Minding Your mAhs – Ep22 – Li-Ion Battery Myths and Facts has been uploaded

[Mooch]

There are a LOT of myths out there about Li-Ion batteries. Some have a bit of fact mixed in but most are completely untrue. I talk about these myths and give you some facts you can use to sort out the exaggerated ratings we too often see on these batteries.

 

[Baditude]

This was needed to dispell a lot of the myths in our community. Probably your most important video to date, IMHO.

 

[Rossum]

I'm questioning myth number 8, especially the adamant, "Don't freeze them". All the datasheets I've looked at allow for storage at -20C or lower, which is as cold as you'll get in a residential freezer. I certainly agree that charging them while they're ice cold is bad news; they must be allowed to come to a reasonable temperature before charging. In fact, modern EVs will actively heat their batteries prior to accepting much charge if they are too cold.

You also say it doesn't make much difference, yet degradation in storage is a chemical process, and all chemical reaction slow down at lower temperatures. Datasheets do indicate more degradation at higher temperatures; anything much above 25-30C seems to be bad. Unfortunately, those datasheets are silent about any slowing of degradation at colder temperatures. Have you seen literature that indicates it's not beneficial over the long term?

 

[Mooch]

I'm questioning myth number 8, especially the adamant, "Don't freeze them". All the datasheets I've looked at allow for storage at -20C or lower, which is as cold as you'll get in a residential freezer. I certainly agree that charging them while they're ice cold is bad news; they must be allowed to come to a reasonable temperature before charging. In fact, modern EVs will actively heat their batteries prior to accepting much charge if they are too cold.

You also say it doesn't make much difference, yet degradation in storage is a chemical process, and all chemical reaction slow down at lower temperatures. Datasheets do indicate more degradation at higher temperatures; anything much above 25-30C seems to be bad. Unfortunately, those datasheets are silent about any slowing of degradation at colder temperatures. Have you seen literature that indicates it's not beneficial over the long term?

There are theoretical benefits to storing at cooler temps but the differences can be minor and issues with condensation can cause rusting. IMO, as a general recommendation for the entire vaping community, there is no real benefit to storing in the refrigerator. It can help slow calendar aging of the cells but does nothing about charge/discharge induced aging (cycle aging).

Is it possible for someone who is especially careful to get some benefit from cool storage? Perhaps. Chemical reactions do slow at lower temps. But the cells are still aging in there and any other abuse (use in hot weather, getting warm during charging, pulsing the cells hard) can cause accelerated aging that would negate any benefits from cool storage.

Charging the cells while they’re below 0°C can be a big safety issue. If someone is in a rush after pulling cells out of a freezer they could be getting into trouble eventually. As someone who is giving advice to a worldwide community I must assume that some will not let the cells rise above 0°C before charging. I must always assume the worst case. Without clear evidence of benefits in extending life vs. the risks it’s better to recommend not freezing cells at all IMO.

Cells hate being stored. It’s just better to cycle through all of them rather than use some and store some. That way you’re not letting some cells age in storage, even if slower, doing nothing.

But, as with anything I recommend, every vaper is free to do something different. Especially if they will be taking the extra care needed to do it right versus the worst case I must assume.

It’s like vaping a mech with a 0.04 ohm coil. Can it be done without issues? Of course. Would we put up a post saying that? Absolutely not. Because we know someone will not do things right and end up venting a cell or forcing it into runaway. So we all advise others not to vape with a 0.04 ohm coil in a mech.

 

[Mooch]

I'm questioning myth number 8, especially the adamant, "Don't freeze them". All the datasheets I've looked at allow for storage at -20C or lower, which is as cold as you'll get in a residential freezer. I certainly agree that charging them while they're ice cold is bad news; they must be allowed to come to a reasonable temperature before charging. In fact, modern EVs will actively heat their batteries prior to accepting much charge if they are too cold.

You also say it doesn't make much difference, yet degradation in storage is a chemical process, and all chemical reaction slow down at lower temperatures. Datasheets do indicate more degradation at higher temperatures; anything much above 25-30C seems to be bad. Unfortunately, those datasheets are silent about any slowing of degradation at colder temperatures. Have you seen literature that indicates it's not beneficial over the long term?

