Maha MH-C9000 SUPPORT / FAQ - continuation

[45/70]

daj013, do a break in and set the capacity as 2600. It's best in these situations to round the number down, so as to not exceed the 0.1 C limit.

Dave

Edit: Black beat me to it. Yes, a discharge first is also recommended.

[jhellwig]

Read the instructions. Also read the Maha Faq http://www.mahaenergy.com/store/mhc9000faq.asp .


270ma will end up being what the c9000 will charge at if you do a break in on the batteries.



Yall beat me to it.

[45/70]

270ma will end up being what the c9000 will charge at if you do a break in on the batteries.

Actually, if you set the battery capacity at 2600, the C-9000 with do the break in charge at 260mA, which is below the 0.1 C maximum, as opposed to setting at 2700 which would charge at 270mA, which is over the 0.1 C maximum.

Dave

[jhellwig]

That may be but Maha says that for odd capacity cells like 2650mAh and 2450Mah to go to the next hundred. Ie 2700mAh.

[Black Rose]

True, but Maha also says to put AA batteries into the charger positive end first, which can result in torn wrappers on the negative end

[45/70]

That may be but Maha says that for odd capacity cells like 2650mAh and 2450Mah to go to the next hundred. Ie 2700mAh.

Yeah, that may be true, and it probably doesn't make any difference, but a standard charge is 0.1C maximum.

As far as the Maha C-9000 manual goes, it's still the one for the very first C-9000's. Nobody's perfect!

Dave

Edit: Black, you're fast!

[Black Rose]

Edit: Black, you're fast!

Just have nothing better to do this afternoon since some parts I ordered have not arrived yet.

Otherwise I'd be wielding my soldering iron of terror against some unsuspecting flashlights

[TONY M]

True, but Maha also says to put AA batteries into the charger positive end first, which can result in torn wrappers on the negative end

Very true this does indeed tear wrappers. Thats "mahatalk".

[Turbo DV8]

I bought two of these just before Christmas for myself (in the spirit of receiving!) I have three BC-900's, and I wanted these Maha's for their ability to do a discharge capacity test without automatically recharging afterwards like the BC-900 does in discharge or refresh. In this respect, it satisfies, but on so many other levels, for me it is a step down from the BC-900's, not the least of which is some of the results are so clearly bogus that my units must be defective. I'll start with the unforgivable stuff first.


I started with sixteen brand new AAA Eneloops I bought along with the Maha. Experience has shown that Eneloop cell-to-cell voltages are extremely close out of the package. I ran a discharge at 500 mA. As soon as all first eight cells were inserted and began to discharge, I immediately noticed that the voltages under load ranged between 0.95 V - 1.24 V. Suspecting this was bogus, I put all eight immediately into the BC-900's at 500 mA discharge, and all eight voltages under that load began at 1.20 V + - 0.03 V and settled at about 1.17 V after two minutes. I put them back in the Maha's at 500 mA load discharge. Again the voltages were between 0.95 V and 1.24 V. First, notice how close the lowest one was to the 0.9 V discharge cut off voltage already, and fresh out of the package. After only a few minutes, the lowest one dipped below 0.9 V and the Maha said "DONE!" I immediately pulled this cell, which the Maha said was now dead, and put it on the BC-900 to continue discharge at 500 mA. The cell delivered 553 mAh on the BC-900 after the Maha said it was dead.


The problem as I see it is, there is some instability in my units in that the beginning indicated voltages under load are all over the place, even on cells that are known to be matched. After 20 minutes, all remaining seven voltages under load had stabilized between 1.04 V - 1.08 V (which, BTW, I know from running Eneloops on the BC-900 is much lower than actual voltage under that load). What I observed during 20 minutes is that the lower indicated voltages came up under load (except the lowest, which terminated before it's voltage had a chance to rise) and the higher indicated voltages came down under load, all to meet somewhere in the middle at 1.04-1.08 V. Now, I don't know about anyone else, but I've not known a cell under load to rise in voltage! All seven other cells discharging on the Maha did ultimately yield between 514 - 532 mAh. I ran the above tests on the remaining eight cells, and again the start voltages were all over the board (and inaccurately low) at the beginning of discharge, then some voltages came up over 20 minutes, and some came down, all to meet at a nice uniform figure in the middle. This is not normal! Incidentally, after charging all sixteen cells on the BC-900's, resting for one hour, and discharging on the BC-900, all sixteen cells delivered between 830 - 879 mAh, so I know the cells are not the cause of the strange results on the Maha.


