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Thread: A look at slow charging

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    Default A look at slow charging

    “What is a good rate to charge my NiMh cells?” and “Is this charger good?” are frequent questions that come up around here. Slow charging has been around forever, and it is used in determining cycle life. However, there is a provision in the testing standard for “Accelerated Test Procedures,” when testing for cycle life. The accelerated procedure involves charging and discharging at 1C. The goal is to achieve more than 500 cycles. The accelerated test procedures have a footnote that states that 1C charging should be done for 1 hour, or with appropriate charge termination, as recommended by the manufacturer.

    The next step is to review what the battery manufacturers have to say about “appropriate charge termination.”

    All of the battery manufacturers recommend charging at either 0.1C for 16 hours, or in the range of 0.5 – 1.0C with proper charge termination. They list peak voltage, change in temperature with respect to time, and total temperature rise as the preferred termination signals, but also recognize that –dV can also be used.

    NiCd charging started with simple timed chargers. From there, they realized that NiCd chemistry was fairly robust, and most of the time cells were being charged from more, or less, empty. They then advanced to 3 or 5 hour charging. Realizing that there was a voltage drop as the cell reached full charge, the fast chargers were ushered in. Now we had 0.5 to 2 hour chargers available. The chip manufacturers jumped in and gave us an abundance of charging chips that allowed us to select charging rates and utilized –dV to terminate the charge.

    When NiMh cells became available, the charger manufacturers realized that they could use the same NiCd chargers to charge NiMh cells. However, further study indicated that the overcharging involved using the –dV values for NiCd cells when charging NiMh cells resulted in around a 20% loss of cycle life. This was a direct result of overcharging and heat.

    The charger manufacturers realized that –dV termination was easy to implement and fairly reliable, so they approached the chip manufacturers about modifying their existing chips to accommodate NiMh charging. The result was increasing the “dead time” where the charger would not accept a fluctuation in voltage as a end of charge termination signal, and reducing the value of the –dV from 10 – 15 mV to 3 – 5 mV. They also come up with charging in pairs, thinking that the combined voltage drop from two cells would be easier to detect.

    With a few exceptions, most of the quick or smart NiMh chargers available today utilize –dV charge termination. If you are checking out a charger, you should probably assume that it utilizes –dV termination, unless it states something different.

    When picking out a charger for NiMh cells, the first thing you need to know is how does it determine charge termination. Once you know that, you can then check to see if the charging rate of the charger is suitable to produce a strong end of charge signal. In the case of chargers that utilize –dV termination, the suitable charge rate is in the range of 0.5 – 1.0C.

    The next thing to look at is the trickle charge rate at the end of the charge. If this rate is too high, and you leave your cells on the charger, you will cook your cells and greatly reduce your cycle life. The optimum is to have the charger trickle for awhile, then shut off. However, some people become over concerned with the self discharge rate of NiMh cells and want a charger that keeps the cell at a full state of charge through trickle charging. In this case, you want a very low trickle charge rate. You can refer to the various battery manufacturers for their definition of what a suitable trickle charge rate is. I don’t recommend leaving cells on the charger trickle charging, but if that is what you think you need to do, I believe the newer Maha chargers trickle charge at a low enough rate that the cells may actually self discharge a little while still on trickle charge. That seems like a suitable rate to me…

    So much for the introduction, now let’s take a look at charging rates.

    The Schulze charger has a long term formatting setting that they label 0.1C charging. There is no time limit or charge termination when using this setting. The charger continues to pulse charge until you manually stop it. The duty cycle is 25 seconds at 400 mA followed by 75 seconds at 0 mA. I use this setting for forming cells and battery packs, and balancing battery packs.

    I have noticed, from time to time, that there is a voltage drop during this slow charge. It doesn’t always show up, but with healthy cells and a balanced pack, it usually does.

    Let’s take a look at one of my 4400 mAh NiMh B90 prototype packs with less than 100 charge/discharge cycles on it. This pack was charged and balanced, then partially discharged for this study. Peak voltage was reached at around 110 minutes into the charge. It took 6300 seconds (105 minutes) to develop a –dV of 2 mV per cell.

    Here is the graph:



    You may be curious why I listed the time to –dV drop in seconds. Checking out the specifications on the charging chips available from the various chip manufacturers, I have observed that when they are looking for a –dV value, they use a time ranging from 100 – 300 seconds. If I was using one of these chips hoping to terminate this charge, I would miss the signal because of the long time (6300 seconds) it took to develop the signal.