I’ve been doing some thinking and I hate not knowing for sure. I’ll buy four each of three different cells. I’ll test the internal resistance and capacity of each.

Two of each cell will then be put in my freezer, typically at about -11C, and two of each will be stored in the kitchen where it’s typically 21C-29C year-round. Then I’ll wait six months and retest all of them.

Since this is a calendar aging test there’s no way to speed it up but at least we’ll have some data eventually.

 

[Rossum]

I’ve been doing some thinking and I hate not knowing for sure. I’ll buy four each of three different cells. I’ll test the internal resistance and capacity of each.

Two of each cell will then be put in my freezer, typically at about -11C, and two of each will be stored in the kitchen where it’s typically 21C-29C year-round. Then I’ll wait six months and retest all of them.

Since this is a calendar aging test there’s no way to speed it up but at least we’ll have some data eventually.

I already started that experiment with VTC5As earlier this summer. Two in the freezer, two in the fridge, two at room temp. All six at storage charge (3.6V). Two more at room temp at 4.2V, which I intend to top off ever few months 'cause I'm curious how negatively that affects them. All were new (unused), although not terribly fresh (manufactured early last year). All were tested for capacity and AC IR. FWIW, they have the lowest AC IR I've ever seen, right around 10 mOhms.

I was planning to re-test them after one year rather than six months, but if you do six month intervals, I will too.

Oh, and I will be adding some LW 18650 LFP cells to this test next month.

 

[Rossum]

Cells hate being stored. It’s just better to cycle through all of them rather than use some and store some. That way you’re not letting some cells age in storage, even if slower, doing nothing.

My qualm with this advice is that it would complicate my life. Cells I have in rotation go in the charger as soon as they come out of a mod. That way, I have a fully charged cell ready to go for each mod the next time I need one, and I keep two (and only two) cells in rotation for each mod.

Because cells ostensibly don't like sitting around fully charged, if I kept more that two cells per mod in rotation, I would either be decreasing their life because each one would spend more time fully charged rather than being a spare sitting in storage at 3.6V, or I would have to constantly keep track of which ones are charged and which ones aren't.

I like to keep my life simple.

 

[puffon]

There are theoretical benefits to storing at cooler temps but the differences can be minor and issues with condensation can cause rusting.

Since the cells are sealed, would there be internal condensation?
Or, are you concerned about rusting under the outer wrap?

 

[Mooch]

Since the cells are sealed, would there be internal condensation?
Or, are you concerned about rusting under the outer wrap?

External rusting. They’re hermetically sealed so no issues with anything internally.

 

[puffon]

External rusting. They’re hermetically sealed so no issues with anything internally.

Gotchya.
Look forward to yours and Rossums cold storage testing.

 

[Punk In Drublic]

Given the many variables I understand it maybe difficult to put a value on degradation, but do we have any kind of estimate? According to the production codes (if the tool is accurate), I’ve purchased cells that have been just a few months from production to well over a year

 

[Rossum]

Given the many variables I understand it maybe difficult to put a value on degradation, but do we have any kind of estimate? According to the production codes (if the tool is accurate), I’ve purchased cells that have been just a few months from production to well over a year

There used to be an old rule of thumb that cells would lose up to 10% of capacity per year in storage. I don't think that's true anymore. I have some VTC5 cells made in January of 2014 that have been used intermittently that still test at slightly above 90% capacity and have very good AC-IR. However, I also have some baby blue 25Rs from 2014 that have not fared as well. They're in the 80% range. Unfortunately, I don't have a good handle on how much I've used them.

FWIW, my capacity tests are at 0.2C and down to 2.8V. I know "spec" is down to 2.5V, but based on the shape of the curve, the capacity that remains between 2.8V and 2.5V is nearly negligible and I prefer not to pull my cells down that low.

