When setting Li-Ion charge for storage - bottom->top or top->bottom?

I have had good results when storing Li-Ion batteries (and cells) at 40% charge and found lots of threads confirming this. What I haven't found is any data on if it makes any difference when setting the charge to 40%. If it should be charged to 40% and stop or discharged from 100% to 40% and then stop.

I have done both, but lately I have been favouring the latter. That is, I charge to 100% and then discharge to 40%. My thinking is that by charging to 100% initially, all of the chemicals have been mixed/distributed better, yielding potentially longer life.

Any one have any thoughts on this?

If state of charge is greater than 40%, discharge down to 40%. If it's less than 40%, charge it up to 40%. Simple as that.

Closet_Flashaholic said:

......What I haven't found is any data on if it makes any difference when setting the charge to 40%. If it should be charged to 40% and stop or discharged from 100% to 40% and then stop.

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Hi Closet. It really doesn't matter either way. With LiCo cells, I use them and when I change them out, if they are between 3.75 and 3.85 Volts, I simply put them in storage. If they are above that, I simply use them a bit more, if below 3.75V, I charge them up a bit, then store them.

My thinking is that by charging to 100% initially, all of the chemicals have been mixed/distributed better, yielding potentially longer life.

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This really doesn't apply to Li-Ion cells. There is no "mixing" that occurs within Li-Ion cells, as with NiCd/NiMH. It is also the reason that no "refresh" or "break-in" cycling is needed. This type of maintenance simply adds wear to the cells with no benefit, and should not be practiced.

Dave

I a new to using the 38650 series of batteries. Do you guys mean 40% of the 3100mAh rating, or 40% of whatever the normal full-charged voltage is for that cell. I'm guessing mA?
Any Lion cells I currently use are in rechargable lights, so I dont worry about it. Thanks.

Hi Light. Concerning LiCo/ICR and LiMn/IMR cells, the following chart is useful for determining the SOC (state of charge) of these two Li-Ion chemistry cells. These are OC (open circuit) voltages of a rested cell.

100% charge = 4.20 Volts
90% = 4.10V
80% = 4.00V
60% = 3.90V
40% = 3.80V
20% = 3.70V
Discharged = ~3.60V

There are several of these charts around and they are not all exactly the same, but give a general idea of the remaining capacity of LiCo and LiMn cells by measuring the OC voltage of a rested cell. Due to the differences in some cells makeup, and other variances in manufacture, the estimation at voltages below 3.80 Volts, is much less accurate. So again, these figures are not exact, but are close enough to use as a general guideline.

Also, this method determines the approximate remaining capacity of a cell as a percentage. So, the capacity of a cell, whether that printed on the side, or "actual", makes no difference.

For LiFePO4​/IFR cells the chart, of course, does not apply. This chemistry in addition to having a different voltage, also has a very flat discharge curve, making remaining capacity estimates more difficult.

I'll also point out that Li-Ion chemistry cells are one of the few types which a general idea of the remaining capacity can be estimated by measuring the OC voltage. Very little can be determined by measuring the OC voltage of NiCd and NiMH cells, for example.

Dave

Great. So I guess while I am waiting another two weeks for either the current PD32 or the new one being released, I should discharge my 38650 cell to 3.80 volts. That's put a light bulb load on the battery, pull the bulb off every once in a while and check to see if the battery is at 3.80? I know if I check the voltage while the 5.2V, 0.85A HPR50 bulb is hooked up, the voltage reads lower. Thanks.

I think precision si not necessary in this case. The point is to avoid full charge and full discharge. Few % of charge really doesn't matter.
I store them with "storage" function on my icharger - it discharges or recharges cell to 3.75V

I think discharging the cell for 2 weeks storage is not necessary - storage is meant at least ~2 months and longer.

EDIT: I just remembered I read somewhere that liions should be stored at low charge at lowest safe voltage possible (above 3.0V but not so much - like 3.5V when they have only few % of charge). Lower voltage=lower electrodes decomposition.

And those 40-60% recommendation is just ideal trade off between cell's degradation and enough energy for covering loss due self-discharge (+ electronic's parasitic drain when they are in battery packs or installed in devices - laptops, cell phones with non-removable cells etc..)

Interesting info, thanks for all of the responses. Even though it seems to make no difference, I actually prefer to charge to 100% and then discharge to 40%. The reason being is that by knowing what the voltage is at 100% and the discharge curve from 100% to 40%, I can extrapolate the overall health and capacity of the battery that might not be as accurate as would be going from 0% to 40%. But it is good to know that either way is okay.