    The result is a missed termination.

    Fortunately, at the 0.1C charge rate, there is little consequence with a missed termination. However, if you increase the charge rate, you can run into problems.

    A Swedish study looked at the affects of a 30% overcharge. It found that charging at 0.3C with a 30% overcharge yielded around 225 charge/discharge cycles before the cell capacity dropped to below 80% of its initial capacity. Charging at 1.0C with proper termination (actually they were using a –dV of 10 mV which I think is high. They would have obtained better results using a –dV of 2 mV, but this study was done a few years ago) yielded around 500 cycles before the capacity dropped to below 80%. The study goes on to illustrate that overcharging at 1.0C is more detrimental than overcharging at 0.3C, but the point is well made that overcharging, even at lower charge rates, is bad for the health of the cell.

    OK, let’s throw some numbers out… If you have a 2000 mAh cell, a 0.3C charge rate would be 600 mA. If your charger misses the end of charge termination, it will continue to charge until the safety timer shuts off. If your safety timer is set to 3000 - 3300 mAh (BC-900) or 4000 mAh (C-9000), or 8050 mAh (Vanson BC1HU), you will end up with an overcharged cell. A 30% overcharge on a 2000 mAh cell is roughly 2600 mAh.

    I have some aged AAA Moden 850 mAh cells. These cells were revived from near death due to low voltage due to improper long term storage. They are not the most vibrant cells, but they do a good job at lower current draws. I use them in my Peak Matterhorn that I EDC on my key chain.

    Here is a graph while charging at 0.25 amps (roughly 0.3C):



    As you can see, there was no –dV signal. I terminated the charge because the cell started to warm up more than “normal.” If I repeatedly charged this cell at this rate on a charger that utilized –dV termination, I would end up with reduced cycle life due to overcharging.

    Here is the same cell charging at 0.5 amps:



    At this rate there is a strong end of charge signal, and the problem with overcharging is eliminated.

    Getting back to the original questions… What is the best rate to charge NiMh cells at? It depends on the method your charger uses to terminate the charge. If your charger utilizes –dV termination, you will get a more reliable signal if you charge in the range of 0.5 – 1.0C.

    Now, if your charger utilizes peak voltage termination, things change. If you look at the graphs, you can see that the voltage rises to a point, then does not rise further. If you terminate the charge when the voltage stops rising, this can be observed regardless of the charge rate. You will notice that it is present in the 0.1C charge of the 4400 mAh battery, in the 0.3C charge of the 850 mAh cell, and also in the 0.6C charge of the 850 mAh cell.

    When using peak voltage termination, you now need to look beyond termination to determine the best charging rate for your cells. Slow charging produces large crystals, and large crystals produce voltage depression. Once again, ultra slow charging may not give you the best performance. Now we are in an area that is application dependent. I have often heard that you should charge at about the same rate that your application uses the cells. There may be a lot of truth in this.

    It is interesting to observe the habits of the RC people. Their NiMh packs used in their cars, trucks, airplanes, and helicopters are often charged at 2 – 3C, but the battery packs in their transmitters and controllers are usually charged at 0.1C for 14 – 16 hours. While their vehicle battery packs are only prime for around 20 – 50 cycles, their transmitter packs seem to last a long time.

    The questions remains, Why does Sanyo in the Eneloop site seem to contradict the recommendations in their main site? Perhaps the Eneloop chargers have changed from –dV termination to peak voltage termination. I think some testing may be in order…

    Tom
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    Flashaholic Luxbright's Avatar
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    Thumbs up Re: A look at slow charging

    Hi Tom,

    As always, great contribution to the CPFers community.

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    Default Re: A look at slow charging

    Your not alone. I usually set my ICE charger to 0.1C charge rate for charging empty cells. Setting the -deltaV charge termination to 20-25mV/cell makes the charger 'blind' and immune to the -deltaV termination. This is a good setting to defeat unstable power supply and early termination.

    The charger will terminate as soon as it reaches the user preset maximum capacity. A good way of turning an analyzer in to a slow charger. 0.1C is actually a good charging rate for balancing NiMH battery packs.

    Regarding Peak Voltage Termination, is the voltage the same for all cells? If not how does it work? Charge will terminate once the cell reaches certain high voltage for certain period of time? There is another type of termination called 0v delta peak termination, where the detection system works by detecting the peak voltage.

