Workplace Solutions Best Books of the Month Amazon Fashion nav_sap_plcc_ascpsc  Kansas Fire TV Stick Halloween Amazon Cash Back Offer Transparent Transparent Transparent  All-New Echo Dot Introducing new colors All-New Kindle for Kids Edition AutoRip in CDs & Vinyl Shop Now 20% off select baby clothing, gear, and more

Sort: Oldest first | Newest first
Showing 1-24 of 24 posts in this discussion
Initial post: Jul 9, 2008 1:01:49 PM PDT
Last edited by the author on Jul 9, 2008 3:50:54 PM PDT
Enthusiast says:
Although it has often been repeated, by a variety of sources, that slow charging is 'gentler' on the batteries, and better for them, this may actually turn out to be something of a rumor. Slow charging can lead to undesirable crystal formations inside the batteries.

One-hour charging may in reality be better for the batteries than slow charging:

http://www.buchmann.ca/Chap4-page5.asp

In reply to an earlier post on Jul 9, 2008 4:26:15 PM PDT
[Deleted by Amazon on Sep 15, 2008 9:00:03 AM PDT]

In reply to an earlier post on Jul 9, 2008 4:33:28 PM PDT
[Deleted by Amazon on Sep 15, 2008 9:00:03 AM PDT]

In reply to an earlier post on Jul 9, 2008 6:17:48 PM PDT
Last edited by the author on Jul 9, 2008 6:25:29 PM PDT
R. Beebe,
First of all, you should not accept everything said in that website without questions. Notice tht the bottom of the web page says "copyright 2001'. As an example of out-dated information, in the FAQ it says the capacity of a NiMH cell is only 30-40% higher than that of a NiCd cell. This may be the case 10 years ago, but presently the capacity of a typical NiMH AA cell is about three times that of a NiCd AA cell (2700mAh vs. 900mAh).

Secondly, all of the smart chargers I have tested (La Crosse BC-900, Maha C9000, Duracell CEF23, Kodak K6200, Sanyo MQN05, etc.) use pulsed DC current to charge batteries. Take the BC-900 for example, when you set the charging current as 500mA, the charger actually puts out a 0-2000mA pulse at 25% duty cycle. This is better than charging at a constant 500mA, because it makes detection of negative dV/dt much more reliable. Another possible benefit is that the crystalization process will be spread uniformly throughout the cell, so it will not suffer from reduced capacity.

The BC-900 actually has four end-of-charge detection mechanisms:
- Negative dV/dt
- maximum terminal voltage
- temperature rise
- time-out after maximum charge of 3700mAh

When charging AA cells in my BC-900, I typically set the current to either 500mA (equivalent to 0.2-0.25C) or 700mA (about 0.3C) . After nearly three years of usage involving dozens of batteries, I have yet to encountered any end-of-charge termination problem. Neither is there any evident that charging at this gentle rate is bad for the cycle life of my batteries.

Here is my line of logic:
- charging at 0.1C with a dumb charge is bad because there is no end-of-charge detection,
- charging at 4C with a '15-minute' charger will send your batteries to an early grave,
- charging at 1C with an 'one-hour'' charger is not as stressful, but still not good in the long run,
- therefore the best charging rate is between 0.2C (5-hour) and 0.5C (2-hour).

In reply to an earlier post on Jul 9, 2008 6:31:17 PM PDT
Kevin M. says:
I've read that dT/dt is the best end-of-charge detection method. NiMH batteries don't have a marked -dv/dt, but the oxygen uptake reactions at the beginning of overcharge always generate heat (0.5C/s marks end-of-charge). I'd imagine dT/dt wouldn't work for very low currents, but those currents would probably be at trickle charge levels anyway.

I see from the pictures the BC-900 has temp sensors, but only two of them, so it probably doesn't do dT/dt on an individual battery basis. Still, you'd think those sensors would cut things off before the meltdowns that people are reporting. Is it a firmware bug? Does anyone know if the meltdown problems have only occurred on version 3.2 and earlier? Has anyone with version 3.3 experienced a meltdown?

In reply to an earlier post on Jul 9, 2008 7:33:07 PM PDT
Last edited by the author on Jul 9, 2008 9:18:12 PM PDT
Kevin,
dT/dt detection probably works best if the temperature sensor is integrated into the battery pack, as in the case of notebook battery packs. For a stand-alone AA charger such as the BC-900, it is impossible to ensure good thermal contact between the battery and the sensor. Also ambient temperature and air flow will affect the accuracy. Therefore the thermal sensors in BC-900 is used only to prevent thermal melt-down.

