Appropriate Charge Rate for NiMH Batteries

[Marduke]

See Sanyo's FAQ page, they clearly recommend against it.

http://us.sanyo.com/Batteries/FAQs

(more on linked page)
Same is true of most battery manufacturers, they make several chargers with under 0.5C rate.

I suggest you read the technical specs and not the dumbed down FAQ written for illiterate consumers.

Sanyo recommends 1C charge...
http://us.sanyo.com/dynamic/product/Downloads/AA_4Pack-49579479.pdf
http://us.sanyo.com/dynamic/product/Downloads/AAA_4Pack-35338904.pdf

Energizer recommends 1C charge (page 19)....
http://data.energizer.com/PDFs/nickelmetalhydride_appman.pdf

Duracell recommends 1C charge...
http://www.duracell.com/oem/rechargeable/Nickel/methods.asp


Believe it or not, there is little correlation to what a company's chargers do (designed purely for cost/profit reasons), and what the best practices say to do (chosen purely for technical reasons). Until the day comes that the bean counters making the decisions are actually knowledgable about the products they sell (fat chance), there will always be a discrepency. However, we here has CPF as knowledgable consumers (usually) have the advantaged of knowing the proper methods, and knowing what equipment will give the best performance for the cells you choose.

Edit:
Panisonic also recommends 0.5-1C charge...
http://www.panasonic.com/industrial/battery/oem/images/pdf/Panasonic_NiMH_ChargeMethods.pdf

Rayovac also recommends 1C charge...
http://www.rayovac.com/technical/pdfs/NM715_2100MAH.pdf
http://www.rayovac.com/technical/pdfs/NM724.pdf

 

[J_C]

There's nothing profitable about their batteries not meeting their published specs. They do not stipulate you must use any one of the methods they recommend to achieve the specifications for their cells.

There are certainly things that shouldn't be done, but anything except 0.5C to 1.0C isn't one of them.

Again I will remind that there is a slight reduction in lifespan from some charge rates and overcharging, but they are $2 batteries not your long lost child or bars of gold. The more important factor is convenience which can lead to rapid charging in some scenarios or not in others. The majority of the planet does not rapid charge and proves the successful use of lower charge rates despite people who obsess about tiny differences and most of them with unscientific tests posed as "fact".

Take for example the crazy supposed facts about battery temperature rise opposed to voltage rise for Delta -V termination. The supposed facts ignore the constants.

If you take one battery 5% away from it's Delta -V threshold and put it into a charger, and compare it to another battery which had been fully drained and recharged up to 5% away from Delta -V, the latter battery is hotter than the former. Delta -V is not temperature dependent, it just happens that a change, a variable one, facilitates it which is a far different situation than predicting end of charge from an absolute temperature or rise over ambient temp.

So it goes with most studies. Only when you take all the information do you find the glaring holes in the research.

What all these battery companies do is guarantee their product when you use their chargers which operate below 0.5C charge rate. I don't make that guarantee, they do. It is not important to try and eek out the last % capacity or life from a battery, we got along fine with less performance for many years and in the years coming will have even more.

Once again, the majority of the world has no problem carrying out their lifes, using myriad types of devices charging at a rate you don't like. Despite it being less than a hypothetically ideal rate, it is fine to use and the world stands constantly ready to prove that every moment.

There is no better proof than almost everything and almost everyone (manages to use their devices and batteries fine without a care in the world about some narrow charge rate).

 

[Marduke]

"That's how everyone is doing it" is not a valid reason for how something is supposed to be done. Lots of people do lots of things incorrectly, that doesn't make it right.

The point of the discussion here of what charge currents to use is not about what lots of people do (incorrectly, even by manufacturer standards), but what is BEST.

And as silverfox described, much of the world using slow charging DO have issues, they just don't know it. They think short life cycles and worsening performance are the norm and don't have the knowledge that the grass really is greener on the other side.

Lots of people drink and drive and get away with it, speed, swim less than 30 minutes after eating, etc. But that doesn't mean any of those are good ideas either.

There is no doubt, given the choice between the two, fast (1C) is better than slow, as proven by documentation above.

Where is your proof of the opposite?

