Maha C9000 readouts, can we trust them?

[NiOOH]

Sorry if this has been discussed before, I couldn't find a thread dedicated to this, so here it goes.

Ha anyone noticed that the readouts off the c9000 screen are not correct?
I believe, my c9000 (0G0E01) displays several values incorrectly.

1. The OCV is too low. If I remove a cell from the charger after done and clamp it quickly to one of my DMMs I get about 10-15 mV higher reading
2. During discharge the voltage reading is too low, perhaps 30-50 mV lower. If I discharge the same cell through a 2 ohms resistor monitoring the voltage, I get higher voltages. I don't believe that all of my AA cells discharged at 0.2C read below 1.2V for most of the cycle. When I use a resistor I get values above 1.2 V for 85% of the cycle.
3. Charged capacity do not seem to be calculated properly. When I charge a cell at say, 700 mA, my charger shows a charging current of 701-705 mA. After 60 min, the calculated charged capacity is around 670 mA. Either the charger does not calculate it properly or there is a hidden pause somewhere that is not reflected on the charging current value.

Anyone...

 

[Power Me Up]

Sorry if this has been discussed before, I couldn't find a thread dedicated to this, so here it goes.

Ha anyone noticed that the readouts off the c9000 screen are not correct?
I believe, my c9000 (0G0E01) displays several values incorrectly.

1. The OCV is too low. If I remove a cell from the charger after done and clamp it quickly to one of my DMMs I get about 10-15 mV higher reading

Not sure about this one - sorry.

2. During discharge the voltage reading is too low, perhaps 30-50 mV lower. If I discharge the same cell through a 2 ohms resistor monitoring the voltage, I get higher voltages. I don't believe that all of my AA cells discharged at 0.2C read below 1.2V for most of the cycle. When I use a resistor I get values above 1.2 V for 85% of the cycle.

That would be because the C9000 uses PWM to achieve the programmed rate - the actual discharge rate is 1 amp. You can test this by checking the voltage during discharge at 100mA and compare it to one at 1A - they'll be pretty much the same - the slight difference will be because the cell has a small resting time between discharge pulses which gives the voltage a chance to recover slightly.

3. Charged capacity do not seem to be calculated properly. When I charge a cell at say, 700 mA, my charger shows a charging current of 701-705 mA. After 60 min, the calculated charged capacity is around 670 mA. Either the charger does not calculate it properly or there is a hidden pause somewhere that is not reflected on the charging current value.

Yes - the effective current is always only 90% of the programmed rate. This was due to the original design measuring the voltage whilst the cell was not under load - it used a slight pause for the voltage measurement. Since the new version measures the voltage under load, they could have removed it with the newer versions, but they probably just kept it the same because they couldn't be bothered to change it...

 

[NiOOH]

So, you mean to say that the cell voltage during discharge is measured under 1 Amp load at any programmed rate. This would explain the lower value and the fact that it is the same regardless of the rate used.
Also, to calculate the real discharged capacity of the cell we have to add 10% to what c9000 displays

 

[TorchBoy]

1. The OCV is too low. If I remove a cell from the charger after done and clamp it quickly to one of my DMMs I get about 10-15 mV higher reading

For the price, 1% margin of error (in either instrument) sounds just dandy to me. :thumbsup:

2. During discharge the voltage reading is too low, perhaps 30-50 mV lower. If I discharge the same cell through a 2 ohms resistor monitoring the voltage, I get higher voltages.

Sounds like you're discharging in the C9000 at more than 600mA. Sounds like Power Me Up is right.

I don't believe that all of my AA cells discharged at 0.2C read below 1.2V for most of the cycle. When I use a resistor I get values above 1.2 V for 85% of the cycle.

Hmmm. Maybe you've got something there. Maybe that's why SilverFox gets higher readings than Curious_Character. It's a bit annoying that it's not going to give us an accurate idea of when a torch might drop out of regulation for not having enough volts.

3. Charged capacity do not seem to be calculated properly. When I charge a cell at say, 700 mA, my charger shows a charging current of 701-705 mA. After 60 min, the calculated charged capacity is around 670 mA.

What Power Me Up said.

 

[TorchBoy]

Also, to calculate the real discharged capacity of the cell we have to add 10% to what c9000 displays

No, Will C from Maha has stated that it integrates correctly.

 

[NiOOH]

Hmmm, looks like these problems arise from the fact that there is no clear convention on when the cell charactersistics are recorded. I mean the cell voltage is recorded apparently offline during charge, but discharge voltage is read online, i.e. with 1 A load. I'm still puzzled though. Just finished a quick experiment, connecting a celll to 1.5 ohm 20W resistor and recording the voltage versus time on a DMM, then setting a discharge at 800 mA on the c9000 on the same cell after it has been charged. Still I get lower voltages on the charger readout compare to the resistive load. Mind that I am recording the voltage with the DMM with the load always on. Something just doesn't add up here.
AFAIK my two DMMs are OK, i.e. it's not that they are measuring higher, and they agree with each other, as well as with what my BC900 records as an OCV when a cell is inserted.

 

[Power Me Up]

Also, to calculate the real discharged capacity of the cell we have to add 10% to what c9000 displays

As others have already said, the value is accurate - at least when doing a discharge at 1 amp. When doing a discharge at lower currents, you would expect a slightly higher capacity, but it doesn't because of the fact that it's still really just a 1 amp discharge that happens to be more interrupted.