So at this point, I was very unhappy about the inconsistent and strange results the Maha was giving me. So I began some tests on eight known good AA Eneloops. I ran more detailed tests and kept even more detailed records than before. I ran the following tests:

1 - (BC900/BC900): Charge @ 500 mA on BC900, rest one hour immediately after termination, discharge @ 500 mA on BC900.

2 - (BC900/Maha): Charge @ 500 mA on BC900, rest one hour immediately after termination, discharge @ 500 mA on Maha.

3 - (Maha/Maha):
(Cells 1-4): Charge @ 500 mA on Maha (no "top off"), rest one hour, then discharge @ 500 mA on Maha.

(Cells 5-8): Charge @ 500 mA on Maha (plus 2 hours "top off"), rest one hour, then discharge @ 500 mA on Maha.

4 - (Maha/BC900):
(Cells 1-4): Charge @ 500 mA on Maha (no "top off"), rest one hour, then discharge @ 500 mA on BC-900.

(Cells 5-8): Charge @ 500 mA on Maha (plus 2 hours "top off"), rest one hour, then discharge @ 500 mA on BC-900.


As you can see, the above tests would answer a few questions. One, how much more capacity does one get from the batteries with and without the two hour "top off" charge on the Maha. Second, to compare remaining capacities as indicated on Maha vs. BC-900. Third, to compare how fully the Maha charges compared to the BC-900. I think I hit all possible combinations with the above tests.


I won't bore with all the details, but will hit upon the notables. First, on Maha discharges, I noted the same strange occurrence of initial voltages varying widely, then over time some voltages rising and some falling under load. For cells that happen to fall at the low end, this may cause the Maha to finish discharge even while up to 75% actual capacity remains (as determined on the BC-900 after "DONE" on the Maha). Finally, as with the AAA cell tests, one of the Maha's had another brain fart during the AA tests. After the BC-900 discharge on test # 4, I put all eight cells in the Maha's to charge @ 500 mA. One cell "terminated" at 1.37 volts with only 978 mAh "into" the cell. The remaining seven continued to completion two hours later. The one which prematurely terminated was immediately placed into the BC-900 @ 500 mA, and it continued charging for about another two hours before terminating. Why did the Maha terminate the charge on that one cell only half way through charging?


So, I'm just throwing this all out for comments and thoughts. From my tests, I learned that if I expect to get discharge capacities to agree between Maha and BC-900, the Maha does need that at least 2 hour top-off after "DONE" (in which case, the results are in close agreement). I learned that the Maha needs an even bigger display, as when the Maha says "DONE" it should really display "Maybe DONE, Maybe NOT." Or, "Nebulously DONE." Finally, in addition to finishing discharges before the cell is actually discharged, the Maha also terminated a charge only half way through the charge process. Why? Granted, both cases of gross misconduct cited occurred on the same individual Maha charger, but I still see the voltage swings occurring on both chargers, and I feel it is only a matter of time before both chargers will indicate "DONE" before it is time. I wanted to love these, but I just do not trust them, so they will be going back. But I'll wait until I hear any thoughts on this. Anybody else had any of these unusual occurrences happen to them on their Maha's?

[jhellwig]

Mybe you just got a bumm charger. Was it both or just one? Contact Maha and see what they say.

Maybe you bc900's are wrong. Maybe the batteries are bad. There are just to many things to be wrong.

[Black Rose]

I don't have a BC900 to compare against, but I'll toss a set of 4 AA Eneloops on the C9000 to discharge and watch the voltages and see what it says.

BTW, what is the "serial number" of your C9000's?

EDIT #1: Immediately after putting the 4 Eneloops in 500 mA discharge mode, all 4 are showing 1.21 volts.

EDIT #2: After 1 hour in 500 mA discharge mode, all 4 Eneloops are now showing 1.17 volts, so they are dropping at similar rates across all channels.

