# What's the best way to store NIMH Cells, charged or discharged?



## BabyDoc (Apr 13, 2008)

After discharging a NIMH cell, is it better to recharge the cell immediately, knowing you are not going to use the cell for a few months, or is ok to leave it in a discharged state, until just before you are going to use it? I have several NIMH cells, some Eneloops and some Duracell 2650 mah cells. While I realize the Eneloops will hold most of their charge for the 3 months, wouldn't I realize a more fully charged cell in 3 months if waited then to recharge it? With the Duracell, it will probably require a recharge in 3 months since most of the capacity will have disapated. So why bother charging it up right away after using it? In other words, is it at all harmful to leave a discharged cell discharged. Is it better to store a NIMH cell in a charged state?


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## Mr Happy (Apr 13, 2008)

I did some research on this a short while ago, looking for information in whatever technical literature I could find from different manufacturers. What I read was that it makes no difference what state of charge a NiMH cell is stored in. It will keep just as well whether fully charged, partially charged or empty.

There was a particular reference that had a very clear statement about this. I'll post the link here when I find it.

Edit: Here are some references:

Section 3.9.2 of this reference (Quest) says that state of charge makes no difference to maximum storage time: http://www.hardingenergy.com/pdfs/NiMH.pdf

Section 3.2 of this reference (Panasonic) implies that self discharge in long term storage (over a year) might cause leakage: http://www.battery-service.de/daten/nimh.pdf It recommends charging once a year to guard against this. I presume low self-discharge cells don't suffer this problem to the same extent.


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## SilverFox (Apr 13, 2008)

Hello BabyDoc,

There is a difference between "consumer" use of batteries and "power" use. If your applications involve draining your batteries over several hours, you can store them anyway you want to. Just make sure they don't drop below an open circuit voltage of 1 volt.

If your applications involve pushing your cells to the limits and voltage retention under load is important, then you need to try to keep your cells "vibrant" during storage.

Keeping your cells vibrant requires more work, but you will be rewarded by much better performance from them.

To keep NiMh cells vibrant, you store them discharged and perform a charge/discharge cycle on them every 30 days. To discharge your cell, you apply a 1C load and stop when the voltage under load hits 1.0 volts. With a battery pack you stop when the voltage hits 1.0 volts per cell.

If your cells have a very high rate of self discharge, this won't work. You need to store them fully charged, and since they quickly loose their capacity, charge them every week to keep them above the 1.0 volt per cell, resting.

The problem with this is that you need to charge your cells before use, and they aren't ready to go, if you need them.

A modification to this is to discharge the cells, and battery packs, and then to just charge them a little. I use this method for my power tool battery packs. I put about 30% of their full charge back in, and that leaves enough in the pack so I can do a quick use or two of it without having to wait for the pack to charge up.

You can also do this with single cells. Discharge them down to 1 volt, then charge them for a short time. When I am charging at 1C, I go for 10 - 15 minutes. In this condition, you can actually get away with longer than 30 days between your periodic charge/discharge cycle, but to keep your cells vibrant, I would not go beyond 90 days.

I have only been using this storage method for about 4 years, but have been amazed at how well it works. I don't keep a lot of extra cells around, but I do have battery packs for my power tools. Like most people, I leave my drill unused for a few months, then need it for a job I am working on. The performance of my battery packs has held steady, using this storage procedure, and I have passed the information on to some contractor friends of mine who also have reported excellent results.

If you store your cells charged and ignore them for an extended period of time, you can sometimes bring them back by using the standard charge and doing several charge/discharge cycles on them. 

Tom


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## Anders (Apr 13, 2008)

Hello BabyDoc.

If you look at: section 4.1.3 Memory effect of Ni-Cd & Ni-MH cells
here 
you see that Schulze also recommend storing cells without charge.


Anders


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## BabyDoc (Apr 13, 2008)

Say you have 4 rechargable Duracells 2650 mah rechargables, but only have one flashlight that uses a single cell. Say that the usage is such that you wouldn't need a fresh cell but every 2 weeks. Is it better to cycle one cell over and over, or is it better to discharge one of the 4 cells at a time before recharging all 4. The advantage of the latter approach is you could recharge all 4 cells at one time. The disadvantage would be some spontaneous discharge of cells sitting waiting to be used. An advantage of charging and discharging only a single cell in use over and over would be less need to do a refresh as you might when using each in a set of mulitple cells less often. What do you think is best?


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## SilverFox (Apr 13, 2008)

Hello BabyDoc,

That's easy... get your self 3 more single cell lights...  

AND, use your lights more often... :devil:

Getting back to your question...

I would charge 2 cells up. This gives you a cell for your light and a spare. At the end of 2 weeks, pull your first cell out and store it discharged. Put the second cell in and charge the third cell for your spare. In another 2 weeks, repeat the process and go on from there.

Tom


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## UnknownVT (Apr 13, 2008)

BabyDoc said:


> Say you have 4 rechargable Duracells 2650 mah rechargables, but only have one flashlight that uses a single cell. Say that the usage is such that you wouldn't need a fresh cell but every 2 weeks. Is it better to cycle one cell over and over, or is it better to discharge one of the 4 cells at a time before recharging all 4.


 
I am not a battery expert, and I would follow the advice by SilverFox and Mr Happy.

Both of them have said for normal consumer usage - NiMH can be stored charged or uncharged - that being the case I would mark each battery -and out of your 2 options - I prefer to use them one at a time in turn, BUT charge a pair at a time, rather than waiting to charge all 4 - this way one always has a spare charged battery for backup.

Waiting to charge all 4 may mean by the time the 4th is used would be about 6-8 weeks which might be marginal for self-discharge range for normal NiMH (although Duracell 2650mAh has a good rep).

However, I'd probably utilize just one pair - 
use one then the other, and charge the depleted one -
and find some other use for the other pair.


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## BabyDoc (Apr 13, 2008)

Thanks for all your suggestions. I guess I'll have to stick around here longer and buy more lights in order to use my cells. I don't think that is going to be very hard.


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