    Can you please post the sanyo link? Interesting stuff...
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    Default Re: A look at slow charging

    Hello Vince,

    I believe that if you set the -dV value to 0 mV you have peak voltage detection.

    Here is the Sanyo Eneloop link. There seems to be some bad information here. I believe my 2 cell and my 4 cell chargers are both independent channel chargers that will charge single cells. The question comes from question 12. It states that you shouldn't use quick charging, but should us a charger that charges in 2 hours or more. I like the 2 hour part, but have problems with the more part. Also, the last time I checked, 2 hour charger is considered quick charging.

    Tom
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    Default Re: A look at slow charging

    Tom, I am always thrilled by your dedication to exploring and learning and sharing with us about the world of rechargeable cells, I learn so much form you I feel I should be paying tuition....

    On a side note, I was thinking... Assuming $2.50 per premium label NIMH AA, even if you do everything in your power to charge them incorrectly, and only manage to get 50 cycles out of them, that's still like 5 cents per cycle.... Still cheaper than alkaline!!!!
    -Eric

  6. #6

    Arrow Re: A look at slow charging

    Tom/SilverFox -
    your comments please on the BatteryUniversity.com page -

    Charging nickel-based batteries

    about 2/3 down the page -under
    Charging nickel-metal-hydride -
    "Nickel-metal-hydride should be rapid charged rather than slow charged. Because of poor overcharge absorption, the trickle charge must be lower than that of nickel-cadmium and is usually around 0.05C. This explains why the original nickel-cadmium charger cannot be used nickel-metal-hydride.

    It is difficult, if not impossible, to slow-charge a nickel-metal-hydride. At a C?rate of 0.1-0.3C, the voltage and temperature profiles fail to exhibit defined characteristics to measure the full charge state accurately and the charger must rely on a timer. Harmful overcharge can occur if a partially or fully charged battery is charged with a fixed timer. The same occurs if the battery has aged and can only hold 50 instead of 100% charge. Overcharge could occur even though the battery feels cool to the touch."

    This explains the preference of fast charge over slow charge on the basis of end of charge detection.

    BUT there is also the third point under Simple Guidelines:
    "nickel-based batteries prefer fast-charge. Lingering slow charges cause crystalline formation (memory)."

    Other references -

    Panasonic NiMH Charging Manual pdf

    Energizer pdf NICKEL-METAL HYDRIDE Application Manual

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    Default Re: A look at slow charging

    hey mdocod - in case you don't know, Tom is try to make up for his Renewable Energy Thesis which is long overdue.

    I haven't seen a picture of crystalline formation. I am wondering if it could be inspected under a microscope. All I have seen is illustration. It would be cool if one could disassemble(with safety precautions) a NiCd and NiMH that has 'memory' then compare them. I would certainly do it if I have a microscope.

    I am pretty sure battery manufacturers do this sort of testing in their lab but they would probably never release such information.
    Last edited by koala; 10-28-2007 at 04:45 AM.
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    Default Re: A look at slow charging

    Hello Vincent,

    My comment on the Battery University information is that they give some good advice...

    Tom
    Behind every Great man there's always a woman rolling her eyes...

    Most batteries don't die - they are tortured to near death, then murdered...

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    Default Re: A look at slow charging

    Hello Vince,

    In this thread I discussed voltage depression and memory issues. I believe there are some references in it that illustrate the difference in crystal size.

    Tom
    Behind every Great man there's always a woman rolling her eyes...

    Most batteries don't die - they are tortured to near death, then murdered...

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    Default Re: A look at slow charging

    I have seen the effects of crystal formation in 12V 2.5 Ah Ni-Cd batteries. The charger/conditioner device would indicate "discard" although the packs were a year old so I kicked it "old school".

    Grabbed a 24V 100W bulb and attached it to the negative and positive output and the voltage started falling rapidly to below 10V or under 1.0V per cell. The light was bright as the voltage kept dropping to 7V then 6V and stabilized at 5.85V. Odd, the bulb was quite bright and putting out some serious heat. Then it happened, the voltage started creeping up for the next 15 minutes and peaked at 6.31V before a slow decline to eventually 0.1V.

    Here is my wild ass guess what happened. The battery was trickle charged continously and built up huge crystals causing major voltage depression. The conditioner detected very little capacity above 1.0V per cell and rejected the pack. The light bulb pulls less current naturally as the voltage falls and more current as the voltage rises. Once the crystals started breaking down in the electrolyte, this boosted the voltage from 5.85V to 6.31V as the crystals have more surface area when breaking even though the current load went up.