[Mr Happy]

Anybody else had any of these unusual occurrences happen to them on their Maha's?

Your results are very strange and perplexing. I wonder if your units are faulty somehow, or if you are getting a bad connection with the cells?

I only have one C9000, but I have never seen it do the kind of things you describe. If I discharge 4 balanced Eneloops I always see discharge voltages that start out the same and which match to within +/- 0.01 V or so during the beginning and main part of the discharge. They vary a bit more at the very end of the discharge when all the cells are racing down towards 0.9 V, but not at the beginning.

Likewise, I have never seen a premature end of charge (with AA cells). With Eneloops the charge always terminates at the high voltage threshold of 1.47 V (which is why you need the top-off for a full charge). This failure to charge all the way to the -dV signal could be considered a shortcoming of the charger, but it is quite systematic and repeatable, and not in any way random.

The C9000 is very sensitive to the electrical connection though, which is why I wonder about connection problems. The slightest contact problem will cause the charger to misbehave. For instance, the connector design for AAA cells is not very good, and I find I need to insert AAA cells very carefully to ensure they charge and discharge properly. Occasionally I have had AAA cells abort part way through, especially ones where the wrapper overlaps the negative end. This is undeniably another shortcoming of the C9000. Maha really should design the AAA cell contacts better.

Overall though, my unit has been consistent and reliable. Since you are having so many problems, I think it would be worth contacting Maha for support and warranty service before giving up entirely.

[Edit: I've just remembered some people have reported contact problems with AA cells, where the positive button is too short to make good contact with the metal contact spring. The shoulder of the battery holds it just too far away from the contact. I think some people have described shaving away a bit of the plastic around the charger contact to bring the cell closer to the spring, or bending the spring slightly so that it is further forward. Some of the older Eneloops in particular have had this problem due to their shorter button height.]

[SilverFox]

Hello Turbo DV8,

If I may offer some comments...

You started with brand new cells and began to discharge them. Do you realize that the BC-900 and C9000 use different pulse currents for discharge? I believe the BC-900 uses 500 mA and the C9000 uses 1000 mA.

The cell voltages under these different loads will be different. If you check the NiMh shoot out thread you can see the voltage differences with the Eneloop AAA cells at 500 mA and 1000 mA of discharge current.

One of the down sides to the low self discharge chemistry is that when the cell is stored for an extended period of time, its internal resistance increases. It is not uncommon to see voltages rise during discharging as a result of this.

Once again the higher current pulses of the C9000 will amplify the higher internal resistance of the cell. This is evident by the cell quickly dropping to 0.9 volts and still having some capacity left when discharged at a lower rate.

The next issue I have is that it is not representative to run tests on cells that haven't been broken in and have a few cycles on them. New cells often false peak and end up showing strange behavior during the first couple of charge/discharge cycles. We have been very fortunate with the Eneloop cells, but the longer they sit on the shelf the more they behave like all of the other NiMh cells. By the way, is there a date code on the cells? Also, what was the open circuit voltage of the cells before you started testing them?

The standard procedure when you run into unexpected behavior with NiMh cells is to run a "standard charge" and "standard discharge" cycle on them, followed by a couple of 0.5C charge/discharge cycles. Now you have cells that are properly broken in and you can run tests to determine if you still have strange behavior.

I might add that "out of the box" performance is interesting, especially with the low self discharge cells, but it is not representative of cell performance. It often takes more time to prepare the cells for testing than it does to actually run the tests. I am constantly testing cells and battery packs. In order to maintain consistency between tests I always follow a break in procedure. Sometimes there is no difference between the initial results and the results after breaking in, but when the cells are somewhat aged, the break in procedure seems to level the playing field and brings the cells back up to optimum performance.

When comparing the performance of the two chargers at the end of the charge (without the top off charge), keep in mind that the BC-900 terminates at 1.52 volts and the C9000 terminates at 1.47 volts. With AA cells and charging at 1000 mA, the C9000 beat out the BC-900 by a small amount.

OK, where do you go from here...

I would suggest that you discharge your new cells at 400 mA, then do a Break-In cycle on them in the C9000. Once you have completed that, do 3 - 5 charge/discharge cycles charging and discharging at 0.5C. Now your cells are ready for testing, and you can proceed to run your comparisons between the BC-900 and the C9000.

