimization.The car industry is a little bit slow to react to new technologies, however, as we all know.QuoteWorks well, expect that UPS LA batteries are costly, extreme weight needs special treatment (building construction, etc.), and they require more maintenance and more frequent replacement than li-ion. Also, short-time UPS is even more insane, the energy density totally plummets for <1h storage, which is often needed. Sometimes even 15 minutes would be enough, but this is not achievable with LA, you pay even more in cost and weight than you already do.UPS industry has been converting to li-ionfor years. It's slow work, yes, it's still mostly LA, but it's absolutely wrong to say that lead acid is somehow more suitable for that purpose. It's just that markets don't change suddenly. Li-ion has not been viable for UPS for long due to high cost just 5-10 years ago.In a nutshell, li-ion is IMO better in every single respect I can think of, but the margin varies, in some fields the difference is not that huge. And anyway, li-ion has been good enough (clearly better than lead acid, justifying transition costs) for those markets for just a few years now, so it's a really new thing, and a changing field. But the gap is widening all the time, LA is getting more and more expensive, and li-ion is getting cheaper and cheaper as we speak.So, as a designer, and a researcher, I don't want to invest (time, learning resources) to an old technology that is clearly losing the game. A few years ago it wasn'tthisclear, but now it really is. You can think it as future-proofing. But YMMV, I can understand that you still need to choose lead-acid now just to get some thing completed simply and quickly in an industry-proven way.