BTW, some people would have you believe the BC-900 is a fire hazard, but so far I have not heard any cases of the charger actaully catching fire. On the other hand, reports of meltdown may or may not be true. All I can say for sure is that I have two BC-900 (one v3.2 and one v3.3), and never experienced end-of-charge termination problem.

In reply to an earlier post on Jul 10, 2008 2:19:31 AM PDT
Kevin M. says:
I agree. Too hot to handle and melting plastic is pretty scary, but it wouldn't be a fire hazard till well above 300 (as in instant 3rd degree burns).

I'll probably buy one this week, provided I can get it from Amazon (not one of their sellers). If it overheats, I'll consider it an opportunity for some sleuthing.

In reply to an earlier post on Jul 10, 2008 11:10:09 AM PDT
Last edited by the author on Jul 10, 2008 3:45:04 PM PDT
Enthusiast says:
NLee,

First, thank you for your contributions and reviews.

In response to the posting above, there are several points:

--Isidor Buchmann's book:

Much of it is still quite relevant. I was not and am not taking his information, nor anyone else's (here or elsewhere), as the final word. However, he is a very highly informed and experienced professional, and an unusually good source of information; and much of it is valuable. If you consider his background and contributions in this field, and look at his company (Cadex), and at his other writings, it is clear that he is an expert source. He has also written many different articles in this field, including technical and professional papers. He has designed sophisticated and innovative chargers and battery analyzers. His experience and expertise in this field probably exceed those of all the participants here put together, many times over.

His designs, and his company's line of products, include high-quality professional-level devices that are used by major corporations with a need for scientific testing and evaluation of large numbers of batteries and battery packs. They go way beyond the BC-900.

There may be some points in his online book that are somewhat dated; but there are many others that are not. Many of the basic principles are as sound today as they were seven years ago, and apply quite well to the points at hand.

--Fast charging:

I see no real (non-hearsay, scientifically based) evidence -- at this point -- that slow chargers are 'gentler' (a better measure would be the effects they have on battery cycle-life or cycle-longevity: 'gentler' doesn't matter much if 'rougher' results in better, equal, or comparable battery life).

There is certainly some credible evidence that some one-hour chargers are just fine.

Differences among charger units and models may also be worth considering. Some of the one-hour chargers actually keep batteries substantially cooler than most of the two-to-five-hour chargers, and do not seem to have real-world negative effects on battery life.

In reply to an earlier post on Jul 10, 2008 12:03:57 PM PDT
Last edited by the author on Jul 10, 2008 3:10:49 PM PDT
Enthusiast says:
You mention four end-of-charge detection mechanisms. If I may ask, what are your sources of information here? I was told by La Crosse that the BC-900 uses negative delta V [or NDV, or negative change ('delta') in voltage (V): or to put it more simply, drop in voltage, or peak detection -- which can be tricky under certain conditions].

This is not necessarily the same as measuring changes in the *rate* of voltage change.

The BC-900s do seem to have temperature sensors. Whether these chargers use temperature change, rate of temperature change, or maximum temperature directly to terminate (or step down) the charging is not yet entirely clear. However, it does seem that they do not always do it well or reliably.

Why, if these chargers have so many systems in place, do they fail repeatedly, and in real-world situations?

You may not have experienced these problems; but others have. Problems seem to be occurring in a significant number of cases that have been reported here; and it is not unlikely that there are many other unreported cases.

In reply to an earlier post on Jul 10, 2008 12:32:46 PM PDT
Last edited by the author on Jul 10, 2008 5:51:18 PM PDT
Enthusiast says:
"- charging at 1C with an 'one-hour'' charger is not as stressful, but still not good in the long run,
- therefore the best charging rate is between 0.2C (5-hour) and 0.5C (2-hour). "

Why is it not good in the long run? Do you have some sources or references for this claim?

Would it not depend to some extent on the charger being used?

Empirical testing would be interesting here. (Buchmann is probably quite familiar with empirical studies, and could give references to the relevant professional literature.)

The final conclusion does not follow: 0.2C to 0.5C is not well supported by any good empirical evidence or logic that has been shown.

In reply to an earlier post on Jul 10, 2008 3:48:28 PM PDT
Last edited by the author on Jul 10, 2008 4:40:20 PM PDT
Enthusiast says:
DARKSKY,

If you read all that he says about slow charging in the online book (or in some of his other writings), it isn't quite as you have put it.

If I may ask, How do you know that slow charging is better (or 'gentler,' in some relevant sense of that word) for the batteries? What is the evidence? Do you know of any good, solid empirical tests or studies?