If you had to add 10% to the reported values, that would be saying that most of my Eneloops have actual capacities of over 2200 mAh!

Discharge capacity doesn't normally vary too much with discharge current for most NiMH cells, but it would make a big difference when it comes to testing alkaline cells. It would also make a big difference when discharging cells with high impedance - I've got some old NiMH cells that won't discharge at all on my newer C9000s but still show a fairly high capacity on my old C9000 which measures the open circuit voltage.

 

[Power Me Up]

Just finished a quick experiment, connecting a celll to 1.5 ohm 20W resistor and recording the voltage versus time on a DMM, then setting a discharge at 800 mA on the c9000 on the same cell after it has been charged. Still I get lower voltages on the charger readout compare to the resistive load.

How much of a voltage difference are you seeing in that case? To truly compare things, you should test with a 1 amp resistive load.

Also take into account that there's probably also a slight voltage drop due to contact resistances, etc.

 

[NiOOH]

How much of a voltage difference are you seeing in that case? To truly compare things, you should test with a 1 amp resistive load.

Also take into account that there's probably also a slight voltage drop due to contact resistances, etc.

About 30-40 mV. It may look small, but that is all the difference between a voltage depressed cell and a healty one. I know that my cells are healty because my lights are bright and my flashguns recharge quickly. Apparently, the voltage readings during discharge should be ignored as a sign of the health of the cells. Also, my c9000 discharges to 0.90 V per cell not 1.00 as stated in the manual.

About contact resistance, I'd believe that the contact resistance on the c9000 is lower than on the setup I'm using. I use 1 AA cell holder soldered to the resistor through a switch, all this in parralel with two female contacts for the DMM probes. Enough contact resistance here too.

 

[Power Me Up]

About 30-40 mV. It may look small, but that is all the difference between a voltage depressed cell and a healty one. I know that my cells are healty because my lights are bright and my flashguns recharge quickly. Apparently, the voltage readings during discharge should be ignored as a sign of the health of the cells.

Maybe, maybe not. If it's consistently 30-40 mV low, you could just take that into consideration.

Perhaps try some unhealthy cells and see what the discharge voltage looks like with them.

Also, my c9000 discharges to 0.90 V per cell not 1.00 as stated in the manual

They just haven't updated the manual since the first version I guess. The original discharged the cells until they had an open circuit voltage of 1.00 - the newer versions discharge until the cell is 0.90 volts under load.

 

[NiOOH]

Let us hear from other 0G... c9000 owners What voltages do you get during discharge?

 

[TorchBoy]

They just haven't updated the manual since the first version I guess. The original discharged the cells until they had an open circuit voltage of 1.00 - the newer versions discharge until the cell is 0.90 volts under load.

Yes. I mean, +1.

 

[TorchBoy]

Let us hear from other 0G... c9000 owners What voltages do you get during discharge?

I don't think I'm the best one to ask. On my 0G0B01:

 

[NiOOH]

I don't think I'm the best one to ask. On my 0G0B01:

Looking at the mode you are in this must be a discharge current? If yes this makes it 18% higher than the set value.

 

[SilverFox]

Hello NiOOH,

I have found the capacity given by the C-9000 as being a little optimistic, but close, but I have not looked at the voltage before.

I ran a check on voltage at three different discharge rates. At 1000 mA, the display shows 1.08 volts and my voltmeter reads 1.13 volts. At 500 mA, the display shows 1.15 volts and my voltmeter reads 1.21 volts. Finally at 100 mA, the display shows 1.15 volts and my voltmeter reads 1.22 volts.

Tom

 

[TorchBoy]

I ran a check on voltage at three different discharge rates. At 1000 mA, the display shows 1.08 volts and my voltmeter reads 1.13 volts. At 500 mA, the display shows 1.15 volts and my voltmeter reads 1.21 volts. Finally at 100 mA, the display shows 1.15 volts and my voltmeter reads 1.22 volts.

So the discharge current set on the C9000 does make a difference to the voltage. But it's still actually 1A?

 

[NiOOH]

Hello NiOOH,

I have found the capacity given by the C-9000 as being a little optimistic, but close, but I have not looked at the voltage before.

I ran a check on voltage at three different discharge rates. At 1000 mA, the display shows 1.08 volts and my voltmeter reads 1.13 volts. At 500 mA, the display shows 1.15 volts and my voltmeter reads 1.21 volts. Finally at 100 mA, the display shows 1.15 volts and my voltmeter reads 1.22 volts.

Tom

That's exactly what I've found too. Haven't checked at differenet rates but at 500 mA my figures are very close to yours as a difference between what the charger and the DMM give. Since it dischargers to 0.90V/cell there should be no risk of underdischarge, if the difference is up to 100 mV. Just to warn the users not to panic if they see too low voltages during discharge.
On another note, has anyone found a way to convert voltage values during internal resistance check to real resistance values. I know that some values have been tabulated. Perhaps William can explain how exactly this check is done. High current pulses have been suggested but for a cell with 20-30 mOhms IR to reach voltages of .45-1.5 V the current needs to be very high indeed.