Tom

[45/70]

I have some experience with both the C-9000 and the BC-900. One thing that comes to mind about Turbo DV8's discharge anomalies, is the fact that the C-9000 discharges @ 1 Amp, regardless of the discharge setting, because the C-9000 uses PWM. I'm not certain, but I believe the BC-900 uses constant current during discharge. Perhaps this is part of the reason why the results were so different?

As for the other noted problems Turbo DV8 experienced, I'm not too sure. Perhaps Mr H's suggestion of poor contact is the culprit.

My biggest complaint about the C-9000 would be the poor way it handles the internal resistance test of AAA's. AAA NiMH cells have a higher internal resistance than AA's when they are new. After some normal wear, they of course develop even higher internal resistance, causing the C-9000 to reject them prematurely. It seems like Maha could have made some arrangement to handle AAA's better in this regard.

There are of course, some other minor inconveniences about the C-9000, like the 2 hour "top off" charge for example, but using it as an analyzer, they're really not that much of a problem, for me.

I've mentioned before, I think the BC-900 is really, in a lot of ways, a better charger. For battery maintenance etc. though, the C-9000 is hard to beat.

Dave

Humm, I see Tom has beat me. I'm just too slow.

[Turbo DV8]

Thanks for all the thoughts. When I add it all up, I guess the Maha is just not for me.

One of the down sides to the low self discharge chemistry is that when the cell is stored for an extended period of time, its internal resistance increases. It is not uncommon to see voltages rise during discharging as a result of this.

I did not know that! Mostly because I have never seen voltage under load go up on my BC-900's... new cells or old.

Once again the higher current pulses of the C9000 will amplify the higher internal resistance of the cell. This is evident by the cell quickly dropping to 0.9 volts and still having some capacity left when discharged at a lower rate.

That makes plenty of sense. But on the other hand, I am left with the fact that my BC-900's never ended a discharge prematurely on new cells, even Eneloops that were manufactured two years earlier. I definitely hear what you're saying about the Maha current pulses at 1000 mA instead of 500 mA. But in my mind, I don't know, I guess my thoughts are if I set the discharge current on both machines at 500 mA, and one rejects cells that are good and the other doesn't, well then I sort of consider that a defective design.

The next issue I have is that it is not representative to run tests on cells that haven't been broken in and have a few cycles on them. The longer they sit on the shelf the more they behave like all of the other NiMh cells. Sometimes there is no difference between the initial results and the results after breaking in, but when the cells are somewhat aged, the break in procedure seems to level the playing field and brings the cells back up to optimum performance. By the way, is there a date code on the cells?

The new AAA cells were all dated April 2008. Not certain I consider that aged to imperfection. Also, I may not have made it clear, but keep in mind that all the tests on the eight AA cells were done on seasoned, known good cells, including the one that prematurely terminated only half way through the charge.

I would suggest that you discharge your new cells at 400 mA, then do a Break-In cycle on them in the C9000. Once you have completed that, do 3 - 5 charge/discharge cycles charging and discharging at 0.5C. Now your cells are ready for testing, and you can proceed to run your comparisons between the BC-900 and the C9000.

I could definitely do that, but you know, I'm really cringing here with the notion that one would have to discharge at this rate, do a break-in cycle, then do 3-5 charge/discharge cycles @ 0.5C before the cells are even ready for testing. I mean, that's pretty nebulous. Don't take this as being smart-ass, but how do I know they're "ready" after 3-5 cycles? Why not 5-8 cycles? I am not trying to be rude, but really, I can't see the necessity in all that ritualistic voodoo when I know the BC-900 would run the tests with aplomb the first time, every time, no questions asked, and no prior pomp and circumstance needed.

Then, I'm still left with that known-good, seasoned AA cell which the Maha decided was done charging at 1.37 volts/978 mAh. The point was made about the contacts having a problem with AAA cells with wrappers extending onto the negative contact, or some AA cells positive nipples not making good contact, suggestions to shave the plastic of the charger, bending the spring... Come on! What can contacting Maha support and warranty service do for a defective design? What can they do for me to make their units less picky about internal resistance, or whatever causes their charger to stumble on a simple task that my BC-900's have performed hundreds of times for me with nary a hiccup? Please don't anybody take my tongue-in-cheek comments as being anything but appreciative of your comments and suggestions, but you can sense my amazement at how much forgiveness I sense the Maha receives here!