In reply to an earlier post on Jul 10, 2008 4:46:35 PM PDT
Last edited by the author on Jul 10, 2008 4:47:10 PM PDT
Enthusiast says:
Buchmann clearly states in his book that it is best to fast charge NiMH batteries.

In reply to an earlier post on Jul 10, 2008 7:45:19 PM PDT
Last edited by the author on Jul 10, 2008 9:02:45 PM PDT
R. Beebe,
It is obvious that your entire argument (that fast charging is better than slow charging for NiMH batteries) is based on whatever you read on Buchmann's website. When visiting any commercial site, you should always ask "What is this guy trying to sell?" In this case, Buchmann's company (Cadex) is selling rapid chargers to the military. Do you really expect this person to tell you that slow charger is better for your batteries?

Now to answer your demand for "empirical testing" (which appearantly is not found on Buchmann's website), see this technical data page on NiMH battery life span provided by Duracell:
http://www.duracell.com/oem/rechargeable/Nickel/cycbatlif.asp

Note that according to this chart, repeated charging/discharging at higher temperature of 45 degree C (which is typical for most 1-hour or faster chargers) can reduce the cycle life of NiMH cell by 50%.

Of course you may argue that a cooling fan can help to reduce the temperature, but here we are talking about using the same charger (be it BC-900 or C9000) at different charging rate. So the bottom line is still: charging at lower rate is better for the life span of NiMH batteries.

In reply to an earlier post on Jul 10, 2008 9:08:26 PM PDT
[Deleted by Amazon on Sep 15, 2008 8:59:55 AM PDT]

In reply to an earlier post on Jul 11, 2008 6:20:08 AM PDT
Last edited by the author on Jul 23, 2008 3:22:19 AM PDT
OldAmazonian says:
What impresses me is the tremendous cycle life reported for the NiMH batteries used in Prius and Civic Hybrids. Charge and discharge currents in these applications must be enormous, yet the batteries seldom fail. From its use in systems that were obviously subject to a great deal of testing and analysis, it is reasonable to conclude that Mr Buchmann is right about the virtue of fast charging within reasonable temperature limits.

My two Duracell CEF90NC 30-minute chargers have fans built in, with temperature detectors on each cell, and all cells are comfortably warm but never hot after charging. After more than a year of frequent use, none of my AA or AAA cells (Duracell, Sanyo, Uniross, and Eneloop brands) have yet weakened or gone toes-up from charging this way. In contrast, a Sony "fast" 4-hour smart charger ruined a set of perfectly good NiMH AAs when at charge completion it overheated them to the point of blistering their plastic sleeves.

In reply to an earlier post on Jul 11, 2008 4:22:34 PM PDT
Last edited by the author on Jul 11, 2008 5:32:58 PM PDT
Enthusiast says:
NLee,
"It is obvious....": Obvious only to those who incorrectly and erroneously project that into what I say. I am also going by other evidence, including Sanyo's information on their batteries, information provided by other companies, and information from reliable and independent third-party sources.

Is he only selling rapid chargers?? And would it really be in the interests of his company to mislead his clients, whether they are individuals, corporations, militaries, or governments?

Would it be in his interests for them to find out that his approaches did not work as well [which would be revealed in testing -- particularly by the battery maintenance specialists who are employed by some of these organizations -- which is often a part of product evaluation at the levels with which he is often concerned)?

***
"...repeated charging/discharging at higher temperature of 45....": I have a one-hour charger that keeps the batteries cooler than most of the slower chargers, and cooler than the BC-900 in many cases.

There are other fast chargers that also keep the batteries relatively cool.

Cooling fans and alternative charging technologies may both contribute to this.

*Do you have any evidence that one-hour charging will reduce the cycle life at significantly lower temperatures? -- or that one-hour charging will reduce cycle life more than slower charging, when charging occurs at the same temperatures?*

Also, what are typical charging temperatures when using the BC-900? According to the Duracell chart, these temperatures (as I have heard them described, both in normal use and during periods of overheating) also shorten battery life.

Also, I suspect that there is some evidence that SLOWER charging at high temperatures reduces cycle life dramatically. (Buchmann and others are pretty clear about this. And the BC-900 does not keep batteries particularly cool -- in some instances, spectacularly not; and it does not to my knowledge have a fan.)

One-hour charging at lower temperatures may actually be *better* for the cycle life of the batteries than slower charging at higher temperatures. And there are one-hour chargers that keep the batteries cooler than the BC-900.

The Duracell chart is about temperature, not charging rate.

(At the bottom, they give recommended and permissible temperature ranges for different charging rates. The temperature ranges are quite similar for all three charging rates. There is no suggestion whatsoever in this material that fast charging, within these temperature ranges, is bad for the batteries.)

