Battery life / cycling

Discussion in 'General' started by Aircooled6, Dec 18, 2017.

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  1. Aircooled6

    Aircooled6 New Member

    I was talking to my brother last night, and he mentioned that Li-ion batteries only have a limited number of "cycles" before they fail. I have been driving my Bolt Premier for the last week commuting between Garland and Dallas. It has over performed my expectations. I believe my research paid off. I'm averaging 4.7 miles per kWh today. My previous before full charge was 419.7 miles on 91.2 kWh. The mileage that would have cost $58.76, cost $9.30! I was calculating for 238 miles / 60 kWh or 3.97 miles / kWh before purchasing the car. Now, should I drive the battery until it is drained and then recharge (less cycling), or continue to recharge every night (I usually drive about 60-70 miles a day - 60- 80 of battery charge left - more cycling) Does it matter as far as extending the battery life goes? PS I couldn't find this in the owner's manual
     
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  3. Aircooled6

    Aircooled6 New Member

  4. Aircooled6

    Aircooled6 New Member

    Another explanation;
    "With properly managed batteries, the number of cycles a "full charge" is ends up being about the proportional fraction. If you use 200 miles a week, one once-weekly charge is one charge cycle. But nightly charges of the 30-40 miles used during the day for a whole week are ALSO one charge cycle, because each one is only adding 20% of a charge. "Proper management" is really key, though. GM gets the capacity cycles it does by rigorous thermal management of the battery pack (It stays between 50 and 90F any time it's being used or charged, and it will use "plugged in but not actually charging" available power to keep the pack there even when not doing much), and avoiding the ends of the charge range (the 100% and 0% charge) like they're Ebola hot zones. A Bolt pack is likely to be able to cycle 300 MWh through before it starts losing capacity, and drive substantially over a million miles with that power. The odds of the battery pack outlasting the rest of the car are very very high."

    I guess I'm answering my own thread. : )
     
  5. JyChevyVolt

    JyChevyVolt Active Member

    Keep your state of charge at 30%-80%. Do a full charge once in a while.
     
  6. Aircooled6

    Aircooled6 New Member

    Thanks. I removed the charger at 85% as soon as I realized that it is better for the battery to do so. I will put the Hilltop reserve function on tomorrow before I drive it. You have helped me in my EV education.
     
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  8. Pushmi-Pullyu

    Pushmi-Pullyu Well-Known Member

    Ideally, to maximize battery life, you should charge daily but only charge as much as needed for that day's trip. Try to balance the charge and discharge around the 50% level. For example, if you typically use 40% of the battery's charge in a day, then ideally you should charge to 70% and discharge to 30%. Or, if you typically use 30% of the battery's charge per day, then charge to 65% and discharge to 35%.

    Obviously this ideal won't always work, and it depends on how much control your car gives you over the maximum charge level.

    In general avoid charging above 80% unless you really need the added range, and charging to 100% should only be done when you plan on an extended trip, or when you will challenge the car's range with extended driving the next day. Now, that's not to say you should go to great lengths to avoid charging to 100%. Occasionally charging to max isn't going to age the pack significantly faster; it's only when that's done frequently that you'll be wearing out the pack faster than necessary.

    That's probably not good advice, unless you don't fully charge the car for 3-4 months or longer. You can see a lot of people suggesting in EV forums that you charge to 100% periodically, but that's at least partly a hold-over from the days of NiMH batteries. NiMH batteries do need to be occasionally charged to 100% to avoid the "memory effect", but it does not help li-ion batteries in any way to charge them to 100%. Some claim that a 100% charge helps reset the BMS (Battery Management System), but at best this only resets the software; it certainly does not help the hardware. I think it's rather questionable to advise someone to wear out their pack faster than necessary just to make sure the BMS is reading correctly, unless they actually have cause to think it's not.

    The reason I specified 3-4 months is that if you charge to 100% only three or four times a year, then that's not going to be charging to 100% often enough to have a significant impact on battery life.
     
