Is 3.92v the best resting voltage for storage?

Over the years, we've all been accustomed to seeing something like storing cells at 40-50% or so, but I believe that may be due *partly* to hazmat shipping issues. You kill two birds with one stone, ie not having cells sitting around in retail fully charged, and low enough to satisfy hazmat.

But once you get cells home, perhaps this low capacity storage is not ideal since we no longer have to deal with hazmat shipping.

Battery U. identifies 3.92v as the ideal voltage between oxidization and SEI growth, and this resting voltage seems to be approximately 70-75% or so of SOC voltages.

Maybe I'm mis-interpreting something - if so, let me know!

The majority of the experts say 40%, cool and dry for long term storage.

I am not a scientist nor do I have the testing equipment and time to find my own verifiable empirical data. Therefore, I am just using approximately 40% SOC and putting them in my refrigerator.

I probably have more batteries than I can actually use. I have an XTAR Dragon 4+, and I do test and track voltages.

Hello IonicBond,

Let me try to complicate things a little...

I have two classes of storage. Active storage and passive storage.

Active storage goes something like this. I have several flashlights and pick one to use on any particular occasion. I have back up cells for these lights. In almost every day use I expect decent run time but don't expect them to last an extended period of time.

Passive storage involves cells that I have no plan on using right now. Not a back up for emergency or any playing around. They are simply excess inventory. My goal for these cells is to store them with minimal degradation and my expectation is to put them into use two or three years from now and I expect them to be vibrant.

The cells in the flashlights in normal use are fully charged.

The backup cells in active storage are charged to 4.0 volts. This is a compromise between runtime and chemical breakdown. In an emergency situation I am comfortable with about 80% of runtime and can ration my use accordingly. These backup cells are rotated into normal use periodically and the plan is to have the whole batch wear out evenly.

The passive storage cells are fully charged then discharged to verify capacity. They are then stored at somewhere between 40 - 50% of actual capacity and kept in a cool place. I cycle these cells through a charge/discharge/charge/partial discharge cycle about once a year just to keep track of them.

Tom

Tom,
I like your method, and I am going to adopt it.

thanks

Can you link to the Battery U article suggesting 3.92V? I've not seen that claim before.

The little bit of research I've seen suggests lower state of charge is better as long the voltage doesn't fall low enough to cause damage, but 40% gives you a good compromise of all factors and a healthy margin for self-discharge without falling below 3V.
http://jes.ecsdl.org/content/163/9/A1872.full.pdf+html

Above roughly 60% there is a jump in rate of capacity loss, and above 80-90%, it continues increasing steadily with state of charge.

So the paper I linked seems to support the 40% recommendation.

The other big thing is temperature. Capacity loss and internal resistance increase both accelerate significantly if stored much above room temperature.

These links should be useful to you:

http://lygte-info.dk/info/BatteryChargePercent UK.html

http://www.candlepowerforums.com/vb/showthread.php?417490-Battery-storage-and-care&highlight=Storage

http://www.candlepowerforums.com/vb...rging-and-ready-use-storage&highlight=Storage

All great info guys!

Tom's procedure differentiating between long-term storage, and "vibrant" ready to go is well noted. I follow much the same with what's left of my nimh cells depending on the situation thanks to his recommendations in the past.

Here's the link at Battery Uni where I got the 3.92v as being the best "neutral" state: (NOT FOR LIFEPO4!)
http://batteryuniversity.com/learn/article/bu_808b_what_causes_li_ion_to_die

It actually contains some great info from one of my favorite Li-ion videos from Professor Jeffrey Dahn who was at Dalhousie University running specialized coulombic efficiency labs:

https://www.youtube.com/watch?v=9qi03QawZEk

I know I'm kind of dancing on the head of a pin, but maybe I'll drop my vibrant-storage down to about 3.9v rather than 4.0 as Tom is doing. I'm sure there are many variables like electrolyte additives that make nailing down the difference between 3.92 and 4.0v somewhat irrelevant, aside from an even lower available vibrant storage capacity right off the bat.

Note that if you have a full battery, and you discharge it to 3.92V, the voltage will probably rise a bit after you stop discharging it.

Also note that using multiple batteries in series can be dangerous, especially if their state of charge is different.

Personally I'm using something comparable to what's already mentioned:

- Short term storage batteries, the ones I want ready for use, are kept in rotation. If a light needs charging, I'll swap the battery, charge the old, and put it back in ready to use pile.

- Long term storage, around 3.92V, a bit below rather than a bit above. Still margins until self discharge becomes an issue, and still lots of useful charge if I'll unexpectedly need them.

- For shipping (not that I ship often), lower, such as 40%.

Short term are used/recharged often enough that I don't do specific checks. Long term are discharged a bit and recharged to 3.92-ish volts every 3-12 months or so (plan is 3-6 months, but you know... ;-) )

Pretty similar to what SilverFox mentioned in other words.

TimeOnTarget said:

Tom,
I like your method, and I am going to adopt it.

Click to expand...

Same here. That really is the way to go. Some lights are used so infrequently, but I still have them sitting around at 4.0-4.05v.

what kind of cells are you speaking of?

Hello Lumen aeternum,

Li-Ion cells.

Tom