In reply to an earlier post on Jul 11, 2008 4:25:06 PM PDT
Last edited by the author on Jul 11, 2008 4:26:25 PM PDT
Enthusiast says:
"Of course you may argue that a cooling fan can help to reduce the temperature, but here we are talking about using the same charger (be it BC-900 or C9000) at different charging rate. So the bottom line is still: charging at lower rate is better for the life span of NiMH batteries. "

We are also talking about the BC-900 in comparison with other options.

The conclusion that charging at a lower rate is better for the life span of the batteries does not logically follow.

In reply to an earlier post on Jul 11, 2008 5:01:08 PM PDT
Last edited by the author on Jul 11, 2008 5:13:32 PM PDT
Enthusiast says:
"Battery Life
The same factors that affect cycle life affect overall battery life. Operation or storage at extreme temperatures, overcharging, cell venting and abusive use will reduce battery life. Operation and storage of batteries at or about room temperature (20C or 68F) will maximize battery life...."

This is from the Duracell site.

There is a clear misstatement: Battery life is maximized (and is significantly improved) at considerably lower temperatures.

In reply to an earlier post on Jul 11, 2008 10:12:16 PM PDT
Kevin M. says:
Maybe for storage. I've seen warnings on multiple sites that you should only trickle charge NiMH at low temperatures. Anything more will damage the battery.

"Nickel-based batteries should only be fast-charged between 10C to 30C (50F to 86F). Below 5C (41F), the ability to recombine oxygen and hydrogen is greatly reduced and the resulting pressure build up may cause the cells to vent."

In reply to an earlier post on Jul 15, 2008 1:32:09 PM PDT
Hollow Man says:
Maha's instruction sheet for the C9000 doesn't recommend charging under .33C and above 1C, and their FAQ on their web page recommends .5 to 1C. Why would they state this considering that the charger is capable of such variable rates? Unless you're going to tell me it's a ploy to fry your batteries so they can sell us more Imedions and PowerEX 2700s...

-HM

In reply to an earlier post on Jul 15, 2008 4:44:27 PM PDT
Kevin M. says:
One manufacturer recommends fast charge (1C), another recommends slow charge (0.1C).

I suspect the reason for the opposing recommendations is that there's little difference. All other things being equal, you might as well fast charge for the convenience of it (as long as it doesn't warm up your batteries too much).

In reply to an earlier post on Jul 16, 2008 3:39:36 PM PDT
Last edited by the author on Jul 23, 2008 3:25:58 AM PDT
OldAmazonian says:
One pertinent difference is the quality of the charger's ΔT/Δt detection. Because rise of rate of ΔT/Δt in nickel batteries is more distinct and easier to detect at higher charge rates, the most reliable charging system is likely to be a fast one that maintains a reasonably low battery temperature. Note that nickel batteries in hybrid vehicles use heat sinks and active cooling to allow charge and discharge at extremely high current.

In reply to an earlier post on Jul 18, 2008 7:58:08 AM PDT
Kevin M. says:
I got my BC-900 yesterday, and I can say that charging double-A's at 1000 mA (0.4-0.5C) the batteries get distinctly warm, even after only 10 minutes. They're not scalding or too hot to handle, but definitely warm (>100 F).

It's my understanding that even mildly warm temperatures like these will prematurely age batteries. Unless you have some special way of keeping your batteries cool (but not too cool) during charging, it's anyone's guess if fast charge is better or worse.

For myself, with NiMH only 4 times the cost of good alkalines, I'm not going to worry about cycle life too much. If I can get 20 decent cycles I'm well ahead.

In reply to an earlier post on Jul 18, 2008 1:10:35 PM PDT
Last edited by the author on Jul 23, 2008 3:27:31 AM PDT
OldAmazonian says:
Having charged several makes of NiMH AA cells a few hundred times at average rates of roughly 2C and 4C with my fan-cooled Duracell CEF90NC and CEF15NC chargers, with no apparent ill effect to any of them, I would guess fast charging isn't necessarily any more destructive than slow charging. It obviously depends on the charger. The only time I've seen a "smart" charger ruin batteries was when a Sony four-hour "quick" charger roasted its included set of four Sony NiMH AAs to the point of blistering their plastic sleeves.
‹ Previous 1 Next ›
[Add comment]
Add your own message to the discussion
To insert a product link use the format: [[ASIN:ASIN product-title]] (What's this?)
Prompts for sign-in
 


 

This discussion

Participants:  6
Total posts:  24
Initial post:  Jul 9, 2008
Latest post:  Jul 18, 2008