 

[Marduke]

As for you temperature dependance question, feel free to view the Energizer document in great detail.

Also, try artificially cooling a cell during charging at 1C and monitoring the temp and voltage. Whoops, the -dV signal just disappeared. So much for that theory....

 

[sygyzy]

Great advice. I wrote it all down.

What's the appropriate DISCHARGE rate?

 

[travelinman]

You really do like to don't you

 

[clintb]

You really do like to don't you

I don't see Marduke as provoking in the least. On the contrary, he dug up plenty of factual information, and presented a distilled version of what the folks (mostly SilverFox) have discovered in actual tests. If you have some good test numbers to backup the one, consumer focused, document you've posted, then please, by all means, give more info.

 

[wapkil]

Hello J C,

I beg to differ...

There is enough evidence to support charging in the 0.5 - 1.0C range that the battery manufacturers actually recommend it.

Can you (or someone else) direct me to the places or documents describing why fast charging is better than standard charging? I have read it many times that fast charging is recommended but I couldn't find any clear explanation why.

It is obvious that if the charging current is too low for the charger to terminate correctly and it overcharges the cell it will be bad for the cell life. I think it may be the most important reason for fast charging current recommendations.

If we assume though that the charging will be terminated correctly, I don't know why 0.1C can be worse than, say, 0.75C. As I understand, every charging is shorting the life of the battery. If fast charging is better it would mean that these negative effects are lower with higher currents. It may be true but it's counterintuitive for me - I would think that it's usually better to perform similar chemical reactions slower, not faster.

 

[Mr Happy]

If we assume though that the charging will be terminated correctly, I don't know why 0.1C can be worse than, say, 0.75C. As I understand, every charging is shorting the life of the battery. If fast charging is better it would mean that these negative effects are lower with higher currents. It may be true but it's counterintuitive for me - I would think that it's usually better to perform similar chemical reactions slower, not faster.

It's a bit of speculation, but here's a hypothesis for you. It is possible that higher charge currents reach more of the chemicals in the battery.

It is commonly observed that flowing things take the path of least resistance. For instance, if you have a trickle of water flowing down a river bed the water will follow a path down the middle and leave much of the channel dry. The reeds at the edge will not get watered. On the other hand if you have a large flow of water the whole river bed well be flooded right up to the edges.

It could be the same with charging an electrochemical cell. A full charge on the battery requires all of the electrode material to be reached by the charging current, but there is no guarantee that all of the paths through the cell have the same resistance. Small currents may follow the path of least resistance and overcharge some parts of the electrodes while leaving other parts undercharged. While larger charging currents may do a better job of flooding the cell and reaching every part of the electrodes.

As I said this is speculation and may be wrong, but it is something to think about.

 

[TakeTheActive]

...It is obvious that if the charging current is too low for the charger to terminate correctly and it overcharges the cell it will be bad for the cell life. I think it may be the most important reason for fast charging current recommendations...

We also have the relationship between Charge Current and Crystal Size:

• Low Charge Current encourages Large Crystals
• High Charge Current encourages Small Crystals

Add to that, Self Discharge also encourages Large Crystals and we start heading towards Separator Damage. :thinking:

 

[TakeTheActive]

It's a bit of speculation, but here's a hypothesis for you. It is possible that higher charge currents reach more of the chemicals in the battery...

I was thinking just the opposite, as evidenced by a recent experiment by Marc999 in the thread Mh-C9000: R&A vs. Cycle dealing with some Storage Box Queens:

Powerex 2700mAh AA [dated 07-08] |   #1    #2    #3    #4
---------------------------------+---------------------------
08/25/09 C9000 Break-In          | 2445  2415  2342  2204 mAh
08/21/09 C9000 Cycle5:  2000/ 100|    ?     ?     ?     ? mAh
08/xx/09 C9000 Cycle4:  2000/ 100| 2109  1917  2080  2281 mAh
08/xx/09 C9000 Cycle3:  2000/ 100| 2125  1960  2061  2298 mAh
08/xx/09 C9000 Cycle2:  2000/ 100| 2160  2022  2103  2316 mAh
08/xx/09 C9000 Cycle1:  2000/ 100| 2239  2168  2153  2351 mAh
08/xx/09 C9000 Break-In          |    ?     ?     ?     ? mAh

AFAICT, it appears that the slow Break-In Charge Current (08/25/09) allowed more of the chemicals (including the areas released by the newly-dissolved crystals) to accept a charge. :thinking:

 

[TakeTheActive]

...What's the appropriate DISCHARGE rate?

