Question about lithium rechargeable storage

[Niconical]

Hello.

There is lots of info on the forum about the long term storage of lithium rechargeables, storing them at a reduced voltage for example. The one thing I've never been able to work out though is the timescale involved. What do "long term" and "storage" mean in this situation?

With my own lithium rechargeables, I might find for example that I charge a cell, and I know I won't be using it for a week or so. Do I need to "store" that cell for a week at a reduced voltage?

Up until now, I have been doing that.
In fact, unless the cell is going straight in a light, I store all lithium rechargeables at a reduced voltage.

So, to take it to either extreme. Charging then using immediately, charge to the full 4.2v. Charging then storing for a year, don't fully charge, or charge then lower the voltage before storage.

Where in between would you say the line is?

A week, a month?

The reason I ask is because although I don't mind putting in the effort to care for my rechargeables, I can't help thinking that a lot of what I'm doing isn't needed. Last night for example, I charged 2 x AW protected 17670 cells. I know I'll be using them both within a month or so, so was it really needed for me to monitor them then put them in a drawer at a reduced voltage, then top them up when used, or for a month timescale, could I have just charged and stored?

Thank you

 

[Niconical]

Is there a battery guru available to shed some light (pun intended) on this for me?

 

[SilverFox]

Hello Niconical,

Battery University gives some information on this. In their tests, a Li-Ion cell stored at full charge lost 20% of its capacity in one year when the cell was stored at room temperature (25 C). If this degradation is linear, we could speculate that you loose around 0.055% of your capacity every storage day.

In contrast, they found that only 4% of the capacity was lost over a year if the storage was done at reduced voltage and the same temperature conditions. Once again speculating a linear degradation, that works out to around 0.011% per day.

The real question may be How much degradation are you willing to put up with?

Tom

 

[Niconical]

Thank you for the reply. I must admit though, I'm not 100% sure what you mean by "degradation".

Do you mean loss of capacity by self-discharge over that time period, which can then be topped up, or permanent damage to the cell, resulting in a xx% lower overall possible capacity?

If it's just self discharge, that wouldn't bother me, my worry is that by storing a cell at 4.2v for up to a month, I am damaging it each time, and each time I then use/charge, the total possible capacity will be lower.

 

[SilverFox]

Hello Niconical,

Sorry for the confusion...

Degradation means permanent loss of capacity.

Tom

 

[Niconical]

In that case I think I'll stick to what I've been doing.
Doing less to care for rechargeables just because it involves a little effort is a slippery slope, and not one I want to go down.

On to another thing though, what would make a big difference is being able to pull a cell off the charger when it reaches a certain voltage. At the moment I fully charge, then drain it in a light before putting it away. This is all quite time consuming. I have the "standard" ultrafire charger than AW sells. I have tried to put the tips of the multimeter on to the + and - of the cell while it is charging, but I can't get them to touch and make contact. Is there a way around that, maybe a bit of wire trailing out or something like that?

I do sometimes just pull the battery out, check voltage, put back for a while, then out/check etc until I get the voltage that I want, but I was wondering if a constant stop/start to the charging process might also be bad for the cell. Is that the case?

Thank you.

 

[SemiMan]

Hello Niconical,

Battery University gives some information on this. In their tests, a Li-Ion cell stored at full charge lost 20% of its capacity in one year when the cell was stored at room temperature (25 C). If this degradation is linear, we could speculate that you loose around 0.055% of your capacity every storage day.

In contrast, they found that only 4% of the capacity was lost over a year if the storage was done at reduced voltage and the same temperature conditions. Once again speculating a linear degradation, that works out to around 0.011% per day.

The real question may be How much degradation are you willing to put up with?

Tom

Unfortunately this is true. You are much better off storing Lithium IONS uncharged for long periods of times. The other alternative which I actually find easier, is to store them in the freezer. That way you can keep them charged, then pull them out when you are ready to use them. Heat also plays a role, so make sure you are keeping that laptop cool. I believe the degradation is as a percentage of current capacity so not perfectly linear, but close in a short time span.

Semiman

 

[SilverFox]

Hello Niconical,

If you are serious about your cells, and value your time, put a figure on the amount of time you spend "adjusting the charge" on your cells.

Now comes the hard part... Does the best value to you come from spending this amount, or would there be better value if you upgraded your charger to one that did it automatically?

Most of the hobby chargers have a storage function. You hook up the cell and it will either charge or discharge it to a safe storage level automatically. Granted, these chargers are more expensive, but what is your time worth?

A less "sophisticated" way to do this is to simply use a timer. When your cell is run down, put it in your charger and set a timer for 30 minutes. When the timer goes off, pull the cell and set it aside for storage. Let the cell rest for around 15 minutes then measure its voltage. You can adjust your timer as needed to reach your target storage voltage.

