Extending Li-ion cycle life: lower cell voltage or depth of discharge or both? - Page 1
Forum section: electronics / projects

author: jt
author_rank: Regular Contributor
timestamp: May 05, 2015, 12:24:24 am
content: I'm working a project that needs to maximize the cycle life of a Li-ion battery.With reference to the following Battery University link, it sounds like limiting the cell voltage to ~3.9V and minimizing depth of discharge are two opportunities to increase cycle life.http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteriesHowever, consider the following:1. If I have an application that consumes 1 W-hr for one hour between charges and I have a battery with a 10 W-hr capacity, then I have a maximum of 1/10 => 10% depth of discharge.2. If the cell voltage is limited to a max of 3.9V, then the same battery will have a ~6 W-hr capacity. My one hour, 1 W-hr load will now deplete the battery to a 1/6 => 17% depth of discharge.So, it seems I'm trading the benefit of the lower voltage for a detriment in increased depth of discharge. Am I understanding this correctly?

author: DanielS
author_rank: Frequent Contributor
timestamp: May 05, 2015, 01:33:18 am
content: Your 1Wh is still 1/10th of the 10Wh nominal battery capacity. What you are doing is operating the battery between the 50% and 60% SoC points instead of 90% to 100% SoC points.

author: amyk
author_rank: Super Contributor
timestamp: May 05, 2015, 01:38:46 am
content: Lions have the longest lifespan between ~40-60% charge, and are not suited to deep-cycling applications. That's only 20% of their maximum capacity though.

author: jt
author_rank: Regular Contributor
timestamp: May 05, 2015, 02:03:00 pm
content: Okay, so for a given amount of energy transfer, the affect on a Li-ion cell's cycle life is constant regardless of where on the discharge curve it occurs?Or if I read amyk's post correctly - operating in the 40-60% region of the discharge curve maximizes cycle life?

author: free_electron
author_rank: Super Contributor
timestamp: May 05, 2015, 03:28:49 pm
content: Which LiIon chemistry ? That is question number one. They are all different.there is two things wou want to avoid : electrolyte breakdown due to temperature and anode poisoning due to growth of pure lithium. this is charge / discharge current driven.

author: jt
author_rank: Regular Contributor
timestamp: May 05, 2015, 04:02:46 pm
content: Our battery is based on Lithium Cobalt Oxide cells. I'm trying to figure out how far we can push the life of these without having to change chemistry to Lithium Iron Phosphate.Does anyone have any intuition (or cited sources) as to what the expected cycle life could be for Lithium Cobalt Oxide cell, presuming I keep depth of discharge to <10% and cell voltage to ~3.9V? I've seen tables that list the benefits of doing either/or but not both.Also, does anyone know of any charts that show Li-ion cell life if depth of discharge is under 10%. All the graphs and tables I am able to find cut off at 10%

author: nukie
author_rank: Frequent Contributor
timestamp: May 07, 2015, 02:25:11 am
content: The easy way is to increase capacity and size of the battery so when it degrades you still have considerable amount of capacity however as the internal resistance increases it will be weak to supply high current this is not useful for power tools. Perfectly fine for other electronics that's sips average amount of power.Manufacturers are adding their own secret additives to the formula to combact parasitic reactions to the lithium metal. Thus delaying deterioration of the Lithium metal. So this makes Lithium battery a little better than those experience by the outdated battery'university'.The best way to extend battery life is to keep it cold. A pelting device attached to bring the temperature down will help. But Lithium perform weak when cold so it's best to look for a good balance between temperature and supply capacity.

author: DanielS
author_rank: Frequent Contributor
timestamp: May 07, 2015, 03:38:56 am
content: If you are lucky, you will be able to replace those batteries with Stanford's Al-ion cells in a few years. If the researchers' claims are true, their prototype has seemingly unlimited lifespan - no apparent capacity degradation after 7500 charge-discharge cycles at 60C rate. You only get about 1/4th the energy density of Li-ion though.