    Upon dropping down to 0.1V with the light bulb, I shorted the battery with a jumper wire and left it shorted over the weekend. Put the pack back on the charger/conditioner and it cycled fine with a "pass" indication the next day. The guys at work pulled out even more packs that failed and so far, I am 10 for 10 with the old school bulb and shorting discharge trick.

    Yes, I told the guys that trick only works with Ni-Cd as it is very robust.

    I have a BC-900 Version 32 and lucked out, it does not start on fire or miss terminations. It is rare I charge at 200mA as my preference is 1000mA with Powerex 2700's. My Eneloop AAA cells were discharged initially at 100mA and charged at 200mA (C/4) with no problems. Recharging is done at 500mA (C/1.6) with no problems noted.

    The only time I prefer a slow charge is for the C/10 forming charge. Have an 8 pack of AA Eneloops still the in package waiting for a Cadex analyzer to arrive at work. I want to discharge at 400mA to 1.0 or 0.9V and form at C/10 (200mA) The cells were manufactured September 2006 so I am curious what their self-discharge rate is.

    Should I charge the Eneloops after the forming cycle at C/2 1000mA or lower? It will do two cycles and finish with C/5 400mA as the discharge rate to 1.0V per cell.

    I will give a detailed report of each AA Eneloop and how it performed after sitting around for 14 months. Just wonder what the "official" CPF way of charging involves. C/5 is the standard discharge and I am assuming 1.0V as the cut-off.
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    Default Re: A look at slow charging

    Hey Tom, somehow I missed that thread. Saw the two pics of the crystalline in the pdf very cool! I remember in the early 90s, battery charger manufacturers started to produce so called "negative pulse charger". They were suppose to blast those whiskers away.

    Then when I got my Triton Charger(maybe ICE), the manual mention that the negative pulse charging algorithm is only suitable for NiCd and not NiMH.

    Hey BentHeadTX, lucky you to have a Cadex to play with. Are they from Battery University?
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    Default Great Information

    Quote Originally Posted by SilverFox View Post
    “What is a good rate to charge my NiMh cells?” and “Is this charger good?” are frequent questions that come up around here.

    The questions remains, Why does Sanyo in the Eneloop site seem to contradict the recommendations in their main site? Perhaps the Eneloop chargers have changed from –dV termination to peak voltage termination. I think some testing may be in order…

    Tom
    Tom,

    That was a great treatise on rechargeable NIMH batteries. I learned a lot.

    However, I'm still of the opinion that a NEW DOCTRINE is going to emerge for charging LSD batteries including the Eneloops. I'll wait for your research to continue. Perhaps, optimimum charging for them will entail procedures not yet discovered. Perhaps, there will be new chargers introduced by Ansmann, LaCrosse and Maha SPECIFICALLY to be used for the new LSD batteries. GE/Sanyo Eneloop DOES have specific chargers available for their batteries. However, their chargers are not as flexible in use as the LaCrosse BC-900.

    In the meantime, I am convinced the LaCrosse Version 33 BC-900 DECISIVELY beats Maha and Ansmann at this moment in time for addressing the needs of the new LSD AA and AAA batteries. Maha stipulates that their charger can never be used below .33C safely. Ansmann has fixed current applications and cannot be adjusted at all. Only LaCrosse stipulates the low charging rates implied as necessary by the Eneloop literature.

    I believe that for Standard AA and AAA NIMH's the Maha C9000 is probably the best out there. For C, D and 9V NIMH's the Ansmann is probably the best out there.
    Last edited by Mitch470; 10-28-2007 at 09:11 AM.

  13. #13

    Default Re: Great Information

    Quote Originally Posted by Mitch470 View Post
    Tom,

    In the meantime, I am convinced the LaCrosse Version 33 BC-900 DECISIVELY beats Maha and Ansmann at this moment in time for addressing the needs of the new LSD AA and AAA batteries. Maha stipulates that their charger can never be used below .33C safely. Ansmann has fixed current applications and cannot be adjusted at all. Only LaCrosse stipulates the low charging rates implied as necessary by the Eneloop literature.
    .

    Oh man, this guy has a major case of choice-bias. When will he stop?