Tom

 

[Niconical]

A less "sophisticated" way to do this is to simply use a timer. When your cell is run down, put it in your charger and set a timer for 30 minutes. When the timer goes off, pull the cell and set it aside for storage. Let the cell rest for around 15 minutes then measure its voltage. You can adjust your timer as needed to reach your target storage voltage.

Tom

Is it bad for the cells to be charged, stopped, then charged some more, particularly as I get used to the times needed, or does repeated in/out of the charger not have any negative effect?

 

[SilverFox]

Hello Niconical,

None whatsoever. No problem at all.

Tom

 

[mdocod]

If you continue to treat your cells with such remarkable care, you'll probably set records for li-ion cell longevity! I certainly don't put that much care into my cells, but I've had pretty good luck, a number of my cells from years ago are still actually performing quite well

Eric

 

[45/70]

A sample of my experience with Li-Ion storage. The oldest Li-Ion cells I have, for flashlight use, are four Powerizer 3.6 Volt unprotected RCR-123's. I bought these new, a little over 3 years ago. They have always been used singly, and never discharged to below 3.7 Volts OC.

Before I bought these cells, I had read up on Li-Ion cell storage characteristics. I decided to do a compromise, and store the cells fully charged, in a refrigerator @ 35-40 degrees F when not in use.

These cells have approximately 50 partial cycles on them, again, never having been discharged below 3.7 Volts OC, and typically not below 3.75-3.80 Volts, often a shallower discharge. I will also point out, that all four cells are not always in the fridge, due to my rotating them through various lights.

I just pulled them from the fridge, the last time a cell was charged was probably about a month ago. I let them warm up to room temperature, and checked the voltage, 4.12, 4.14, 4.14, and 4.16 Volts. Again, these cells are over 3 years old and have been used to some degree. They all, are still at 90% or better capacity. I'm quite satisfied with this method of storing Li-Ion's. They're always ready to go, and I think I'll stay with this simple method.

Dave

 

[Ajay]

Hello Niconical,



A less "sophisticated" way to do this is to simply use a timer. When your cell is run down, put it in your charger and set a timer for 30 minutes. When the timer goes off, pull the cell and set it aside for storage. Let the cell rest for around 15 minutes then measure its voltage. You can adjust your timer as needed to reach your target storage voltage.

Tom

SilverFox and others,
correct me if I am wrong but
I believe 40% charge is the recommended charge for storing your lithiums. My question is how do you know when you have reached 40%?

I have some 2600mah Sanyo 18650 cells from laptops and I need to store them in the fridge but I am not sure if there is a voltage that corresponds to 40% charge. Please enlighten me.:thinking:

Thanks

 

[Mr Happy]

Here is a data sheet for an 18650 cell (it is a Sanyo 2150 mAh cell): http://www.eneloop.info/fileadmin/EDITORS/BATTERIES/industrial/datasheets/li-ion_li-po/UR18650A.pdf

If you look at the low rate 0.2C discharge curve, you can see it starts out fully charged at about 4.2 V, and is just about empty when it reaches the knee of the discharge curve at about 3.3 V.

If you look back along the curve to the point where 40% of charge remains (i.e. 60% x 2150 ~ 1200 mAh) the voltage is about 3.7 V. You get a slightly higher number of about 3.8 V if you do the same thing with the charge curve (about 800 mAh of charge is about 40%).

So extrapolating from those charts a voltage of about 3.7-3.8 would do it. However, there is no need to be too precise about it. The storage life does not depend on exact voltages. It is only a guideline.

 

[jeffosborne]

This is a great thread! I have many 18650's waiting for future projects, and some are now a year old. I will discharge all my storage Li-ion cells to 3.7 volts or so, and have longer life for these cells. Thanks everyone for the info! Jeff O.

 

[Ajay]

Here is a data sheet for an 18650 cell (it is a Sanyo 2150 mAh cell): http://www.eneloop.info/fileadmin/EDITORS/BATTERIES/industrial/datasheets/li-ion_li-po/UR18650A.pdf

If you look at the low rate 0.2C discharge curve, you can see it starts out fully charged at about 4.2 V, and is just about empty when it reaches the knee of the discharge curve at about 3.3 V.

If you look back along the curve to the point where 40% of charge remains (i.e. 60% x 2150 ~ 1200 mAh) the voltage is about 3.7 V. You get a slightly higher number of about 3.8 V if you do the same thing with the charge curve (about 800 mAh of charge is about 40%).

So extrapolating from those charts a voltage of about 3.7-3.8 would do it. However, there is no need to be too precise about it. The storage life does not depend on exact voltages. It is only a guideline.

Mr Happy, you are da man! Thank you very much that is exactly the info I was looking for.:twothumbs