author: Fsck
author_rank: Super Contributor
timestamp: May 07, 2015, 04:23:31 am
content: Quote from: jt on May 05, 2015, 04:02:46 pmprobably because cell life is a ballpark estimate and variability between cells makes estimated cycles for such low depth discharge meaningless. you could extrapolate if you really wanted to though.

author: eas
author_rank: Frequent Contributor
timestamp: May 12, 2015, 04:01:12 am
content: Quote from: DanielS on May 07, 2015, 03:38:56 amThey don't sound particularly compelling, even if they were commercially available today. Who needs 60C discharge rates from batteries with 1/4 the energy density of lithium ion? Moreover, if you only cycled within the optimal 25% of a lithium ion batteries capacity, what will its lifespan be?Nukie, Lithium ion batteries don't have metallic lithium unless something has gone wrong.There are probably better options than LiFEPO4 if your current chemistry doesn't work for you. NCA seems to have a nice balance of durability, energy density and power density.

author: mzzj
author_rank: Super Contributor
timestamp: May 12, 2015, 07:32:53 am
content: Quote from: jt on May 05, 2015, 12:24:24 amAs above and limit charge and discharge currents to rather low values like less than 0.2C and keep temperature between 10-25CWith reduced voltage and 10% DOD you may get something like 5000-25000 cycles (or more if your cells dont die because of old age before that!)25000 cycles 1h/1h charging and discharging non-stop would be 6 years.These things are bit hard to predict, after 10 years you find out failure mode that is unknown at the moment etc.How many cycles you would like to get?

author: Psi
author_rank: Super Contributor
timestamp: May 12, 2015, 07:35:29 am
content: One things you have to watch is the quality of the cell. A cheap cell can lose 20% capacity per year even if you're not using it.They also get rather terrible self discharge after a year or two.If you charge up to 4.1V and discharge down to 3.7V its a good trade off between capacity and livespan

author: DanielS
author_rank: Frequent Contributor
timestamp: May 12, 2015, 02:13:00 pm
content: Quote from: eas on May 12, 2015, 04:01:12 amThe same places that still use lead-acid, NiCd and NiMH mainly due to cost. Imagine replacing those by cheaper, lighter, higher performance aluminum-based batteries that have 1000X better cycling endurance - the Stanford article said their prototype aluminum cell showed no significant performance degradation after 7500 charge-discharge cycles. If those claims survive commercialization, this would mean never having to replace a dead UPS, car, remote, power tool, flashlight, electric toothbrush/shaver, etc. battery ever again.If you are only using your lithium battery across its optimal endurance 25% sweet-spot due to endurance concerns, you could replace it with an aluminum battery 1/4th the nominal capacity of similar dimensions that you run 0-100% instead.

author: jt
author_rank: Regular Contributor
timestamp: May 13, 2015, 05:49:34 am
content: Quote from: Fsck on May 07, 2015, 04:23:31 amThat makes sense. If I were to extrapolate - any idea what fitting function is a realistic model? Some of the plots I've seen make it look like an exponential curve.Quote from: mzzj on May 12, 2015, 07:32:53 amHoping to get up to 30k cycles. Or about 5 years of 90 minute charge-recharge periods (can anyone guess what the application is based on that info?)Quote from: eas on May 12, 2015, 04:01:12 amThanks, I will look into NCA options as well.

author: mzzj
author_rank: Super Contributor
timestamp: May 13, 2015, 09:16:26 am
content: Quote from: jt on May 13, 2015, 05:49:34 amSmells like LEO satellite?Have you done digging on NASA website? they have loads of information on batteries but digging it out from NASA website is sometimes tedious. Previously Nickel–hydrogen battery was the king of cyclic applications in satellites, but I bet it's too expensive if you are asking here for battery optionsAFAIK for Nickel–hydrogen battery something like 100k cycles is not unheard of.

author: Fungus
author_rank: Super Contributor
timestamp: May 13, 2015, 12:24:30 pm
content: Quote from: jt on May 13, 2015, 05:49:34 amMight also be worth looking into supercapacitors.