Appropriate for what?

I use:

• 0.2C for comparing Capacity
• 1.0C for evaluating vibrancy
• 100mA for dissolving crystals

 

[wapkil]

I was thinking just the opposite, as evidenced by a recent experiment by Marc999 in the thread Mh-C9000: R&A vs. Cycle dealing with some Storage Box Queens:

Powerex 2700mAh AA [dated 07-08] |   #1    #2    #3    #4
---------------------------------+---------------------------
08/25/09 C9000 Break-In          | 2445  2415  2342  2204 mAh
08/21/09 C9000 Cycle5:  2000/ 100|    ?     ?     ?     ? mAh
08/xx/09 C9000 Cycle4:  2000/ 100| 2109  1917  2080  2281 mAh
08/xx/09 C9000 Cycle3:  2000/ 100| 2125  1960  2061  2298 mAh
08/xx/09 C9000 Cycle2:  2000/ 100| 2160  2022  2103  2316 mAh
08/xx/09 C9000 Cycle1:  2000/ 100| 2239  2168  2153  2351 mAh
08/xx/09 C9000 Break-In          |    ?     ?     ?     ? mAh

AFAICT, it appears that the slow Break-In Charge Current (08/25/09) allowed more of the chemicals (including the areas released by the newly-dissolved crystals) to accept a charge. :thinking:

I'm not sure how the values were compared but if it's the comparison between the C9000 "cycle" charge and "break-in" charge results, I think it may not be easy to draw any conclusions from it. The C9000 undercharges good batteries with its strange 1.47V (IIRC) voltage termination but the break-in charge fully charges them.

Anyway, I also think that it's quite possible for the standard charge (0.1C) to provide higher charge levels than with the fast charging currents. I remember seeing the results confirming it but it's also what the data sheets suggest. The battery capacity is always given in accordance to the standard that uses 0.1C. If there was a way to get better charge with higher currents, the manufacturers would use a standard with this better method I've read that batteries charged with 0.1C may have problems giving higher currents but with moderate currents their charge capacity should be higher.

It's still not clear for me how the standard and fast charge methods compare in relation to the cells longevity, assuming that no errors are made (overcharge, overdischarge, keeping batteries unused etc.). I think the separator damage you wrote about in the previous post is an example of such an usage error (letting the battery self-discharge for an extremely prolonged period). These errors are important but I wanted to start from discussing how the standard charge and fast charge rates compare in the ideal conditions.

 

[travelinman]

OK, I waited until today hoping my buddy would be at "coffee" and sure enough, there he was. The coffee is a gathering of ham radio types on Friday mornings at 10am. We sit around for an hour or so and talk electronics, and politics.

This morning I raised the topic of "how best to charge Nimh cells" This one particular guy is the tech for the regional RCMP detachment, and is responsible (among other things) for the care and feeding of 250+ cells and packs of Nimhs used in a wide assortment of electrical devices daily.

There was no hesitation on his part, as he has made charging method comparisons over the last few years on several different types of packs, individual cells and manufacturers. All his individual cells are colour coded, and the packs are numbered for data gathering.

He now exclusively charges (16+ hours) at ~0.1C and does NOT depend on any of his chargers (even the so-called computer controlled "smart" chargers) to terminate properly at any voltage. He has found this method gives him the most capacity immediately after charging, the best longevity of cells, and the fewest malfunctions of cells.

He is 100% positive that this method has saved his department thousands of dollars per year on replacement batteries.

I don't know how many of you out there have his breadth of experience, or how much the sale of new cells has influenced the manufacturer's data being quoted here, but I'm inclined to base my future charging on his advice.

