NiMH cells go dead on shelf too quickly

[richfloe]

I have quite a few hand-held two-way radios, of course I am using rechargeable batteries. most have been upgraded to NiMH from NiCad to get improved capacity. What I seem to be finding is that some of these super high capacity NiMH cells (2000mAH AA size) seem to self-discharge rather quickly just sitting on the shelf while the NiCads are still fine. After about a month on the shelf, the NiMH are lucky to last half as long as they normally would while the NiCads still have practically full power in them (though of course due the lower capacity they wont run as long as the NiMH but it's getting close).<p>Has anyone else noticed that the super high capacity NiMH cells will self-discharge quickly?
Comments? Ideas?<p>Rich

[k7elp60]

I am not sure what the shelf life of the NiMH is now, but a little over 5 years ago the published shelf life was about 10 days. Over five years ago I did a lot of research on rechargeable batteries and wrote a series of articles on them.
I recently had some 1800 MA NiMH, AA batteries for my digital camera. I felt lucky as they seemed to stay pretty well charged for about a month. The shelf life my well depend on who manufactured the cells. Even thou the chemistry my be the same the manufacturing process my affect the self discharge rate.

[richfloe]

K7, thanks for the response.
I had some literature from panasonic that told all about the batteries, primarily the various charging methods etc but I don't recall them giving me the bad news that they were so "leaky". Most the cells I have are not panasonic (though some are) but I would assume them to be similar. The panasonics that I have are being used constantly in the company 2-ways, rarely would one sit on the shelf for as long as a week before someone takes it.<p>The ones that are going dead are my personal batteries for my work and ham radios. I have been rotating thru the two pools (different types) on the shelf and they are likely to sit for a month before I use them again, this is where I'm running into trouble. Guess I'll just have to live with it.<p>Rich nØvl

[Chris Smith]

I have several cell phone batteries [NiMH] that I have modified for use with LEDS and a flash light.<p> The charge in these batteries last all year, and I only recharge them by solar twice a year, or more, if I use them heavily.

[VIRAND]

Take advantage of the "no memory" characteristic and leave them trickle charging on the shelf until you need them. I mean like 20ma up to 1.4 Volts.

[peter-f]

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by k7elp60:
...
The shelf life my well depend on who manufactured the cells. Even thou the chemistry my be the same the manufacturing process my affect the self discharge rate.<hr></blockquote><p>I find the same (mostly with alkalines)...
The chemistry is pretty common.
The "stack" of chemicals is actually a film rolled around the central axis, and the quality of the film determines a lot of performance characteristics.

[Dean Huster]

Besides the lack or reduction of the "memory" problem, I thought that I'd originally read several years ago that part of the hype for NiMH cells was that they self-discharged at a much lower rate than NiCd cells. Rich, I'd say try switching the cells for one of the radios to a different brand and compare.<p>And I think that we need to watch our term "shelf life", since with rechargeables, that can be confusing. Shelf life for primary cells is how long they can retain their charge, unaltered manufactured chemistry and maintain their package integrity, for once they're dead, they're dead.<p>Secondary cells, on the other hand, will have two factors: how long they'll stay charged under no load conditions and how long they'll last (MTBF if you will, although that's usually under operating conditions) before they'll no longer hold a charge, short or generally fail in their function to be a battery. Shelf life is the term we lay on electrolytic capacitors in the latter description since they don't remain as pretty as manufactured just laying around in the shipping container for a few years.<p>So, regarding "shelf life" in the sense of long term storage of a rechargeable battery (at least for NiCd cells), I've read somewhere that they should be stored in a discharged state. Are any of you knowledgeable on that issue?<p>And in a similar vein, will a cell maintain more total energy with a "top off" charge in a frequently-charged situation or on a full charge from a nearly-dead state?<p>My experience with rechargeable batteries of any kind is that for my infrequent use, such as for handheld transceivers, I prefer the reliability, extra voltage, power and longetivity of alkalines.<p>Dean

[John Brown]

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr> Take advantage of the "no memory" characteristic and leave them trickle charging on the shelf until you need them. I mean like 20ma up to 1.4 Volts. <hr></blockquote>
From what I have read, it is not a good idea to leave NiMH permanently on trickle charge. I think NiCd may be more tolerant of this.