author: jt
author_rank: Regular Contributor
timestamp: May 13, 2015, 02:24:22 pm
content: Quote from: mzzj on May 13, 2015, 09:16:26 amYup, you got it. The field is pico/nano satellites; a Nickel-hydrogen battery would blow the mass and volume budget in addition to the $ budget. We normally deal with mission life of ~1 year; looking at options to stretch that for some mission concepts on the horizon. Ideally we could get there by narrowing the region on the charge/discharge curve of our existing battery module, or if need-be, by swapping our our 18650's to a different Li-ion chemistry.Does anyone know of methods for accelerated life testing of batteries? It seems that because cycle life is so dependent on depth of discharge and charge/discharge rates, there is isn't a great way to get empirical data without being patient.Quote from: Fungus on May 13, 2015, 12:24:30 pmI have recently looked into supercaps. They are an attractive option, but unfortunately the volumetric energy density just doesn't look like it can cut it.

author: sacherjj
author_rank: Frequent Contributor
timestamp: May 13, 2015, 02:32:00 pm
content: If you are wanting long life out of Lithium batteries, you need to be using good quality LiFePO4 cells. These have 10 years of life if you treat them right. The rub is this:Lithium Cobalt is 3.7-4.2, LiFePO4 is 3.0-3.6Sometimes that is a problem for systems that need >= 3.3V as you loose some efficiency on a boost/buck stage to get to 3.3V.We are using two cells in series for a 6-7.2V cell that does not need balancing (if made with the same cells at the factory). This loses a little in dropping to 3.3V, but we same more than that with less of a boost to get up to backlight voltage.LiFePO4 has about 30% less energy density than Li Cobalt at first, but equal and surpass in energy density after 18-24 months, then it is a run away win.For general long life, you are much better to cycle a Li cell to 50% twice than cycle to 100% once. Depth of common discharge is a big factor in life of a Lithium cell for traditional Cobalt. It is still and issue, but a little less so for LiFePO4 chemistries.The other issue is that you can't really use output voltage to give a rough estimate of capacity, because the discharge curve for LiFePO4 is really flat. We are using a fuel gauge chip that measures charge and discharge for more accurate battery state.Battery life is a function of both cycling and age. A battery fully charged and sitting on the shelf will age poorly, without cycles. The best storage is charged 40-60%. So a true cycle life includes both cycles and time and temperature. If this is satellite, I assume you are providing some heating so the cell isn't operating very cold.

author: mzzj
author_rank: Super Contributor
timestamp: May 13, 2015, 06:33:13 pm
content: Quote from: sacherjj on May 13, 2015, 02:32:00 pmCan you name a few manufacturers for good quality LiFePO4 cells?Is A123 systems up and running again?30% less energy density? more like 30% energy density of li-ion18650 size 3100mAh Li-ion is around 10-11Wh of energy and same size LiFePO4 is around 3-4WhIf you reduce the li-ion charge voltage to 3.9 or 3.8 volts and and use it with 30% DOD its not so clear wich one is the winner, li-ion at 30% DOD or LifePO4 at 100% discharge.

author: sacherjj
author_rank: Frequent Contributor
timestamp: May 20, 2015, 03:53:22 pm
content: Quote from: mzzj on May 13, 2015, 06:33:13 pmSorry, didn't come back here for a while. Those numbers are way off. Either the Li-ion numbers are lies or the LiFePO4 cells are crap. Hmm. Some more research seems to show that preformed cylindrical cells for LiFePO4 just suck. We are using Li-polymer style pack batteries as you would use in tablets, etc.We are using custom packet cells from a Chinese factory that makes batteries for military missiles, and hybrid vehicles. We are getting right at 30% energy density for a typical li-ion polymer pack. You might not be able to use these for your application, due to forces, I don't know.

author: Howardlong
author_rank: Super Contributor
timestamp: May 20, 2015, 06:03:07 pm
content: Quote from: mzzj on May 13, 2015, 09:16:26 amIndeed it does. Designing your own electrical system is hard, but it's not only the charge/discharge cycles you also need to get the thermal analysis reasonably good. For Cubesats we frequently use Gomspace's unitshttp://gomspace.com/index.php?p=products-p31u.We have had less success with some other manufacturers, mostly due to bad documentation and/or firmware, if you can believe someone in this industry would dare to release half finished products as flight-ready (they do). If you can use an off the shelf solution like Gomspace's, I'd recommend it.

