Longterm Battery Storage - What is 40%?

[UncLeJunkLe]

I keep reading people suggesting that you should store batteries you won't using for a while "at 40%".

40% of what?

40% of 3.7v is 1.48v - so that's not it.
40% of 4.2v is 1.68v - so that's not it.

So what's this 40% I keep reading about?

Why not just suggest 3.5v-3.6v (which is what it seems warehouse storage voltage is from my experience)?

 

[Rossum]

Voltage and SOC don't have a totally linear relationship. If they did, I'd say 4.2V is fully charged and 2.8V is fully discharged. So 1.4V difference. 4.2V less 60% of 1.4V is 3.36V.

Personally I've settled on 3.6V as a storage charge. It works well.

Oh, and keep 'em cool. I'm doing a long-term test on this. After one year, refrigerator temperatures seem to be the best compromise. Room temperature lost more capacity. Freezer lost slightly less capacity, but had more of an increase in internal resistance. One of these days, I really need to write this up in detail...

 

[UncLeJunkLe]

Oh, and keep 'em cool. I'm doing a long-term test on this. After one year, refrigerator temperatures seem to be the best compromise. Room temperature lost more capacity. Freezer lost slightly less capacity, but had more of an increase in internal resistance. One of these days, I really need to write this up in detail...

I'm planning on keeping mine in the fridge, but I find it interesting that you say that freezer batts lost less capacity yet increased in internal resistance. I always thought that an increase in IR = less capacity.

Personally I've settled on 3.6V as a storage charge. It works well.

Yeah, I just go for 3.5v to 3.6v because that's what almost all the batts are charged at when I receive them in the mail...with the exception of the 18350s I recently bought, they came to me at 3.8v. But I discharged them to 3.5-ish volts.

But that 40% thing confuses me.

 

[Superuser187]

Havent done any tests but when i store a battery whatever type of battery that is i will fist charge it to max and then store it in fridge in a sealed container if its a small one that can fit ofc.....and if its larger battery i store in a cool place....car batteries must be charged into full capacity to be stored for sure....
@Rossum yeaaaaaaaaaa make a detailed post TEACH US MASTAAAAAAAAAAAAAAAAAA

 

[Mooch]

Havent done any tests but when i store a battery whatever type of battery that is i will fist charge it to max and then store it in fridge in a sealed container if its a small one that can fit ofc.....and if its larger battery i store in a cool place....car batteries must be charged into full capacity to be stored for sure....
@Rossum yeaaaaaaaaaa make a detailed post TEACH US MASTAAAAAAAAAAAAAAAAAA

Storing a Li-ion battery at full charge results in faster aging of the battery. It’s best to store them at somewhere roughly around 50%. Anything between 3.3V and 3.7V or so is fine, it doesn’t matter much.

Check them every couple of months to make sure the voltage hasn’t dropped below 3V or so, 2.5V absolute minimum. Give them a bit of charge if they’re getting near 3V.

 

[Superuser187]

Storing a Li-ion battery at full charge results in faster aging of the battery. It’s best to store them at somewhere roughly around 50%. Anything between 3.3V and 3.7V or so is fine, it doesn’t matter much.

Check them every couple of months to make sure the voltage hasn’t dropped below 3V or so, 2.5V absolute minimum. Give them a bit of charge if they’re getting near 3V.

Hmmm Soo it has to do with balancing the electrons I guess that's why half charge....if I am not wrong....makes sense if it's that the cause....doesn't matter why tho
...
Either way I trust the legendary Mooch....thanks

 

[zoiDman]

I have Read, though I'm Not sure how accurate it is, that Room Temperature (72F) storage is about as effective as storage in a Refrigerator If the Room Temperature/Humidity remains Stable.

And that Fluctuations in Temperature/Humidity for say a Battery stored in a something like a Garage where Seasonal Changes and Night/Day swings can play a role in Battery Lifespan/Performance.

Not sure what kinda of Quantitative Changes or Similarities there are? Or if they are something a User would need to be Concerned with though?

I have a Few Built-In Battery Mods that I put away in Non AC/Heated Hall Closet at about 3.5v. I pull them out every couple of Months or so and check on them. And give them One or Two Discharge/Charge Cycles.

Aside for that, I figure they are just going to Die when they Die. And I did what I can to Prolong their Life.

 

[Mooch]

Hmmm Soo it has to do with balancing the electrons I guess that's why half charge....if I am not wrong....makes sense if it's that the cause....doesn't matter why tho
...
Either way I trust the legendary Mooch....thanks

No electron balancing. The higher the voltage the faster some of the aging mechanisms work.