  9. JyChevyVolt

    JyChevyVolt Active Member

    Need to charge to 100% for battery management software to recalibrate.

    https://forums.tesla.com/fr_CA/forum/forums/senior-tesla-executives-comforting-answer-concerns-re-loss-range
     
  10. Pushmi-Pullyu

    Pushmi-Pullyu Well-Known Member

    I did address that in my post. You've linked to a 2014 discussion of battery management in the Tesla Model S; a discussion in which the Tesla rep said that they would be issuing a software update to deal with the problem.

    Is this still a problem with the Model S? Does what applies to the Model S apply to the Bolt EV, which is what this question concerned? I'd say it's rather questionable that the answer to both questions is "Yes".

    As I said, there's nothing wrong with charging to 100% very occasionally, like a few times a year. It does concern me that people might think they need to charge to 100% on a regular basis "to recalibrate the BMS", even when there is no indication that it needs to be recalibrated. Sure, if it looks like the battery pack is losing capacity noticeably over just a few weeks or a few months, then I'd do the 100% charge to see what happens. I just don't think it's a good idea to be advising people that they need to charge the pack to 100% on a frequent basis when there's no good reason to do so, and a good reason not to do so.
     
  11. JyChevyVolt

    JyChevyVolt Active Member

    "Do a full charge once in a while."

    Does that sound like frequent basis?
     
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  13. Pushmi-Pullyu

    Pushmi-Pullyu Well-Known Member

    What I think, based on my life experience, is that it's such a vague description that different people will interpret it in wildly different ways. I think it's good to inject a note of caution about doing that frequently, especially when there is serious doubt that it will actually have any benefit in most cases.

    One thing is certain: Charging the battery to 100% and letting it sit at that state for days or longer is very bad for battery life. It would concern me that people think it's a good idea to charge to 100% on some sort of regular schedule, because if it's regular thing they might charge to 100% and then forget about it. If for some reason the car was left unused after charging to 100%, leaving the pack at 100% for days or weeks, that would be very bad for battery life.

    Or, to summarize: There are a lot of good reasons not to charge the car to 100% unless you're going on a long trip the next day. I haven't seen any compelling reason to do this charge to 100% balancing thing unless you think something abnormal is going on with your battery pack's capacity. So far as I can see, it's just something people think will be good merely because they've been told so.

    The internet is unfortunately better at spreading bad memes than good ones. :(
    -
     
  14. Domenick

    Domenick Administrator Staff Member

    Oddly, until just a month or two ago, Zero Motorcycles advised owners to keep their bikes plugged in when they weren't in use. I imagine it was to allow the BMS to balance the cells. It struck me as contrary to what I had learned about maintaining battery health, but figured they knew something I didn't. Now, it seems, maybe they didn't.
     
  15. Pushmi-Pullyu

    Pushmi-Pullyu Well-Known Member

    It would make sense if they're talking about setting the controls to charge the bike's pack to a pre-set level and then stop. If that's what they are talking about, then leaving it on the charger all the time makes sens, especially if it has a battery heater, as that would keep the battery warm if the external temperature drops too far.

    But if they're talking about charging the batteries until they're full as much as possible... then no, that seems like very bad advice, if they are using li-ion batteries. But do they? Maybe they're using NiMH batteries or something else?

    All the advice I've seen on long-term storage of li-ion batteries (whether inside an EV or not) say that is best done at 50% state of charge, and that you absolutely don't want to charge a li-ion pack to 100% and let it sit at that state for an extended period of time. On that at least, the experts seem to agree.
     
  16. Domenick

    Domenick Administrator Staff Member

    It doesn't have heating (or cooling, for that matter).

    Zero Motorcycles uses lithium batteries with cells from Farasis. Just tracked down the new charging recommendations. >Here<

    There shouldn't really be any problem with a pack sitting at 100%. Electrodes may see a little damage getting to full charge (though the tapering off of the power really mitigates most of that), but once there, there's no mechanism for further damage that I'm aware of.
     
  17. Pushmi-Pullyu

    Pushmi-Pullyu Well-Known Member

    Battery University says: "Li-ion has higher losses if stored fully charged rather than at a SoC of 40 percent."