 

[Punk In Drublic]

There used to be an old rule of thumb that cells would lose up to 10% of capacity per year in storage. I don't think that's true anymore. I have some VTC5 cells made in January of 2014 that have been used intermittently that still test at slightly above 90% capacity and have very good AC-IR. However, I also have some baby blue 25Rs from 2014 that have not fared as well. They're in the 80% range. Unfortunately, I don't have a good handle on how much I've used them.

FWIW, my capacity tests are at 0.2C and down to 2.8V. I know "spec" is down to 2.5V, but based on the shape of the curve, there capacity that remains between 2.8V and 2.5V is nearly negligible and I prefer not to pull my cells down that low.

Interesting. Thanks for sharing

 

[Rossum]

Interesting. Thanks for sharing

One of these days, I really should start a thread with all the data I've gathered so far. I tested pretty much every cell I've got this summer.

 

[CMD-Ky]

One of these days, I really should start a thread with all the data I've gathered so far. I tested pretty much every cell I've got this summer.

Please do.

 

[Punk In Drublic]

One of these days, I really should start a thread with all the data I've gathered so far. I tested pretty much every cell I've got this summer.

I would be interested in seeing this data

 

[Mooch]

Given the many variables I understand it maybe difficult to put a value on degradation, but do we have any kind of estimate? According to the production codes (if the tool is accurate), I’ve purchased cells that have been just a few months from production to well over a year

It varies wildly between different cells and different storage conditions (temperature and state-of-charge) and the degree of previous damage which can increase the self-discharge rate. It could be a 1% per month or 20% per month.

 

[Punk In Drublic]

It varies wildly between different cells and different storage conditions (temperature and state-of-charge) and the degree of previous damage which can increase the self-discharge rate. It could be a 1% per month or 20% per month.

Thanks Mooch. Figured it would be difficult to place a value given the many variables. Was looking for a guestimate average, under average conditions for the typical cells we use – all to feed the curiosity of an average user.

Given we do not know the distribution of these cells (I’m sure many assume they come straight from the manufacture and are exposed to little shelf life), I figured the information would be beneficial in the event we find 2 cells that were purchased at the same time but with a noticeable performance difference.

 

[Mooch]

Thanks Mooch. Figured it would be difficult to place a value given the many variables. Was looking for a guestimate average, under average conditions for the typical cells we use – all to feed the curiosity of an average user.

Given we do not know the distribution of these cells (I’m sure many assume they come straight from the manufacture and are exposed to little shelf life), I figured the information would be beneficial in the event we find 2 cells that were purchased at the same time but with a noticeable performance difference.

I fully understand and share the desire to have even just a rough estimate but if anyone has one of the cells that degrades much slower or much faster than my estimate then my number is useless. There are just too many different cells in too wide a range of storage (external) and internal conditions to try to pin it down to one number.

Take vehicles as an example. I think the average is about 27MPG. But that number is completely useless if I buy a Hummer or a used and badly maintained vehicle, classic car, or a hybrid or electric vehicle.

Having said all that, I’ve seen a couple of research papers where the drop in capacity for cells from the big manufacturers was about 5% the first year for new cells at room temp stored at about 50% charge. Double or triple that at higher temps. Increase it a lot more for used cells. Increase it more if the cells are charged more than that.

There’s the inevitable cell-to-cell differences (when new) we need to also deal with in addition to just storage time differences. Some cells, particularly ones from the smaller China factories, can have a 1%-3% capacity difference between new cells.

Performance differences can also be caused by internal resistance differences, which cause different amounts of voltage sag. Greater voltage sag = less vaping time.

Without testing we can't know if the difference in performance between two new cells was due to storage conditions, typical cell-to-cell differences, or internal resistance differences. This assumes we can read the date codes, otherwise the date of manufacture can be a variable too.

 

[Rossum]

I’ve seen a couple of research papers where the drop in capacity for cells from the big manufacturers was about 5% the first year for new cells at room temp stored at about 50% charge. Double or triple that at higher temps. Increase it a lot more for used cells.

If that's true, would it not make sense to keep spare cells in un-used condition, rather than including them in one's rotation?