Again, thanks for the suggestions, and also the education. I did learn some things, and as usual, I can blame SilverFox for some of that! But, taking all into consideration, I just don't trust the Maha when, on the other hand, the BC-900 never disappoints...

...until it melts down.

[Mr Happy]

I still feel you have somehow ended up with two faulty samples of the charger. I hear your doubts about contacting Maha for warranty support if the basic design is faulty, but unless people do complain Maha will not get the necessary customer feedback to fix it.

There is something I can do to give you a reference point. I have an unopened pack of Duracell Pre-Charged AAA that I bought a few months ago. Tomorrow morning I will measure the open circuit voltages and then put them on a discharge at 500 mA. I will make a note of the voltages during discharge and the final capacity readings and let you know what happens.

[Black Rose]

But, taking all into consideration, I just don't trust the Maha when, on the other hand, the BC-900 never disappoints...

...until it melts down.

...and that's the #1 reason why I can't trust the LaCrosse BC-900 and don't have one.

If it didn't have that nasty feature of possibly turning into a puddle of melted plastic, I'd have one.

[SilverFox]

Hello Turbo DV8,

I agree with you. It seems the best thing for you to do is to send the C9000 chargers back...

I still think that what you observed has everything to do with the cells and does not indicate problems with the chargers. The reason I hold this position is because I have observed the very same things with many different cells (including Eneloop cells) and it is not unusual for a charger to terminate the charge on a false termination signal.

I have observed missed terminations on all of the chargers I have used, including my Schulze and my BC-900. Newer cells will false peak giving an incomplete charge and older cells will never peak resulting in over charging, and sometimes visa versa. Take the same cell, after it did not terminate properly, and it will do fine on the next charge. I don't know why, it just happens from time to time. Fortunately, it is not a regular occurrence.

What you refer to as "ritualistic voodoo," I call "conditioning."

You described unusual things happening during charging. Since I have observed those same things before, and have identified them as being cell issues rather than charger issues (in most cases), I gave you instructions on how to condition your cells. After conditioning, you could run the tests again eliminating the cells as a variable. This involves a lot of time and effort, and I can fully understand why it wouldn't be a satisfactory solution for you.

However, I wonder what your comments would have been if you had done the charger comparison with "conditioned" cells and the understanding that missed terminations are simply a fact of charging...

At any rate, when you do a "standard charge" and "standard discharge" you get a capacity for the cell. When you run the 3 - 5 cycles, you compare your discharge results with those obtained from the "standard discharge." You are looking for similar results, and know that the cells are "ready" when the the results stabilize.

I might also point out that Sanyo, the manufacturer of the Eneloop cells, recommends charging at 0.5-1.0C. They give performance curves after charging the cells at 1C. They recommend 1C charging when using a change in cell temperature to terminate the charge, or when using -dV to terminate. Since both the BC-900 and the C9000 use -dV, you may have observed different results had you conducted your tests following Sanyo's recommendations.

If I may offer a suggestion... Before you send the C9000 units back you may want to consider doing a "Break-In" cycle on your "seasoned" AA cells to "condition" them...

AND, I sincerely hope that your BC-900 never melts down.

Tom

[Mr Happy]

OK, here are the results of my test with the four "Duraloop" AAA cells.

Open circuit volts
1.299 1.300 1.298 1.300

First I tried a discharge at 500 mA.
volts after one minute
1.09 Done Done 1.10

Clearly these cells were in no condition for a 500 mA discharge. The voltage plummeted and the two middle cells stopped discharging straight away.

I pulled the cells and switched to a discharge at 200 mA.
This proceeded in a stable manner:
volts after 3 mins
1.12 1.14 1.09 1.11
volts after 15 mins
1.12 1.13 1.08 1.10
volts after 45 mins
1.13 1.13 1.09 1.11
volts after 90 mins
1.13 1.13 1.09 1.11

It looks like the 3rd cell is a bit weak compared to the other three. The discharge continues, and I will report the recorded capacities when it finishes.