Of course, everyone in our society is encouraged to their own beliefs. :twothumbs

 

[Mr Happy]

He now exclusively charges (16+ hours) at ~0.1C and does NOT depend on any of his chargers (even the so-called computer controlled "smart" chargers) to terminate properly at any voltage. He has found this method gives him the most capacity immediately after charging, the best longevity of cells, and the fewest malfunctions of cells.

most capacity immediately after charging - Yes, certainly. A 16 hour 0.1C charge is the gold standard for achieving a 100% full charge.

the fewest malfunctions of cells - Quite believable. This charging method is very fail safe for cells of all ages and conditions and for both packs and single cells.

the best longevity of cells - On balance, maybe yes. It depends on many factors concerning the exact application of fast charging and the quality of the fast charger used. A fast charger only has to overheat your batteries a few times to undo the benefit that fast charging theoretically brings.

It's worth noting that industrial NiMH battery packs tend to use dT/dt charge termination rather than dV/dt since the batteries are so sensitive to temperature.

On balance, nice feedback.

 

[wapkil]

This morning I raised the topic of "how best to charge Nimh cells" This one particular guy is the tech for the regional RCMP detachment, and is responsible (among other things) for the care and feeding of 250+ cells and packs of Nimhs used in a wide assortment of electrical devices daily.

There was no hesitation on his part, as he has made charging method comparisons over the last few years on several different types of packs, individual cells and manufacturers. All his individual cells are colour coded, and the packs are numbered for data gathering.

He now exclusively charges (16+ hours) at ~0.1C and does NOT depend on any of his chargers (even the so-called computer controlled "smart" chargers) to terminate properly at any voltage. He has found this method gives him the most capacity immediately after charging, the best longevity of cells, and the fewest malfunctions of cells.

He is 100% positive that this method has saved his department thousands of dollars per year on replacement batteries.

I don't know how many of you out there have his breadth of experience, or how much the sale of new cells has influenced the manufacturer's data being quoted here, but I'm inclined to base my future charging on his advice.

Of course, everyone in our society is encouraged to their own beliefs. :twothumbs

I hope we don't have to rely on beliefs. Batteries may be somehow complicated but I'm sure there is enough demand for data and research about them to find the comparison of the standard and fast charge rates. Of course there may be no simple answer which one is better (it may depend on requirements and additional conditions) but some discussion and experimental data is probably available.

Has your buddy mentioned some research documents about the subject or was he working only by doing his own experiments? I don't doubt his results validity. In my previous posts I wrote that my gut feeling is that 0.1C charge rate should be better for longevity and moderate current applications. It would be even better though to see some hard, published data accompanied by a theory explaining it (even if it would require too much chemical knowledge for me to understand it).

BTW, I don't think it can be said that manufacturers recommend fast charge rates because they are "better". In most of the longer manufacturer's documents that I read this recommendation was prefixed by an explanation that it is because of the contemporary trend to charge batteries faster or because it reduces back to service time. I haven't seen anywhere an explanation that it is actually better for batteries...

 

[J_C]

"That's how everyone is doing it" is not a valid reason for how something is supposed to be done. Lots of people do lots of things incorrectly, that doesn't make it right.

The point of the discussion here of what charge currents to use is not about what lots of people do (incorrectly, even by manufacturer standards), but what is BEST.

Exactly, sort-of. It's not "incorrect" to not do what is absolutely best. It's not "supposed" to be done only one way, manufacturers don't claim it will "fry" or "cook" a battery to simply charge it at a lower rate. In most areas of life, every day we all do things which may not be hypothetically the "best" way possible, for various reasons and in the end we only care if the need was met, OUR need not your hypothetical about what we should want or need.

And as silverfox described, much of the world using slow charging DO have issues, they just don't know it. They think short life cycles and worsening performance are the norm and don't have the knowledge that the grass really is greener on the other side.

Short life cycles and worsening performance would be things that people do notice. Many people do not notice problems as suggested.

Further, worsening performance as a cell ages is typical with any charge method. Further, a common resolution for worsening performance is to leave a battery trickle charging once it had supposedly reached max capacity, resolving the pseudo-memory effect which is yet another myth many consumers repeat.

Lots of people drink and drive and get away with it, speed, swim less than 30 minutes after eating, etc. But that doesn't mean any of those are good ideas either.

