Got a MH-C9000

[Xe54]

Hi:

I also have a few LaCrosse BC-900s. This is my first MH-C9000.

I'm not particulary impressed with the pulsed business. The discharge of LSD (Eneloop) cells is incomplete.

It is also quite a different matter to charge/discharge cells with pulses where the peak current differs dramatically from the average current. It appears the thing uses the voltage while the pulsed discharge is active to determine cutoff. Very peculiar.

The main problem here is consistency of results. One cannot assume that the discharge measurement done with PWM will agree with a constant current, even if the average currents equal.

Further, with the PWM method, there is a challenge of defining a suitable metric for the cutoff voltage. Should it be open circuit voltage, or loaded voltage? We already see what happens with the loaded voltage is used.

Granted, I am a not particularly easy to please electronic instrument designer. So for the price, these consumer chargers with analysis and programmable features are still impressive. I could do something better, but only for several $100s.

I guess if I ever do get to building my "dream charger" project, that it should include both constant current, and pulsed current options. Since particularly for charging, there could be some value to high peak currents, but reasonable average current.

 

[Mr Happy]

However, many applications of NiMH cells use high current pulses (digital cameras) or continuous high drain currents (high power lights). Therefore the pulsed discharge mode of the C9000 is representative of demanding real world devices.

NiMH cells are also very capable of high current discharge by design. I have found very little difference in measured capacity of my eneloops when I compare a discharge test at 1000 mA with a discharge test at 100 mA. In both cases the cell gives results very close to spec.

If I find a cell to test with a low capacity on the C9000 it means the cell is underperforming and is either in need of conditioning or disposal.

There have been other reports of people coming to the C9000 after using the BC900 for a time and finding the C9000 to give poor discharge readings. I sometimes wonder if this is because extensive use of the BC900 at the default charge rate of 200 mA damages cells, and this damage is not seen until exposed to the more demanding test environment of the C9000?

 

[Bones]

There is also this from William Chueu of MahaEnergy:

...

6) Capacity difference between the MH-C9000 and BC-900

BC-900, according to our measurement, tends to overestimate the capacity quite a bit (for instance, the Powerex 2700mAh capacity (1A charge/discharge) typically gets 2750mAh on the BC-900, when our laboratory-grade instrument consistently give about 2550mAh). This has to do with two things: 1) accuracy of the discharge current measurement, and 2) accuracy of the time base. The MH-C9000 has a 1% tolerance in the discharge current measurement. Furthermore, the MH-C9000 uses a quartz oscilattor time base (you can see them if you open the charger, I think there are some photos here on CPF) whereas the BC-900 uses a resistor-capacitor (RC) network as a time base which is less accurate.

I trust the BREAK-IN capacity on the MH-C9000. It typically is within 2% of the capacity measured on a calibrated battery analyzer.

I hope this answers some of the questions raised.

William

 

[bcwang]

I have found very little difference in measured capacity of my eneloops when I compare a discharge test at 1000 mA with a discharge test at 100 mA. In both cases the cell gives results very close to spec.

This is exactly the problem I find with the C9000 with the new firmware. The 100ma discharge rate is terminating like it was a 1000ma discharge because that's what it is using when it measures the cell has dropped below 0.9v during the 1000ma pulse load.

xe54,
I totally agree they need a real constant current discharge to really get the juice out of the cell. If what I read that slow discharge can help break up large crystals, I don't think 1 amp pulses even if averaged to 100ma is really going to act the same as a nice slow 100ma constant current for that purpose. I certainly hope maha releases something in the future with constant current in mind.

 

[Bones]

This is exactly the problem I find with the C9000 with the new firmware. The 100ma discharge rate is terminating like it was a 1000ma discharge because that's what it is using when it measures the cell has dropped below 0.9v during the 1000ma pulse load.

xe54,
I totally agree they need a real constant current discharge to really get the juice out of the cell. If what I read that slow discharge can help break up large crystals, I don't think 1 amp pulses even if averaged to 100ma is really going to act the same as a nice slow 100ma constant current for that purpose. I certainly hope maha releases something in the future with constant current in mind.

If the capacity measurements of the revised MH-C9000 are as accurate as claimed, than its discharge has to fall within the very strict tolerances required by the IEC convention. Wouldn't it therefore have to achieve the same state of discharge as that provided by the original firmware?

And while it would be great to have an option to drain a cell much deeper for extreme rehabilitation purposes, I don't know how feasible it is for a device that has to be priced to compete with the supposedly equivilant offerings from LaCrosse.

I also wonder if it's even advisable in a device that's intended for the average consumer. Imagine the noise when worn-out cells fail to respond and marginal cells are damaged even further from being left in a deeply discharged state.

Incidentally, how solid is our supposition that marginal cells will actually benefit from a deeper discharge than that already provided by the MH-C9000?

 

[bcwang]

I wouldn't say the c9000 meets IEC testing standards. This can be assumed by the simple fact that older firmware and newer firmware versions give different capacity readings already on the same cell, especially one with high internal resistance.

I'm not sure if the IEC standard specifies constant current, but I'd venture a guess that it would be as anything else would be hard to standardize against.

And you are correct, who knows if a deep slow discharge does help the cells. But what the constant current would help is in attaining true capacity under a lowered load to verify if the battery is still good for your intended application.

 

[TorchBoy]

And you are correct, who knows if a deep slow discharge does help the cells. But what the constant current would help is in attaining true capacity under a lowered load to verify if the battery is still good for your intended application.

Are you drawing a distinction between "true capacity" and "relevant capacity"? Surely the "truest" (or most relevant) capacity would subjectively depend on the intended application? Whatever. At least it sets some sort of benchmark. :shrug:

 

[bcwang]

Are you drawing a distinction between "true capacity" and "relevant capacity"? Surely the "truest" (or most relevant) capacity would subjectively depend on the intended application? Whatever. At least it sets some sort of benchmark. :shrug:

I mean true capacity for IEC standards testing. If IEC specifies 1/5 C discharge rate, that should be a fixed standard someone can use to test their cells for IEC rated capacity. The fact that two firmware revisions give completely different capacity ratings on batteries leads me to believe it isn't really an IEC accurate device.

Someone have access to a Cadex 7000 series and know if it uses constant current discharge?

 

[TorchBoy]

Oh that true capacity. Does the IEC standard specify when the discharge should stop? And if it's a voltage condition, should it be a loaded or unloaded battery state? Because of the change made to the C9000 termination (which you reminded me of last week) its algorithm then or now can't be right. (Actually I think it can only approximate the IEC standard anyway, since it's on a 90% duty cycle.)

 

[clintb]

Someone have access to a Cadex 7000 series and know if it uses constant current discharge?

MattK, at batteryjunction.com has one.