Appropriate Charge Rate for NiMH Batteries

I think some people have missed the entire point of the original question. Asking for the "Appropriate Charge Rate for NiMH Batteries" implies you have a charger which actually gives you a choice.

Obviously if you have a slower smart charger and that is all you have, that is what you use and make the best of it. However, if given the CHOICE using a good charger such as the BC-900, BC-9009, or C9000, the appropriate rate to CHOOSE would be 0.5-1.0C to ensure a reliable termination.

Since the OP has a C9000, his appropriate choice would be 0.5-1.0C for typical use. For basic charging, there is no reason to choose a slower rate, as it does not "give" you anything extra. It would take longer, and increases the chances of a missed termination.
 
exactally, what marduke said. What charger if you only got One?
the 9000 maha
which does digital pulsing (PWM) to achieve different averaged currents, and if they have a later model of it, it slows down on voltage peaking.
and it has a seperate break-in method.

if it was a lacross, it is relying on a V-drop only then times out, and still uses an averaged PWM current method. and will melt into a pile of plastic blob on the floor if you miss one step :crazy:

while there are only tiny differences in the two models, what it can do, and does to batteries is entirely different. what it Cant do, might be even more important 🙂
 
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VidPro said:
if it was a lacross, it is relying on a V-drop only then times out, and still uses an averaged PWM current method. and will melt into a pile of plastic blob on the floor if you miss one step :crazy:
Click to expand...

It has a temp sensor guard against that also.
 
Marduke said:
It has a temp sensor guard against that also.
Click to expand...

the temp sencors being only emergency meltdown indicators, nowhere near capable of a temperature based inclusion in the charging alogrythm.
and stopping then restarting again, think about what the does to something trying to indicate a V-drop, its no wonder why its not part of the alogrythm.

meaning, if they were using temperature sencing , first they have to have the sencor on the Battery not hiding down in the case somewhere, then they have to include it into the charging alogrythm, they dont.
and if a battery is overheating when it gets close to a v-drop, and it pauses, then restarts again, then it can do that over and over again, never reaching the v-drop. that means it just keeps ramping the temps up and down like a yo yo.

its so many things to concider, it is no wonder why they dont concider them. poor little microcontrollers :-(
mine melted down because the microcontroller wasnt even running anymore, so much for it even being a emergency shutdown.
 
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TorchBoy said:
Are you referring to the top-up charge (after DONE is shown)?
Click to expand...

the maha "7th generation" (or whatever cute name) charge alogrythm, has included in it now, a voltage peak indication, when the voltage of the battery reaches some set point, they slow down.
you can see this being said many times by people tracking the voltages on the 9000 itself , along with other newer model Maha chargers.

it is what is keeping the batteries cool(er), it keeps OLD cells with loose resistance from going crasy, and solves the problems of digital current controls and v-drop detection (which it also uses) and can keep meltdowns from happening.
but it also means that batteries are not usually heavily overcharged either. which is also indicated by the reduced charge capacity people get on a discharge tests. especially if they Pull before all the topping is done.

its in the book, i dont yet own the 9000, so me not being the one to talk 😳 but i have the other Maha, and i should use a scope to see things even with it, because its got all kinds of complex stuff going on.

see now i have stepped out on a ledge of logic based only on hours of user data, it could be wrong. or you could just Believe 🙂
 
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VidPro said:
see now i have stepped out on a ledge of logic based only on hours of user data, it could be wrong. or you could just Believe 🙂
Click to expand...
Or you could be just smoking something, because that's what happens after the DONE is shown, because of the top-up charge. What model number is this "7th generation"?
 
TorchBoy said:
Or you could be just smoking something, because that's what happens after the DONE is shown, because of the top-up charge. What model number is this "7th generation"?
Click to expand...

ok it says DONE when it goes into topping and isnt done, and . . .

i dont know what you want in model numbers, the original first 9000 that people tested, is different than todays 9000 that people are usually buying. check out silverfoxes original thread on it.
 
