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Old Mar 15, 2007, 3:09 PM   #11
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Hello TwoStep,

There is a large debate over what charge level to store Li-Ion cells at. The RC people have cases where they have stored their Li-Ion Polymer cells at 100% charge for over a year (in the refrigerator) with virtually no loss in capacity.

I believe that a 40 - 50% charge level limits the oxidation potential within the cell. Since Li-Ion cells fail through oxidation from the inside, it seems reasonable to store them in a way to limit this.

My practice is to store cells at less than a full charge, but usually more than 40% charged. This allows some use if they are needed, while at the same time I am close to the idea storage conditions.

If you are interested in the best performance from your NiMh or NiCd cells, store them in a discharged condition and do a charge/discharge cycle on them every 30 days. Cells stored like this need no priming to get up to speed, and are ready to give full performance immediately after charging.

If this is too much "work" to keep track of, and you can handle a little less performance from your cells, stretch the charge/discharge cycle out to 3 months.

This is for cells stored for 3 - 30 days, with a stretch to 90 days.

If you are going for long term storage, discharge your cells and add back around 20% of a full charge. This is the way cells come from the factory, and it is usually good for around a year of storage. Cells stored like this will need to be "formed" or cycled to come up to full performance.

The worst way to store your cells is on trickle charge on the charger.

The next worst way is to store them fully charged. Fully charged cells stored for a month will take about 3 cycles to come up to full performance. The loss in performance is reversible, but why waste cycles reconditioning your cells when you can charge them up and have them ready to go with their full performance available simply by storing them discharged.

Oops, let me back up for a moment. The absolute worst way to store your cells is in a hot ambient condition. NiMh cells prefer temperatures from 35 F to 86 F. The ideal is 40 F. When temperatures get above 86 F, there can be difficulties with charging, cycle life, and the cells will have a higher rate of self discharge.

The best life for a rechargeable cell or battery is to be used every day. We carry extras "just in case," and feel that they should be at the highest state of charge to insure the longest runtime. Yet, there are times when we switch to these back up batteries and find that they don't perform as well as our "regular" cells do. The back up batteries would perform better, if they were stored at a less than full state of charge.

One way to do this is to charge your backup batteries up and use them in your camera for a few shots. Then, switch them out for your regular batteries.

I know this is a radical idea, but your NiMh batteries will love you for it.

Tom
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Old Mar 15, 2007, 3:22 PM   #12
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Hello Steve,

The low self discharge NiMh batteries are really proving themselves.

The Eneloop cells have been out for almost a year now, and I have not heard of any problems with them at all.

They seem to enjoy being fully charged and hold up very well to room temperature storage. I have noticed that their impedance increases during storage, but it immediately drops when you start using the cells. Perhaps this is a piece of the puzzle of how they accomplish their low self discharge rate.

These are excellent cells for back up. You can use your high capacity cells during your main shooting, knowing that if you run out, your low self discharge cells are ready to go and at a full charge state. If your not into taking a lot of shots, you can use them all the time as primary and as back up.

We are continuing to test these in a wide variety of applications. So far, so good. I am very impressed.

Tom
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Old Mar 15, 2007, 3:36 PM   #13
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Steve,

Thanks for the great NiMH info. It is starting to sound like I need to get a few of each (NiMH and Lithium Ion) and see which fits my usage habits.

Tom,

The storage information is very helpful. I have a tendency toput down my cameras (and other rechargables) for random periods of time. I may use a camera every day for a month and then for some reason just decide to put it down and won't even look at it for three months. (of course I think my new D50 will be in my hand as often as I can manage, I have missed the SLR school of photography).

When I am using my camcorder a lot I do tend to rotate batteries, so I am doing that part. I have seen the batteries appear to discharge quickly on occaision but it does smooth out after a few charge discharge cycles like you said they will.

I knew there was quite a bit of science behind how batteries work but I had no idea how much science is involved in using them!

Unfortunately 86F is considered a cold front down here, so would I be better off with L-Ion batteries?

TwoStep
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Old Mar 15, 2007, 6:23 PM   #14
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amazingthailand wrote:
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Violet sky wrote:
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Quote:
2. What charge level should you store a rechargeable?
About 40% for Lithium-ion or Lithium-polymer, irrelevant for NiMH.
If by "store" you mean leave it unused for months, that is. If you use them often don't bother.
I would tend to disagree with this answer. If you plan to store your batteries for a long time, then I would give them a full charge before mothballing them.

Probably doesn't matter too much for NiMH as they self discharge anyway, but LiIon's tend to hold onto their charge very well.
You can disagree Violet Sky want but the answer is correct. Li NiMHs are completely different so anything said about NiCd or Li does not apply. All but the latest NiMH self discharge in storage sostorage charge wasnot relevant. The latest NiMH do retain a charge and hence the Mfr charge to 100% prior to shipment, mostly to prove they hold a charge.

My practice with my Li rechargeables is to use the Li in the camera until the camera tells me it's about gone then drop in my other one, recharge to original fully and return it to my bag. This works out to be about a three cycle at my shooting rate.
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Old Mar 15, 2007, 6:32 PM   #15
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Li rechargeables have internal circuitry to shut themselves of when the charge level approaches 20%. If they drop below this level they self discharge which mean selfdestruct (smoke/fire). The Sony's in the news a few months back was the result of mis-calibrated self protection circuitry.
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Old Mar 16, 2007, 7:16 PM   #16
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SilverFoxCPF wrote:
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....The next worst way is to store them fully charged.* Fully charged cells stored for a month will take about 3 cycles to come up to full performance....*
That true for NiMh?.... Rats! I've always read that for Li-ion, but never read that for NiMh. Thought the self-discharge might have made it a non-issue for NiMh [excluding the Eneloops].

I know you guys from Candlepower Forums know your stuff.
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Old Mar 16, 2007, 10:27 PM   #17
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Hello Reppans,

That is correct.

Keep in mind that the low self discharge cells are different. At least the early testing on them seems to show that they are different. They are still toonew to know for sure. They seem to work well even when stored at full charge. Cells stored for 6 months perform just as well as cells recently charged.

Tom
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Old Mar 17, 2007, 10:55 AM   #18
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I have read that the low self-discharge cells are shipped from the manufacturer at 100% charged. If that's so it probably means that it OK for the consumer to store them at full charge.
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Old Mar 17, 2007, 12:52 PM   #19
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Hello Ac.smith,

Testing is underway, but it is too early for any definitive statements.

I have only been testing the Eneloop cells for about a year now, and so far I haven't run into any "issues." The only area where they are a little weak is when they are stored at elevated temperatures. At higher temperatures, they self discharge quickly, but still a little slower than normal NiMh cells.

Charge them up and store them until they are needed. This is a true break through in technology.

Tom
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