[Robert Reed]

Back in the 70s, Eveready had a free book just for the asking. It was sort of a "data book", similar to semiconductor books only for battery info. It had everything you would want to know about any of their products (and they manufactured most of the products at that time) This was a great reference book for these type of questions and more. Unfotunately mine dissapeared about 10 years ago, which wasn't a great tradgedy as half the produts were obsolete by that time. But I sure would like an up to date one now. Does anyone know if these books are available now or a good website with this tyoe of info ?

[Chris Smith]

Robert<p>They moved to India [http://www.evereadyindustries.com/] and they opened up a tea shop, as well as they sell batteries and Flashlights? <p>Who could could have guessed?

[Robert Reed]

I swear--the whole world is going to hell!

[Chris Smith]

I know one country that is, thanks to the politicians. All for a buck.

[awright]

The comment I remember is that NiMH cells characteristically have fairly high self-discharge rates - the price you pay for higher energy density than Nicads - and that NiMH cells are VERY intolerant of overcharging and can be damaged or destroyed by charging above a certain voltage related to the chemistry (the value of which I do not remember). Trickle charging is definitely contraindicated for NiMH cells.<p>Cell voltage should be monitored and charging discontinued when the critical voltage is reached. This is why chargers specifically designed for NiMH charging are desirable. I think some of the cheaper chargers may continue trickle charging after charge termination. <p>As I recall, IC makers like Maxim have good tutorials on charging of the various cell types and sell chips specifically designed for optimum charging control.<p>awright

[Rodney]

Being heavily involved in flying RC aircraft where "in use failure" is very expensive, we have found that NiMh self discharges about twice as fast as NiCads do and also have only about half the life time in number of charge/discharge cycles. Also, NiMhs are much more apt to be damaged by trickle charging than NiCads, in fact some manufacturers go to special effort to warn against trickle charging NiMh's. NiMh is also more easily damaged by overcharge.

[richfloe]

I appreciate the comments, glad to know I'm not the only one who notices this.<p>I have noticed that some of my older NiMH cells that are rated 1200mAH don't self-discharge as quickly as the 2000's, must be something about how thin the chemical sandwich is or something.<p>Panasonic says not to trickle charge NiMH, I think it has something to do with how it handles recombining the gasses generated when the cell is overcharged or something. Given the high rate of self-discharge I would be tempted to think that it would be acceptable to apply a continous slow (really slow...) trickle charge, just fast enough to keep up with the self-discharge. This is probably pointless as I dont want to have to build a multi-pack trickle charger for 20 packs, it would be easier to just get the pack out the night before I need it and put it on the charger.<p>I understood that storing a NiCad cell in a discharged condition for an extended period of time would cause dendrites (whiskers) to form between the electrodes and short the cell. I've had this happen quite a few times and have either applied a brief high-current charge (several amps for a few seconds) or discharged a large cap into the cell to blow the short. I've only had limited luck in getting these to work after this has happened. I was told that leaving NiMH in a dischaged condition causes something similar to sulphation in lead-acid batteries which increases the internal resistance of the cell.<p>Onto the subject of memory, according to the panasonic lit sheet, you can get NiMH to succumb to the dreaded memory effect just like NiCad. They say all you need to do is discharge the cell to below 0.9 or 1.0v and recharge, repeat once or twice and you're good to go. If you want "no memory", try lead acid.<p>Off in another direction, I also use "gel-cells" for a couple of products at work (solar power) and have found these can sit in the dark for 6-12 months and still be nearly fully charged. Self-discharge of these will increase with higher temperatures, I wonder if that also applies to NiMH?<p> Rich