...while I appreciate the Battery University website ha a lot of good info, sometimes I come across some questionable statements.
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 One thing we might ask is just how often the info there is updated; perhaps some of it is outdated.
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.
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.
Because, there is no mechanism but which for them to do so.
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.
Finally, yes, 50% is considered to be the ideal charge level for long term storage.
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.