I think these cells need a bit of conditioning as advised by SilverFox. I hope the 3rd cell will wake up a bit after a break-in. Once I have exercised them a bit, I will try again to see if they can manage a 500 mA discharge.

Perhaps we can say that the C9000 is a very rigorous and sensitive instrument? It would be a good thing if it can pick out weak and poorly conditioned cells that you otherwise might not suspect as being bad.

Update #1

The discharge completed and this was the state of charge recorded:
mAh at end of discharge
564 585 575 574
volts after resting
1.21 1.20 1.21 1.20

The readings between cells are more or less evenly balanced, with an average charge 72% of the label capacity.

I am now putting them on a break-in charge set at 800 mAh:
volts at start of break-in charge
1.26 1.26 1.26 1.26

[SilverFox]

Hello Mr Happy,

If I am reading your results correctly... it appears that you also saw a couple of cases of voltage increase during the discharge.

I will go a step further. The C9000 not only is aggressive at weeding out marginal cells, but it also offers the functions needed to give the cell every chance of recovering and returning to optimum performance.

Tom

Edit, to add: While the C9000 offers may advanced features and does a good job, it is far from being perfect. I still find myself "pushing" it into more advanced functions. Perhaps we can hope for an even more advanced charger from Maha in the future...

[Turbo DV8]

What you refer to as "ritualistic voodoo," I call "conditioning."

Nope, it's right there on my displays:

MODE
CHARGE
REFRESH/ANALYZE
BREAK-IN
DISCHG
CYCLE
RITUALISTIC VOODOO

[pobox1475]

I recently opened a 4-pack of Monster Power 1800's that I purchased about 3-4 years ago. I have run two back-to-back break in cycles after first discharging them. The results after each were an average of 1280 mAh. Can anything be done to improve their capacity? I know the rating of most cells is a little optimistic, but -30% seems rather poor.

[SilverFox]

Hello Turbo DV8,



Tom

[SilverFox]

Hello Pobox,

I have seen off brand cells in the 1400 - 1800 mAh range that come in at about 70% of their labeled capacity when new.

The key to your cells health is the voltage under load. If you watch the discharge, note the voltage when you are at 600 mAh of discharge. This is about half way through the discharge. Under a 500 mA discharge load, on the C9000, you should see voltages at 1.2 or 1.19 volts. If your voltage is much below that, the cells are crap.

Tom

[Mr Happy]

I recently opened a 4-pack of Monster Power 1800's the I purchased about 3-4 years ago. I have run two back-to-back break in cycles after first discharging them. The results after each were an average of 1280 mAh. Can anything be done to improve their capacity? I know the rating of most cells is a little optimistic, but -30% seems rather poor.

There may not be much you can do to rescue them. Regular (non-LSD) NiMH cells do not have a long shelf life. In an ideal world they should be used and refreshed within a year of manufacture to keep them healthy. After 3-4 years unopened and untouched, they may have suffered chemical degradation that cannot be reversed.

For future reference if you have other NiMH cells, make a note on the calendar to refresh them every six months if they are not being cycled through regular use.

[TakeTheActive]

...The key to your cells health is the voltage under load. If you watch the discharge, note the voltage when you are at 600 mAh of discharge. This is about half way through the discharge. Under a 500 mA discharge load, on the C9000, you should see voltages at 1.2 or 1.19 volts. If your voltage is much below that, the cells are crap...

I'm going to re-word your post to confirm that I'm interpreting it correctly:

If you monitor the discharge process, note the voltage when you are at the 0.5C point (i.e. 1000mAh for 2000mAh cells; 640mAh for 1280mAh cells; 350mAh for 700mAh cells; etc...). Under a 0.4C discharge load (i.e. 800mA for 2000mAh cells; 512mA for 1280mAh cells; 280mA for 700mAh cells; etc...) on the C9000, you should see voltages at 1.2 or 1.19 volts. If your voltage is much below that, the cells are crap...

...I have seen off brand cells in the 1400 - 1800 mAh range that come in at about 70% of their labeled capacity when new...