I think i see the problem here.
Let me restate it.

Many chargers have a high voltage termination, or just a voltage max (useless usually).
The high voltage termination on the 9000 is now set at a "lower" level than other chargers, including older version of the 9000.
Most of the chargers will do final topping after the initial charge termination anyway, so nothing changes there.

The 9000 though will slow down ON this high voltage, WHERAS many V-drop chargers (with high peak and max voltages) will not REACH their high voltage termination or max, and instead be in the overcharge state, then V-drop terminate, and then continue to finish up by slow topping.

The difference being, the 9000 is much more capable of terminating on high voltage (being lower), and much more likly to do so.
Slam a battery with different currents, averaged or not, and check the peak voltages reached.
The higher the input current the higher the battery voltages will reach. also dependant totally on the resistance of the cell.
SO
With a lower "high voltage termination" level, and depending on the current, the Maha charger will Terminate a charge Earlier than other devices, prior to going on to topping.
This Can, in some situations at some charge rates, mean that the Maha charged batteries will not read as "full" capacity , as seen by a discharge on a SEPERATE discharger. even after full topping.

So the batteries are not always "fully" charged, and the batteries are also not overcharged as much (or ever), at fast rates especially where it is more important.
I see all this as a GOOD thing.
I am not knocking the charger, or its style , its great. keeping away from overcharging , treating bad cells better, and working at different currents without V-drop Only termination issues.

Given high rate, and a low resistance battery, it is likely that a battery charged in the 9000 will not be charged "as far" as it can be on other chargers, reguardless of final topping.

The voltage termination level is not changed or adjusted for different resistances or charge rates, it was just changed in the rest of the pages of code the microcontroller uses , to "improve" it. it is just one non-moving indicator, it is not a voltage "alogrythm" it is just "peak" voltage.

That is my analisis from the data provided by the users, and the manufacture themselves, and the many tests the users have shown data on.

Anyone with a 9000, could pull the cells "fast" charged, and after Full topping, and prove it by discharge on a different device.
then charging the cell in something else, or using a slower rate, or a break-in rate, and testing the same cell again.

What am i supposed to do when the data doesnt add up, put 2+2 together and make a logic choice on the shown data, then test it here, i have , i just dont have an actual 9000.

notes: everything is just more likely or less likely , with so many possible variables, rate, temperature, resistance of battery, capacity of battery, there is no absolutes. so that is why i put in all the more thans and less thans.
 
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VidPro said:
Most of the chargers will do final topping after the initial charge termination anyway, so nothing changes there.

The 9000 though will slow down ON this high voltage, WHERAS many V-drop chargers (with high peak and max voltages) will not REACH their high voltage termination or max, and instead be in the overcharge state, then V-drop, and then continue to finish up by slow topping.

[...]

What am i supposed to do when the data doesnt add up, put 2+2 together and make a logic choice on the shown data, then test it here, i have , i just dont have an actual 9000.
Click to expand...

Am I correct that by "V-drop" you mean the -deltaV algorithm? Are you sure that most chargers will top off the batteries after it? -deltaV when finished will already overcharge the battery, topping off afterwards will make it even worse...

I have a C9000 and I believe your description of its behavior is generally correct. I think though that maxV at 1.47V doesn't make overcharging impossible. It makes it less likely but if I correctly remember as the cells age their peak voltage before overcharge gets lower. With old cells it may get below 1.47V and you'd have to trust -deltaV which is also not completely reliable.

From what I read -deltaV, although sometimes recommended by manufacturers, is definitely worse for the batteries life than dT/dt and probably worse than the standard 0.1C charge.

The algorithm used by C9000 with many termination conditions still doesn't guarantee that the cell will not be overcharged (although it is usually unlikely).