  14. #14

    Arrow Re: A look at slow charging

    Quote Originally Posted by SilverFox View Post
    The questions remains, Why does Sanyo in the Eneloop site seem to contradict the recommendations in their main site? Perhaps the Eneloop chargers have changed from –dV termination to peak voltage termination. I think some testing may be in order…
    Even eneloop.info (the earlier European site) seems to offer a differing take on this page:

    "Other Chargers
    Basically eneloop is a modern Ni-MH battery, which can be charged like any other Ni-MH battery.
    Therefore eneloop can be charged also with other, modern chargers, which are suitable to charge Ni-MH batteries.
    However, SANYO cannot accept any liability for the function or safety of chargers made by other manufacturers.
    Also SANYO cannot be held responsible for any damage to eneloop batteries caused by unsuitable chargers."

    This is very much in line with the entry at Wikipedia 5.1.1 Low Self Discharge Batteries (I could not find the source reference to that information, but generally the Wikipedia is accurate because of the open exposure and correction/refereeing)
    "Besides the longer shelf life, they are otherwise similar to normal NiMH batteries of equivalent capacity, and can be charged in typical NiMH chargers. "

    Of course eneloop.info on the same page also do say -
    "SANYO Chargers
    SANYO offer special chargers for the eneloop. These chargers have been developed by SANYO and have been tested for charging eneloop batteries.
    With the charger MDR03 you can charge two eneloops of size AA in 250 minutes or two eneloops of size AAA in 170 minutes. The charger works with AC-voltages from 100V to 240V and is therefore ready to join you on your trips to other countries. "

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    Default Re: A look at slow charging

    Quote Originally Posted by Mitch470 View Post
    Only LaCrosse stipulates the low charging rates implied as necessary by the Eneloop literature.
    Quote Originally Posted by Mitch470 View Post
    So far as Termination Detection, I have my own ad hoc way of preventing a miss. I charge 4 batteries or more at one time and take the last one to terminate out of the charger when all the others have already reached a termination detection. I don't really care if it was slightly undercharged. I then check its voltage. If it is less than the others, I put it back in the charger for another hour.
    Do you still charge 4 batteries at a time and terminate the charge manually? It seems like a lot of trouble, especially for someone planning to throw their batteries out in a year due to obsolescence. My Eneloop literature recommends charging at a rate that will take 2 hours or more, so I charge at 1000mA and don't worry about supervising the process. FWIW, the batteries barely get warm, and I expect them to last many years.

    I guess if Sanyo ever comes out with a "NEW DOCTRINE" for recharginging Eneloops, then we will need to pay attention. But thus far, everything points to them being regular NiMH chemistry with a few design optimizations which do not require special chargers or charging routines.

    Do you have any thoughts on large crystal formation during slow charging, and what is it about Eneloops or the LaCrosse charger that you think would allow them to avoid this problem? Anything more than a hunch?

  16. #16

    Default Re: A look at slow charging

    Quote Originally Posted by GaryF View Post
    Do you have any thoughts on large crystal formation during slow charging, and what is it about Eneloops or the LaCrosse charger that you think would allow them to avoid this problem? Anything more than a hunch?
    I am NOT an Electrical Engineer. Is anyone else here an Electrical Engineer? I am just a gadget hobbyist. I used to just buy regular batteries and THROW them away. Then I proceded to get Standard NIMH's due to their more advanced technology. I only used the Ansmann Energy 8 with them. Frankly, if someone introduces a 6000 mAh non-rechargeable Lithium, I'll go with that one and discard all the rest. I'd even pay a premium price for it. Right now I still mostly use 3000 mAh Lithiums - nonrechargeable.

    Now, I have become a fan of the LaCrosse BC-900 Version 33. If I don't see new gadget batteries and chargers in 12 months, I will be very disappointed. This is all GADGETRY for me.

    I know that you MH-C9000 fans are very upset that I much prefer the LaCrosse BC-900. Live with it.

    I expect to be getting involved with the new Garmin Nuvi 750 GPS when it comes out this week. I hope to see even newer gadgets next month.

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    Default Re: A look at slow charging

    Quote Originally Posted by Mitch470 View Post
    I know that you MH-C9000 fans are very upset that I much prefer the LaCrosse BC-900. Live with it.
    You know, Mitch, it's not so much that (I think most MH-C9000 owners would happily admit it's a good charger) as how you are using it, while authoritatively claiming that it's the best way to use it and that it's THE best there is. Based on what? Feelings and misunderstandings? There are people here who do understand very well what NiMH cells are doing when they charge and you're going to have to come with some very good reasons to convince them that their genuine good understanding is bad misunderstanding. I suggest you listen to them and figure out why they're saying what they do.