    And also:

    Battery University screen shot.jpg

    Full article here: http://batteryuniversity.com/learn/article/how_to_store_batteries

    Battery University seems to be suggesting long-term storage should be at 40% or 40-50%; other sources recommend storing at 50% state of charge.
    -
     
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  18. JyChevyVolt

    JyChevyVolt Active Member

    That's unrealistic! Nobody in their right mind will store at 40-50% SOC.

    EVs have the greatest efficiency at 32 miles per hour. Nobody is going to drive at 32 mph/hr in the freeway.

    I got 80,000 EV miles on my Volt. I have a 80 miles commute and charge twice a day with no degradation. Stop spreading FUD.
     
  19. Pushmi-Pullyu

    Pushmi-Pullyu Well-Known Member

    That's talking about long-term storage of li-ion batteries in general, not just EV batteries. I'm not suggesting that you only charge your EV to 40-50% on a daily basis, and neither is Battery University.

    For example, if you plan to let your EV sit for weeks, such as sitting in long-term parking at an airport while you're on vacation, then yes you are advised to leave it charged to 50% capacity. That may not be good advice for a Model S or Model X, because of the "vampire drain" issue, but it certainly should apply to most plug-in EVs.

    You don't know how much degradation your Volt has, since GM has engineered the car to hide the degradation. What you're referring to is that what the car reports as the usable capacity has not shown any reduction, but if we could see what the pack's full capacity was, and how much that has been reduced since the car was new, then you can be sure there is some slight loss of capacity.

    But this does bring up another can of worms: What the car's instrument panel reports as a "100%" or a "0%" charge isn't actually a 100% charge, and probably not a 0% charge. EV makers reserve a few percent at the "top" and the "bottom" to prevent battery packs from being prematurely aged by being either charged to 100% or completely discharged to 0%.

    The ideal of charging around 50% for daily driving** is made more complex with PHEVs, especially the Volt, since they have a lot of reserve capacity, so what the car reports as a "50% charge" is actually somewhat less than 50%, just like a 100% charge is in reality rather less than that.

    But then, with all that extra reserve capacity, the issue of degradation over time is also less important for the Volt, and possibly for other PHEVs if they also keep a lot of the pack's capacity in reserve.

    **For example, if your daily drive uses 40% of the battery pack's capacity, then ideally you should charge the pack to 70% overnight and discharge to 30% by the time you return home the next day. But that means balancing around 50% of the battery pack's actual capacity... which may be significantly different than what the instrument panel reads. It's a complex issue.

    I don't appreciate you throwing unjustified insults in my direction. I have never knowingly posted to InsideEVs anything I knew wasn't true, let alone FUD. I can't imagine any situation in which I would.

    If I take the time and effort to share my knowledge with people here, then the least you can do is give me credit for trying to spread good memes, not bad ones.
     
    Last edited: Dec 21, 2017
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  20. Domenick

    Domenick Administrator Staff Member

    I understand the mechanism for losing charge in lead acid batteries, but not lithium ones. Andd, while I appreciate the Battery University website ha a lot of good info, sometimes I come across some questionable statements.

    Also, I have seen people take batteries that have been sitting on the shelf for a year or two and they haven't lost charge. Because, there is no mechanism but which for them to do so. (Ok, aging could possibly have an effect, but that would be over a significant period of time, not likely to affect anyone with an EV that isn't squirreled away in long term storage.)

    Finally, yes, 50% is considered to be the ideal charge level for long term storage.
     
  21. Pushmi-Pullyu

    Pushmi-Pullyu Well-Known Member

    One lesson I learned by a great deal of participation in the (now defunct) TheEEStory forum is that when it comes to highly technical issues like this, experts disagree. Sometimes they disagree a lot! I certainly make no claim to expertise in this field, and I certainly don't know enough to challenge what's at the Battery University. As you say, some of the claims seem questionable. One thing we might ask is just how often the info there is updated; perhaps some of it is outdated.

    May I suggest, Domenick, that you look more carefully at what it actually says:

    Table 2: Estimated recoverable capacity when storing a battery for one year... Depending on battery type, lithium-ion is also sensitive to charge levels.
    It's not saying that the batteries lose charge when stored on a shelf. It's saying that they lose capacity when stored on a shelf. It also says "depending on battery type", so perhaps not all li-ion chemistries are sensitive to SoC level in long-term storage.