I think that the standard 0.1C charge, if the correct capacity is supplied, guarantees that the cell won't be overcharged. It offers good charge level and prolongs the cells life. If the time is not a limiting factor (which would require a fast charging method) I think the standard 0.1C charge can be considered the best approach. I may be wrong but I still couldn't find any argument to the contrary.
 
delta , i am not greek 🙂 yes same thing.
When hit really hard and the battery voltage (eventually) drops, that does not mean ALL the chemicals have converted over to the charged state.
because that can occur in a fast charge and it is just because MOST of the chemicals are already converted. Then it gets too complicated for me to repeat correctally. but the gas re-integration stuff cant occur as fast or whatever. The charge speed is to fast for the DESIRED chemical reaction.
So there is room for more charge, even after the battery cant take any more at that speed.

On .1c charging I would tend to agree, there is NOT a problem with it and ni-mhy cells, but Silverfox is the expert, and if he says there is , then he has been right like 99.99% of the time, and when he wasnt he corrected it quick. you dont mess with those odds 🙂 Add to that a Pulsed 1000ma or 2000ma Averaged charge speed is NOT slow, so depending on the machine, there is no real slow.
EX: If we try and get a Lacross to do 200ma it does 1000/5 not 200ma.

The question becomes, is long term slow charging worse than the slam of the overcharge V-drop (and respetive evil gasses and temperature)? I also want to form up the stuff for fast discharges even on slow discharges , because we want to keep the voltages up , and we want to knock the stuff back into Place on the plates and all. (we dont want pits in our metal, or whatever)

with the Maha stuff doing the voltage peak termination, we basically can have a nice fast initial charge and still not wack it so hard when it reaches overcharge.

so that is why i say it Is a good thing, but any of this stuff is still doing cheap tricks. because really extensive multi sencing would be much better, and so much more problematic if it wasnt done correct. the Rate of charging Should (in a perfect world) match perfectally the ammount of charge acceptance, or chemicals that are ready to move.
 
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then the 1.47v , that must have been thought up by somone paying attention, because when Manually charging i have always thought that around there somewhere, is a beutifull place for a Voltage only type of alogrythm. at 1.47-9v that is about where i want my ramping pulse voltage based charger to be at .1c or less.

and i have tested that many times and it works better than these computers. even in series. . . well most of the time

older cells usually have higher resistance, it is rare for them to have lower resistance , IN this chemistry , that means they are a more "open curcuit" to the voltage being poked at them. like many older (crap) cells V-drop terminate around 1.5-1.8v at high rate charges, like 1C if i remember right. so on these junk cells that have high resistance and would be heating up badly at high voltages, the voltage termination stops thier torture and further overheating.

then if your going really SLOW with the charging , the 1.47 might not be reached, BUT then again it is also probably going slow enough to handle the overcharge.

Tough, robust cells be they lower capacity or just low resistance dont V-drop as easily, and because of low resistance dont rise in voltage as high. so they might not make it to the 1.47v but they also can handle the rates easier. so it still keeps working out . . . most of the time 😀
 
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VidPro said:
On .1c charging I would tend to agree, there is NOT a problem with it and ni-mhy cells, but Silverfox is the expert, and if he says there is , then he has been right like 99.99% of the time, and when he wasnt he corrected it quick. you dont mess with those odds 🙂 Add to that a Pulsed 1000ma or 2000ma Averaged charge speed is NOT slow, so depending on the machine, there is no real slow.
Click to expand...

Do you happen to remember where Silverfox described the problems with 0.1C? I've read a few of the discussions where he participated but I haven't seen such a description.

The standard 0.1C charge is of course not ideal. It is slow, requires you to know the cell capacity and it shouldn't be used on a partially charged cell (or alternatively you should use shorter time for a partially charged cell). You are also right that a pulsed charger does not perform the standard charge (although I don't know how different will be the results). While these factors may be inconvenient in some situations IMO none of them can be seen as a drawback of the 0.1C charge when the cells performance and longevity are considered.