Hardly an equivalent example. How about lots of people eat different foods, not the same hypothetically "best" meal they possibly could every time. Lots of people accelerate and decelerate their automobile in a way that is not the hypothetically "best" way to get maximum fuel economy. Lots of people cook food in a way that is not the hypothetically "best" way to get best energy efficiency and lowest power bill.

Why? Because the difference between best and the rest isn't enough to matter relative to other factors.

There is no doubt, given the choice between the two, fast (1C) is better than slow, as proven by documentation above.

I never wrote that fast wasn't better. I wrote that slow isn't a problem (to reasonable people who do not have significant problems every day). You are incorrect and have now tried to twist the topic away from that into claiming I am against fast charging when I am not. It is not a problem if a $2 battery doesn't reach the ultimate heights of performance when the difference is usually quite small. If the slower charging rates people use weren't sufficient they would change the rate or the device but they don't in most cases because it JUST WORKS.

Battery manufacturers are well aware that the majority slow charge their batteries, if it were the problem you suggest they would strongly warn against it, even put the clear directions on the product package. The last thing a battery manufacturer wants is people becoming disenchanted with their product and avoiding it in favor of the competitors' product.

 

[wapkil]

Battery manufacturers are well aware that the majority slow charge their batteries, if it were the problem you suggest they would strongly warn against it, even put the clear directions on the product package. The last thing a battery manufacturer wants is people becoming disenchanted with their product and avoiding it in favor of the competitors' product.

I'm not sure if it is correct. I think that for many years the recommended charge rate was the standard charge rate. In recent years fast chargers became more popular than they used to be. It seems possible to me than now the manufacturers are aware that fast charge rates are becoming more popular, hence their recommendations. Reading various manufacturers documents I had an impression that the recommendations for fast currents sounded more like "if they absolutely have to use fast charging, let's at least tell them to use an algorithm that shouldn't overcharge with 0.5C-1C".

I may be wrong of course. It is only my impression. Maybe there were some changes in the batteries technology that made fast charge rates better than the standard rate. I haven't seen though anything confirming it and the more I read, the more it seems that my impression may be correct.

EDIT: I meant that I'm not sure whether the majority still slow charges their batteries. I agree that the manufacturers would probably warn against the standard charging if it was bad, although I don't think they would warn against fast charging. It wouldn't look good and could scare off the people with "15min" chargers. BTW some manufacturers still put the text "standard charge rate XmA" on their batteries.

 

[45/70]

Of course, everyone in our society is encouraged to their own beliefs. :twothumbs

I'm glad you're giving us a break travelinman.

It doesn't really surprise me that your RCMP friend is using the 0.1C charge rate. Since I'm guessing most of the batteries they use are cells in packs, this is the best way to charge them. You run the risk of overcharging the cells in an unbalanced pack, when charging at a faster rate. The 0.1C rate also balances the cells as well, in a series pack. Most radio equipment and such that I'm aware of, that use nickel based chemistry batteries, use the 0.1C rate for these reasons.

Another thing I curious about, is if all the batteries and cells they're using are NiMH. A lot of the LEO and military equipment here in the States is still run on NiCD's. I remember reading an article here recently about the U.S. Navy trying out NiMH packs for their two way radios. The conclusion was that the NiCD's were less trouble, more tolerant to abuse, and lasted longer in the long run. Sorry, I don't have any links.......well, this one.

Dave

 

[VidPro]

good point, them radios and stuff are in series, and while you can fast charge them in series using hobby chargers and other things like those super expencive charge racks that goverments might use, it still cant get it perfect, because whats the poor charger supposed to read.
without a perfected series charge alogrythm, series V-drops are just bad guesses, because some are going up while others are finished.

https://www.candlepowerforums.com/threads/187526&highlight=torture
as far as slow charging i did this test, which could be concidered far from slow, for over 300 hours, only the first few hours are in a time lapse video.
the battery still doesnt act squat different from the others, if i could just remember which one it was because now i forgot how i marked it, and they all test out the same.

but this is NOT some digitally averaged 1000ma+ high pulse charge, it is stable same current.
http://home.comcast.net/~trivalvid/ENLTor00-20hrDx.avi