author: HKJ
author_rank: Super Contributor
timestamp: May 20, 2015, 06:30:32 pm
content: Quote from: jt on May 13, 2015, 02:24:22 pmI have seen a video with some researchers that measures charge and discharge capacity, batteries with small difference had longer lifetime. They used it to find additives to the battery chemistry.

author: Siwastaja
author_rank: Super Contributor
timestamp: May 20, 2015, 06:56:46 pm
content: I have been doing research on this but don't have time to elaborate right now, so I hope this quick summary helps.High end of the SoC is by far the most important. Every 0.1V (or about 10% of SoC) approximately doubles the cycle life, and avoiding storage at high SoC, high temperature, or most importantly, the combination of two, will increase the calendar life manyfolds. This makes a difference between a two-year or ten-year life, or 300 vs. 2000 cycles.Low-end SoC is not that important, but I lack real data on this and will do my research on this. But if you need to choose between 90%<->20% and 80%<->10%, the latter is definitely better for lifetime.Another thing, less commonly known, is the aging effect caused by high charging currents. Typical LCO/NCA/NCM energy cells are specified up to 0.5C charge typical, and while they can be charged faster, fast charging really hurts athigh SoC. So you need to start tapering off the charging currentbeforethe CV phase. I have done cycle testing to show this, but basically, when fast charging energy cells (not designed for very fast charging), charge at max 1.0C up to 4.0V, and then normally at 0.5C max until 4.2V. Preferably, use even lower currents if you can. The 0.5C spec really doesn't have any leeway at high SoCs!Discharge current is not overly important, as long as the cells do not heat up excessively; although, heating once in a while doesn't hurt, as long as they are mostly kept cool.

author: Howardlong
author_rank: Super Contributor
timestamp: May 21, 2015, 04:53:30 pm
content: Do LTO have heritage in space? That's usually the first questions that gets asked when a different technology is suggested. While it wouldn't necessarily be a show stopper, it is an additional risk particularly if it's the only technology in orbit.There are many strategies used to prolong battery life for space applications. Nicads are still used, there are birds with many tens of thousands of orbits and charge/discharge cycles still operating on nicads. Some of this is down to the charging methodolgies used and some is due to the selection process, including xraying the cells and performing electrical measurements for performance and balancing as well as the usual thermal cycling and vacuum and outgassing testing.mBarely five years ago it was often frowned upon to be considering LiIon for space, but now there is peoven heritage it's much more mainstream.

author: Howardlong
author_rank: Super Contributor
timestamp: May 21, 2015, 06:21:41 pm
content: It will depend on the project and what they are intending to do, it's not for you or I to determine whether they want to try out new battery technologies.Personally speaking, I would put trying a new battery technology as a very high risk. I would not be recommending such a technology on any mission unless the risks were agreed and understood, and had mitigation, such as it not being a single point of failure. In this case would mean sending up other traditional battery technologies, and having an EPS that would deal with charging multiple battery technologies.I am sure that they probably already have a primary mission. While you can tinker with stuff, I would be very wary of risking an entire mission because someone wants to play with a new toy, it's not like you can fix it once it's been launched and deployed.Like it or not, parts and subsystems with proven history have a lot of value in this business.

author: djQUAN
author_rank: Frequent Contributor
timestamp: May 22, 2015, 02:30:16 am
content: Quote from: DiligentMinds.com on May 21, 2015, 04:10:35 pmWhere is this source you speak of? I want to test them for renewable energy use but I can only find them in prototype cells that cost an arm and a leg.

author: Mr.B
author_rank: Unknown
timestamp: May 22, 2015, 03:47:24 am
content: Quote from: djQUAN on May 22, 2015, 02:30:16 amIndeed, where please DiligentMinds.com?I see specs that rate them very well at low temperatures.I have an application that may be ideal if they perform well at minus 20 deg C.