VidPro said:
older cells usually have higher resistance, it is rare for them to have lower resistance , IN this chemistry , that means they are a more "open curcuit" to the voltage being poked at them. like many older (crap) cells V-drop terminate around 1.5-1.8v at high rate charges, like 1C if i remember right. so on these junk cells that have high resistance and would be heating up badly at high voltages, the voltage termination stops thier torture and further overheating.
Click to expand...

Heh, I remembered that the peak voltage moves but now when I read what you wrote, I admit that I'm not sure in which direction 😱

Interesting thing is that if, as you describe, the peak voltage becomes higher for older cells, it means that the C9000 will end up at 1.47 maxV no matter how old are the cells. It would be consistent with my measurements and other people tests but it would make other conditions rather useless for most.
 
will end up at 1.47 maxV no matter how old are the cells.

yes IF I also included the post termination TOPPING, so when reading along with what they are saying, there will also be something like "it floated up to 1.51 , then back down again after a few hours after it said Done"
meaning (to me) that on topping it doesnt have that restriction. (nor would it be needed).
and
most of the testers pointed out that on the break-in it did not seem to have the peak voltage termination, which is logical that on a "slow" charge it wouldnt be as critical.
 
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Wow, this thread is really getting deep!

There are a couple things I'd like to point out relevant to the discussion.

Concering NiMH cells, when a battery cell manufacturer or charger manufacturer refers to a "Standard Charge", this is a reference to the IEC 61951 standard for determining the capacity of NiMH cells, nothing more, nothing less. That is to say, it is not a reference as to how a NiMH cell should normally be charged, but rather an industry standard by which the capacity of NiMH cells is determined.

The other point I'd like to bring up is the Maha C-9000's "top off charge". All but the initial version of the C-9000 have this feature, which applies a 100mA charge for two hours after "DONE" when in charging mode. This rate is 0.1C or less, for all cells except those of <1000mAh capacity. It seems to me this is a good way to accomplish a full charge however the C-9000 terminates, whether it's -dV, by temperature, or the 1.47 Volt max V method. It's a way to in effect, finish the charge in a way that resembles a "Standard Charge".

As for Scooby Doo's original question, what is the appropriate charge rate for NiMH batteries, I think there may very well be more than one answer. As I've said elsewhere, part of the reason I changed over to NiCd's back in the early 80's (from alkalines) was their increased performance, yes at that time capacity suffered, but the performance was superior, ie. less voltage drop under load.

I used the 14-16hr chargers for 10+ years, that's pretty much all that was commonly available. Then the 4-6hr chargers showed up around the same time that NiMH chemistry did. Now we have chargers that will charge at a 4C rate (15min chargers). I've stayed away from the 4C chargers and after researching a few years ago, decided that the often recommended 0.5-1C rate was a good choice.

From the early days to now, one thing I've noticed is that with faster charge rates, cells seem not to develop voltage depression as often. In some cases over a period of time, the same batch of cells were charged both ways, some fast and some slow, so improvement in cell technology wasn't playing a part.

One aspect of faster charging that might be of significance, is that faster charge rates are known to retard large crystal formation in cells better than slow charge rates. Actually very slow rates, somewhere below 0.05C I believe, actually are a cause of large crystal formation in cells, although this may only happen when the cell is in a state of overcharge (trickle).

What I've seen in my cells may not be readily apparent, unless you frequently use your NiMH cells at 0.5-3C discharge rates. In most medium/low discharge rate applications it's possible you would not be aware that your cells were suffering from voltage depression.

I can only report what I've experienced personally. I find the discussion here of the ins and outs of charge rates interesting. I also suspect both sides are correct, in some ways, but for me, from a high performance aspect, I'm going to continue with the 0.5-1C charge rate, as it seems to work the best for me.

Dave
 
Good summation, 45/70. Your conclusions seem to be supported by my experience and, certainly, SilverFox's many explanations on this subject. The NiMH cells I've charged in this manner have not suffered voltage depression and have maintained their capacity. No over heating has occurred or missed termination - ever.
 
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