    It's cool that you like gadgets. I can appreciate that since I've got a slow but steady stream of new toys coming from DX and KD. It's like Christmas all year round.
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    Default Re: A look at slow charging

    Quote Originally Posted by koala View Post
    Hey Tom, somehow I missed that thread. Saw the two pics of the crystalline in the pdf very cool! I remember in the early 90s, battery charger manufacturers started to produce so called "negative pulse charger". They were suppose to blast those whiskers away.

    Then when I got my Triton Charger(maybe ICE), the manual mention that the negative pulse charging algorithm is only suitable for NiCd and not NiMH.

    Hey BentHeadTX, lucky you to have a Cadex to play with. Are they from Battery University?

    Hi Vince,

    I am starting to favor the temperature rise method because that can
    detect end of charge sooner than minus delta V, which i have proved
    with my battery monitor chip and software.
    BTW, you will have a battery analyzer too as soon as i get the check
    from Tom. He said his wife was sending it and that was last weekend
    but i havent seen it yet.

    Also BTW, anyone else interested in a battery analyser circuit for under
    20 dollars (more like 10 dollars) just PM me.
    Take care,
    Al
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    Default Re: A look at slow charging

    Quote Originally Posted by BentHeadTX View Post
    ... I kicked it "old school".

    Grabbed a 24V 100W bulb and attached it to the negative and positive output and the voltage started falling rapidly to below 10V or under 1.0V per cell. The light was bright as the voltage kept dropping to 7V then 6V and stabilized at 5.85V. Odd, the bulb was quite bright and putting out some serious heat. Then it happened, the voltage started creeping up for the next 15 minutes and peaked at 6.31V before a slow decline to eventually 0.1V.
    I can understand it would work wonders, but doesn't it seriously reverse charge some of the cells?
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    Default Re: A look at slow charging

    Firstly, thank you for yet another very educational treatise Silverfox.

    Quote Originally Posted by SilverFox View Post
    The questions remains, Why does Sanyo in the Eneloop site seem to contradict the recommendations in their main site? Perhaps the Eneloop chargers have changed from –dV termination to peak voltage termination. I think some testing may be in order…
    Quote Originally Posted by MrAl View Post
    I am starting to favor the temperature rise method because that can detect end of charge sooner than minus delta V, which i have proved with my battery monitor chip and software.
    The manual for the model MQN05 contains the following statement under the 'How to Charge' heading which appears to bear some relevance here:

    Do not plug-in upside down. If plugged in upside down, it will finish charging before it is fully charged.
    Given my very limited knowledge on this subject, I have only been able to surmise that it pertains to temperature, ie: that there may be sensors in a location where they would reach a given temperature prematurely when the charger is upside down.

    On a side note, the statement infers that the premature termination will happen in every instance, indicating that there is also a fairly narrow tolerance at play, so the sliding cover may serve as more than just a decorative touch if the statement is indeed temperature related.

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    Default Re: A look at slow charging

    Quote Originally Posted by Bones View Post
    ... the sliding cover may serve as more than just a decorative touch if the statement is indeed temperature related.
    That's interesting. The manual of a NiMH charger I have says make sure the cover is closed before charging. Would that be to help sense heat too? All other charger manuals I've seen say leave the cover open - in case of venting and to help keep things cool is my guess.
    No, a torch does not always mean flames.
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    Default Re: A look at slow charging

    Quote Originally Posted by TorchBoy View Post
    I can understand it would work wonders, but doesn't it seriously reverse charge some of the cells?
    Yes, it will do that but not for a very long time... all cells will eventually go to 0.00V under load and stay shorted for a day or two. Back in the 80's, we used to drive 120 cell 144V Ni-Cd packs and attach them to large banks of light bulbs to run them down overnight. After that, we would place shorting bars across each cell and let them sit like that for a day. The packs would go through this every 6 months and would last 5 to 8 years before replacement.

    That is why I love Ni-Cd for harsh use... they keep going and can be revived. Could be the reason so many defibrillators still use 12V 2.5 Ah Ni-Cd cells.

    One of the cells gave me hell overseas, it would constantly trip the Cadex with various errors. I then did the light bulb trick and left it dead shorted over the weekend. It still failed but later in the cycle. Performed the dead short trick again and it almost passed. Got a wild hair and did very bad things to that battery!