Thus, to determine "crap cells":

• For NEW off-brand cells, after 1 'Break-In' cycle and 3-5 'Refresh & Analyze' cycles, IGNORE the capacity printed on the cells and use the CALCULATED capacity from the C9000 for the above "crap" determination.
  .
• For AGED brand-name cells, use the capacity printed on the cells.
  - Another indicator is when the CALCULATED capacity from the C9000 is below 80% of the capacity printed on the cells.

[SilverFox]

Hello TakeTheActive,

The 1.2 volts under load is specifically for AA cells at or under 2000 mAh capacity. Cells of higher capacity tend to have higher internal resistance and that causes the voltage to drop a little more.

Technically, this is called "mid point voltage." As you have correctly interpreted, this is the voltage half way through the discharge.

I referenced a 500 mA discharge current because I happen to have data on that rate. This is not necessarily a 0.4C rate, its just 500 mA, regardless of the cells actual capacity.

Crap cells are defined as cells that:

Have less than 80% of their initial capacity. Sometimes the labeled capacity is close to correct, other times it is simply an "optimistic guess."

When you initially get new cells the first thing you should do is to check their voltages. Cells that are above 1.0 volts can be given a standard charge and the capacity from a standard discharge can be used as a baseline. These cells are generally in good condition.

Cells that arrive below 1.0 volts are suspect. Sometimes they come back and other times they don't. It may take several standard charges and discharges to bring them up to their optimum capacity, and they are suspect for having higher internal resistance, and developing higher rates of self discharge.

The next thing to look at is voltage under load. This is where the mid point voltage comes in. When comparing cells it is good to standardize on a discharge rate, but with varying capacities, this can be a little misleading. I, informally, look at how the cell performs at 500 mA, but formally I look at two different rates. One is 0.5C and the other is 1.0C. If the application involves high currents, I will adjust the discharge current to check the voltage under the load the cell will be seeing, but still formalize on 1.0C voltage under load performance.

The next thing that qualifies a cell as a crap cells is its self discharge rate.

The final thing is its general condition. No rust spots, dents, torn (or melted) label, etc.

Keep in mind that there are others who are less picky about their cells that use some different numbers. For example, some people keep the cell until it is simple below 60% of the labeled capacity, and don't pay attention to mid point voltage. Others simply keep moving the cell to less demanding applications and finally into the drawer for those "just in case a disaster happens" moments.

I am not sure if one way of looking at this is right or wrong, but I know that my way is better. In an emergency, I know that I can grab the cells from my remote thermometer or remote control, and they will work just fine in my high current draw flashlight. Also, if anyone in the household besides me grabs cells to use, I know that they will be vibrant and will give good performance.

Tom

[Turbo DV8]

Hello TakeTheActive,

The 1.2 volts under load is specifically for AA cells at or under 2000 mAh capacity.

What would you suggest the minimum mid-point voltage be for the 800 mAh AAA cells, and taken at what discharge current?

[SilverFox]

Hello Turbo DV8,

Using the C9000 and discharging at 400 mA, you should see voltages of 1.16 or higher. Very good cells will be at 1.17 or 1.18 volts. Once they drop below 1.15, they are well on their way to being crap cells.

The reason for the lower voltages is the higher internal resistance of AAA cells.

Tom

[45/70]

What would you suggest the minimum mid-point voltage be for the 800 mAh AAA cells, and taken at what discharge current?

I sorta look at all this in reverse. The "ideal" voltage to be maintained under load, is the spec voltage for a NiMH cell, or about 1.2 Volts (or above). So, what you are looking for is the maximum current at which the cell (regardless of size or capacity) can maintain 1.2 Volts.

The 1.2 Volt level may be variable to some extent, depending on your particular application, but 1.2 Volts is a good benchmark, as it is a "standard". As I said, this is somewhat dependent on the device you're using. In a hotwire, for example, it is important to keep the cell voltages, under load, at or above 1.2 Volts per cell for best performance. On the other hand, an LED light with a decent boost circuit, may still work well with a cell or cells that won't hold their voltage at 1.2 Volts, at the required current. Still, the cells that are incapable of holding their voltage up as well are not, obviously, as fit for the job, and may better be relegated to use in lower drain devices.

This is my method of determining what constitutes "crap cells". YMMV.

Dave