 

[Rossum]

I have Read, though I'm Not sure how accurate it is, that Room Temperature (72F) storage is about as effective as storage in a Refrigerator If the Room Temperature/Humidity remains Stable.

There seems to be some controversy over this, which is why I decided to do some actual testing.

 

[sonicbomb]

Putting batteries in the fridge/freezer makes me uncomfortable. As discussed many times here, most of the batteries we use for vaping are in no danger of leaving the market any time soon.

 

[Rossum]

Putting batteries in the fridge/freezer makes me uncomfortable.

Why? Look at some battery data sheets. They all allow storage at very low temperatures.

Note that they must be allowed to come back up to operating temperature before being charged. Charging a cold battery is bad news, so don't do that.

 

[zoiDman]

... As discussed many times here, most of the batteries we use for vaping are in no danger of leaving the market any time soon.

Completely Agree.

Though I see much of the Information in this thread applying towards someone with a Mod with a Built-In Battery.

 

[zoiDman]

There seems to be some controversy over this, which is why I decided to do some actual testing.

Yeah... A Lot depends on how one sees "About" as it pertains to Real World numbers.

How many Cells are in your Comparison Test?

 

[Rossum]

How many Cells are in your Comparison Test

Eight VTC5As, two under each storage condition. This batch started July of 2019. I tested them both before and after after one year in storage. I'll test them again after an additional year has passed, so next summer.

This July I added eight more cells, P26As, again two under each condition. These have been tested before being put in storage.

I selected VTC5As and P26As because Mooch rates them as "best all-around" 18650s.

Conditions are:
1) Room temperature, fully charged.
2) Room temperature, 3.6V
3) Fridge, 3.6V
4) Freezer, 3.6V

 

[zoiDman]

Eight VTC5As, two under each storage condition. This batch started July of 2019. I tested them both before and after after one year in storage. I'll test them again after an additional year has passed, so next summer.

This July I added eight more cells, P26As, again two under each condition. These have been tested before being put in storage.

I selected VTC5As and P26As because Mooch rates them as "best all-around" 18650s.

Conditions are:
1) Room temperature, fully charged.
2) Room temperature, 3.6V
3) Fridge, 3.6V
4) Freezer, 3.6V

Very Cool.

I remember when you bought that Hardware Device that can measure battery Internal Resistance and such. Looking forwards to seeing what you find with these Batteries.

 

[Rossum]

I remember when you bought that Hardware Device that can measure battery Internal Resistance and such.

Yeah, that one got me curious. AC/IR clearly increases with user (and to a much lesser extent age). So I started wondering how that correlates to capacity and got this as well:

 

[Mooch]

Yeah, that one got me curious. AC/IR clearly increases with user (and to a much lesser extent age). So I started wondering how that correlates to capacity and got this as well:

The loss of capacity is much easier to measure and is a great way to track aging. It’s not linear over time but you’ll see that as the testing proceeds.

 

[Rossum]

The loss of capacity is much easier to measure

Easier to measure than AC IR?

I respectfully disagree. To properly measure capacity, one must fully charge the cell, and then discharge it at 0.2C. The latter intrinsically takes 4-5 hours. Measuring AC IR takes a few seconds.

 

[Mooch]

Easier to measure than AC IR?

I respectfully disagree. To properly measure capacity, one must fully charge the cell, and then discharge it at 0.2C. The latter intrinsically takes 4-5 hours. Measuring AC IR takes a few seconds.

Not the capacity itself, the loss of capacity. I could have phrased that better.
The change in capacity as a cell ages is much more obvious and easier to measure than the change in AC IR. Yes, it takes much longer to measure though.

Measuring AC IR requires a dedicated device and a good testing methodology to eliminate the variables that affect the results (state-of-charge, temperature, test jig resistance, contact resistance). The AC IR is low and easily affected by a number of different things and only varies a little as the cell ages.

Measuring changes in capacity can be done in a number of ways.

 

[DaveP]

I look at 18650 cells as a consumable item. Internal resistance dissipates voltage more rapidly over time in storage/standby as the cell ages. When IR exceeds 100 milliohms I just duct tape the ends, recycle, and buy replacements. My Miboxer C4 charger tells me when that threshold is reached.

I ordered 10 replacement VTC6's a couple of weeks ago. For my 10W vape style, the high MAH nature of the VTC6 suits me well. I keep them in cases that hold 6 cells and just rotate left to right as I use and recharge them. They get a rest between uses and I get 2 to 3 years of daily use.