    I noticed it would not charge all the way up and give a low voltage error. I charged it at 3.6 amps which is C3 considering it has 1.2 Ah cells. The thing hit 1.37V and stalled and tripped the heat sensor at 45C. I let it cool down for a few hours and hammered it at C4 (4.8 amps) and it charged to 1.56V and shut down. The temp was at 43C so I juuuuust made it. Let it cool for several hours and did a 2.4 amp discharge to 0.4V per cell followed by two cycles of C1 1.2A charge/discharge. The battery finally spewed out a result of 93% and was returned to service.

    The downside of that was the time it took to get it going, almost two weeks! The unit was being replaced in three months and it takes at least a month for a replacement battery to make it to that side of the world.

    If I was going to be trapped on a desert island for 10 years and had a choice of batteries to get me through... make mine Ni-Cd... they can be revived. When the Cadex makes it to work I have a special battery, a 9.6V Makita drill battery manufactured in April 1992... it could be time for "ride the lightning" but that is another story.

    Another chemistry that I find can handle a beating is Lithium Iron Nano-Phosphate from A123 Systems. Have four of those batteries and they work very well and no problems after a year of using four of them. Time will tell their shelf-life though... I am thinking positive on LiFePO4 in general so things are looking up.

    BTW, don't beat the hell out of NiMH or Li-ion, they will fail on you.
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    Default Re: A look at slow charging

    Quote Originally Posted by BentHeadTX View Post
    If I was going to be trapped on a desert island for 10 years and had a choice of batteries to get me through... make mine Ni-Cd... they can be revived. When the Cadex makes it to work I have a special battery, a 9.6V Makita drill battery manufactured in April 1992... it could be time for "ride the lightning" but that is another story.
    Its doubtful you will have electricity on a desert island. Non-rechargeable Lithium batteries in a large supply would be best. Chargers would be simply JUNK there unless you get a solar charger.

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    Quote Originally Posted by TorchBoy View Post
    It's cool that you like gadgets. I can appreciate that since I've got a slow but steady stream of new toys coming from DX and KD. It's like Christmas all year round.
    Get that Garmin Nuvi 750 GPS coming on November 1, 2007. Its the best new toy out this week. IMHO it beats both the LaCrosse BC-900 AND the Maha MH-C9000 for pure enjoyment.

    I tend to be fickle about my toys.

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    Default Re: A look at slow charging

    Quote Originally Posted by Mitch470 View Post
    Its doubtful you will have electricity on a desert island. Non-rechargeable Lithium batteries in a large supply would be best. Chargers would be simply JUNK there unless you get a solar charger.

    Solar cells are easily packable for use with a low-amp-draw charger such as those from maha...........

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    Default Re: A look at slow charging

    Quote Originally Posted by SilverFox View Post




    Tom
    some of this slow junk that could never happen. This continual rise into oblivion, because they have a peak total voltage available for each cell unit.
    that means when they reach the say 1.53V the charge tapers as the voltage differential is lower, the current flow slows down.

    so there is no termination on some chargers but it never gets completly out of control because they dont have 2V on each channel, so it cant even do that.
    example of that are them dumb series timed things with about 3.2-3.5v for 2 cells max.


    when you have a 2volt channel if your computerised stuff goes braindead, there is nothing to prevent what occured in this graph.
    but was it really so high? read the actual battery voltage, its the LOW part of the line, not when the chargers pulse is ON, and it does 0V a few times.

    so although the rate never goes over the overcharge rate, the voltage is going higher here, far beyond what the voltage is in lots of the cheap chargers.
    besides the rate on computerised stuff is an averaged PWM, not a low current rate, with the averaged PWM you have (again) a completly different charge alogrythm, for better or for worse.

    for better, it is not charging at 250ma , its charging at 1000ma, so therefore its not making different crystals than 1000ma, for better , its IS charging at 1000ma, not 250, but pausing between which keeps the cell cool, and keeps it from reaching an overcharge state.
    for worse, keeping the cells cool, and having such a low rate, keeps it from Vdrop, but its not really at a low rate, each charge pulse is at a high rate, course that could be better or worse, because this cell still didnt collapse, it didnt vdrop, so it still didnt reach gassing up and all.
    (assuming this is a computerised charger thing your using)

    so basically different charge alogrythms, which really cant be compared DIRECTALLY to similar charge alogrythms on devices that dont use PWM, or high voltage maximums.
    so you cant use a PWM charger to determine the negative effects of a Dumb slow charger.

    stuff like these new wizz bang computerised analising chargers DO have the 2V max per cell capacity, DO use PWM ,and they CAN incur this kind of wrath.