    If it was me, Domenick, I would have qualified that by saying "I don't know of any mechanism..."

    Again, I'm not an expert on this subject. But I asked Mr. Google, and he found the following question and two answers on Quora:

    Q: Can a battery hold a charge "forever"? If you take a standard lithium ion battery and let it sit for years, will it run out and why?​

    A1: by Karl Young, M.E., Li-ion batteries, supercapacitors, cell-balancing, nano-materials
    Answered Sep 17, 2016​

    Let me answer this from another angle. Pretty much all things in the universe (at least on Earth) seek equilibrium. When you charge up a battery, you are adding energy to increase the desire for opposite poles to want to come together (discharging).

    This is not a perfect analogy, but think of filling up a balloon with helium gas as compared to charging up a battery. In time, the helium gas will slowly diffuse through the balloon material and the pinched opening of the balloon.

    The gap between the two electrodes is the thickness of the separator film, which is only about 25um thick and porous. The ions from each electrode are motivated to migrate through the separator to recombine with each other to a lower energy state.

    The warmer the material, the easier it is for ions to move around (think of honey). So, storing at a lower temperature will reduce the leakage rate. However, you should only use the cells after they are warmed up to the ambient temperature or above -10C; better if between 10–30C. You must only charge above 0C to avoid damaging any of the chemical structures; better if between 15–30C.

    So, just like a glass of water on the table (at 25C, w/o adding water), the water will eventually evaporate (ok, another not-too-great analogy).

    * * * * *

    A2: by Anton Wilhelm, Specialized in li-ion 18650 cells
    Answered Jul 26, 2016

    Unfortunately, commercial lithium-ion batteries lose a small percentage (1–3%) of their voltage every month they sit in storage. This is called self-discharge, and it’s caused by electrolyte oxidation in the cathode.

    Furthermore, the steel can and other chemicals are not rated to last more than around 7 years of normal use - regardless of cycle life at the time. However, in space and other applications, we can see them lasting substantially longer.​

    If there was no mechanism that caused li-ion batteries to lose either charge or capacity when stored in a highly unbalanced state (that is, much closer to 100% SoC than 50%), then why would the ideal be 50% SoC?

    * * * * *

    Now, I don't know if any of this applies to daily or weekly charge/discharge cycling in an EV. Perhaps the effects described by Karl Young and Anton Wilhelm, in the citations above, don't apply to batteries which are cycled daily or weekly.

    On the other hand, maybe they do apply. It seems to me that the rule of thumb that long-term storage of PEVs should be done with the battery pack SoC at 50%, indicates the same rules do apply to PEVs, or to any appliance using li-ion batteries.

    Unfortunately, as I noted above, figuring out what the true 50% SoC is for your EV's battery pack may not be easy. What the driver is interacting with is not the battery cells themselves, but the pack's BMS (Battery Management System), which reports percentage of usable capacity... not full capacity. This is perhaps most notable in the Volt, because the apparently large reserve capacity means the true SoC may be quite a bit different than what the car's SoC gauge indicates. That same caveat may apply to other PHEVs, too.

    This is a very complex subject, and I question that anyone posting here has sufficient expertise to answer that definitively and for all EVs, regardless of what specific li-ion chemistry their batteries contain.
     
    Last edited: Jan 2, 2018
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  22. Domenick

    Domenick Administrator Staff Member

    Good points all.

    I guess this, "...it’s caused by electrolyte oxidation in the cathode" would be the mechanism. And yes, potential capacity in stored batteries may be less due to deterioration from age.

    Still, as I've seen it demonstrated that batteries sitting in a cupboard for a couple years have pretty much the same charge, I have to think that any change would be negligible.

    As for why wouldn't we use 100% as a storage rate, well, that's a good point. I'm sure I've heard the answer to that previously, but it's not coming to mind. Possibly, though, if there is some anode degradation, being fully charged may create some problems (I dunno, really).
     
  23. Pushmi-Pullyu

    Pushmi-Pullyu Well-Known Member

    Okay, but perhaps that applies only to the specific chemistry in that particular type of li-ion battery cell. Not all chemistries are created equal!
     

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