    this cell probably has lower resistance, and it is a AAA, which have cruddy resistance. but it does show that after TIME, when something even like an Enloop itself gets higher in resistance or sits on a shelf or in a light for ages, it will have properties similar to this. but a new one working great, will be of low resistance and would not act that way.
    besides the voltage on the cell itself isnt to 1.6v yet even, what its at 1.58 or something? and may not ever reach that, a AAA with low resistance could have terminated higher than that with a higher current, so was it toasting it, or about to heat up and go into an overcharge state?

    ya still didnt show that it Ruined the cell, although anything ruins AAA cells :-) i can slow charge a AA enloop for a month, did it, and it didnt give a crud. but my charger doesnt have 2Volt channels. but my AAA enloops are becomming offset from eachother (in capacity), and i treated them all nice :-( used all that proper termination junk and 2 of them are at about 750 and 2 are at 800, and i am guessing it cause of a minor reverse charge.

    then "here is the same cell at .5amps"
    Is it the same cell? it IS if it was done at the .5amps first, but if it was just "Formed" :-) and cycled on the last slow charge you did, then it didnt have the same resistance anymore :-)
    so the only "proof" would be if the one test was done after the other, or that the first test was done again. because this time that same cell might terminate. for a AAA cell to not terminate at 250ma???? its either a crap cell (throw it out:-) or your charger sucks the charger must have been expecting that the cell suffer miserably first before it determines that it is beaten down enough. whatasa matter cant it detect the big fat 0 there.

    suuuureee you got a big fat drop there when your cycling the cell around on that day or week, but ya dont get such pretty huge drops on cells that are all unused and un cared for.

    my charger would terminate most AAA cells with 250ma, at a high voltage, but well within the "capacity" of the cell. meaning the readout or the time is never soooo different than the tested discharge capacity.


    (ok i edited the heck out of that, just soes you know)
    Last edited by VidPro; 10-29-2007 at 06:13 AM.

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    Default Re: A look at slow charging

    Quote Originally Posted by koala View Post
    Then when I got my Triton Charger(maybe ICE), the manual mention that the negative pulse charging algorithm is only suitable for NiCd and not NiMH.
    them negative pulse charging did accomplish one thing. or at least on the idea behind it.
    the idea was to load the battery after a charge pulse, i donno how "negative" it was, but basically it would charge and put a load on Draining some of that charge, then charge again, drain again, on and on.
    and this type of alogrythm is very similar to Cycling a battery if you think about it.
    at each charge pulse there is a Use pulse, while countering eachother, the ammount of USE or load, is insuring that section or part of the battery is getting fully cycled.
    for a battery that was uncycled and low resistance and all that other stuff, it should help it out by 1/4 or so which was then cycled. because 1/4 is the most they would re-drain what they just charged.

    plus it does pulse charging.
    it would be usefull for some situations, and totally useless in others.

  28. #28
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    Default Re: A look at slow charging

    Hey Mitch470 - I am an eletronic design engineer! Mostly digital stuff, never had to design a battery charger I must say.Thanks a ton Silverfox for your fine writeup! To be honest, I thought the better chargers used a short DISCHARGE periodically to 'burp' the chemistry, especialy for the rapid chargers. Anyway this post and set of informed replys is why I keep coming back. Good stuff!Jeff O.

  29. #29
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    Default Re: A look at slow charging

    Tom...first of all

    So great to take the time to read this thread. Great additional points raised from others also.
    • On a practical basis, it seems the 0.1C x 16 hrs with a timed termination (I usually just set an alarm)j would not be long enough to lead to large crystal formation if you made sure they were discharged to 0.9V/cell first. Is that correct?
    (I just want to make sure that no one comes away from this discussion with a downside to 0.1C forming charge, and how important it is.)
    • Ignoring the Eneloop link controvery, it appears that the best general charging rate for most users is 0.5C in terms of prompting adequate -dV signal for proper termination, and thereby avoiding (smaller) crystal formation. Is that your NiMH recommendation for general uses?
    • This again underscores for me the importance of attending to my charges. Watching the time, increasing mA and voltage, looking for a plateau & possible failed termination, feeling the cell temps, etc. For those readers who are not as able to absorb the technical aspects of this discussion, I think it is always useful to emphasize/reinforce the optimal general guidelines.
    Many thanks again!

  30. #30
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    Default Re: A look at slow charging

    Tom:

    Not near my manuals but I don't think that a ICE or Triton allow a zero volt setting for termination.
    Bill

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