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Batteries & Chargers Discuss Li-P, Li-Ion, NiMh, Nicad battery technology and the chargers that juice 'em up!

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Old 03-23-2014, 05:00 PM   #1
Merlotmaker
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Default When is IR too much or too different?

I am flying my 450 helicopter with a set of 6 2.2A 3 cell 11.1V 30C Lipo Battery Packs which I monitor as carefully as possible. Post every flight I measure output voltage, IR of each cell, internal temperature and then I measure these same parameters pre and post charge.

This possibly seems a bit of an overkill but I am able to carefully see if any trends are occurring in any of my battery packs and I believe I have saved my helicopter exactly for this reason as one of my pack failed as I was spooling up the motor - as expected according to my recorded parameters.

I was expecting this failure because the one pack's IR different between the 3 cells was getting progressively further apart and progressively more. IR for the 3 new cells started of at 4,4,1 failed at 18,15,12. Knowing perilously little about battery electronics, I saw this as a sign of pending failure and I was, possibly by coincidence, correct.

But here is the real question.
If this parameter is as important as I think it is, How much difference between the 3 cells is too much? I am currently getting concerned when the max and minimum cell are 50% of each other. Is this too much or too little or am I barking up the totally incorrect tree here?
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Old 03-23-2014, 05:10 PM   #2
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It's not only difference but just as important, the actual value. For a heli, and for this size of battery (2200mAh), if any of the cells are over 12mOhm then the packs probably should be retired as voltage sag will be excessive.

You probably know already but the best way to ensure a long life from your LiPos is to avoid over discharge (around 3.7v per cell when resting minimum) and avoid leaving batteries fully charged while stored.
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Old 03-24-2014, 03:38 AM   #3
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Originally Posted by JetPlaneFlyer View Post
It's not only difference but just as important, the actual value. For a heli, and for this size of battery (2200mAh), if any of the cells are over 12mOhm then the packs probably should be retired as voltage sag will be excessive.

You probably know already but the best way to ensure a long life from your LiPos is to avoid over discharge (around 3.7v per cell when resting minimum) and avoid leaving batteries fully charged while stored.
IMHO, that calculated value of 12 milliohms is not an exact science. But the relative value between cells is important.

As an example, if you're pulling 60 Amps out of your 2.2 Amp Hour battery, your voltage drop due to that calculated 12 mOhm series resistance would be E=IR, or E = 60 X 0.012 X number of cells. That would be 2.88 Volts lost inside your battery pack due to its internal resistance. And, power lost inside that battery pack is P = E X I, or 2.88 times 60 or 172 Watts. That's why those packs can get hot when running current levels of 30C or so.

Down to a more reasonable 4 milliohms, you get 60 X 0.004 ohms per cell X 4 cells or 0.96 Volts lost, with internal power lost at 0.96 X 60 or 57 Watts. Makes a big difference.

The way I calculate internal resistance is by first applying a load current of about 10 Amps, measure the voltage on the battery, and then apply a load current of about 30 Amps, and again measure the voltage on the battery. This is more the "Real World Value" of internal resistance, rather than the AC type of calculation that some LiPo battery chargers use. I've got a big stack of power resistors that can be switched in or out to pull up to 50 Amps on a 6S battery pack. A simpler way would be to use a wattmeter, hooked up to a battery pack/ESC and motor. Run the motor at about 30% throttle, write down voltage and current, and repeat again at 100% throttle, and again write down voltage and current. (It would be difficult on a chopper! But if you have an ESC with a data recording feature such as the Castle Creations line, you can download the information after a days flying.)

The internal resistance would then be R = (Volts at 10 Amps, minus the volts at 30 Amps)/Current at 10 Amps-Current at 30 Amps)

So, if you measure voltage of 10.3 Volts at 35 Amps, and a voltage of 10.9 Volts at 9 Amps, that is R = (10.9-10.3)/(35-9) or 0.023 Ohms total. Divide that 0.023 ohms total by the number of cells to get each cells reading. For four cells that would be 0.023/4 or 0.0057 ohms, which translates to 5.7 Milliohms per cell.

All of this is a lot of work. And, likely won't prove a lot. It's much simpler to hang a wattmeter on your power system, measure volts/amps/watts at full power on your motor with a prop, and write it down on the battery pack. Or, as a minimum, just write down RPM on the pack. Assuming you use the same motor and prop size a few months from now, any battery that is loosing juice will quickly show up on your volts/amps/watt/RPM readings.

As for me, I've been using those over weight, over sized, under voltage A123 cells. Those cells hang in there at the same RPM on the same power system, even after several hundreds of flights. Or, 5 years, which ever comes first.

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Old 03-24-2014, 08:18 AM   #4
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Originally Posted by kyleservicetech View Post
As an example, if you're pulling 60 Amps out of your 2.2 Amp Hour battery, your voltage drop due to that calculated 12 mOhm series resistance would be E=IR, or E = 60 X 0.012 X number of cells. That would be 2.88 Volts lost inside your battery pack due to its internal resistance. And, power lost inside that battery pack is P = E X I, or 2.88 times 60 or 172 Watts. That's why those packs can get hot when running current levels of 30C or so.

Down to a more reasonable 4 milliohms, you get 60 X 0.004 ohms per cell X 4 cells or 0.96 Volts lost, with internal power lost at 0.96 X 60 or 57 Watts. Makes a big difference.
Which is why I suggested 12mOhm as an upper limit for any single cell. Personally I'd probably retire them sooner. In reality you know when thay are ready for retirement because you start noticing the drop in performance and often start to hit LVC before the end of the flight, but LVC is something you really don't want to happen on a helicopter.

If all cells were all 12mOhm then the battery would in reality be more like 15c rated rather than whatever it said on the label. Using a 4mOhm as an upper limit is rather unrealistic because, at 2200mAh cell size, you would often be retiring brand new batteries.
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Old 03-24-2014, 06:16 PM   #5
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Originally Posted by JetPlaneFlyer View Post
Which is why I suggested 12mOhm as an upper limit for any single cell. Personally I'd probably retire them sooner. In reality you know when thay are ready for retirement because you start noticing the drop in performance and often start to hit LVC before the end of the flight, but LVC is something you really don't want to happen on a helicopter.

If all cells were all 12mOhm then the battery would in reality be more like 15c rated rather than whatever it said on the label. Using a 4mOhm as an upper limit is rather unrealistic because, at 2200mAh cell size, you would often be retiring brand new batteries.
Yup
As pointed out, these calculations are nice, but the bottom of the line is, How much RPM is your motor putting out, as compared to a new battery pack? Or, if your pack really went South, as indicated by jetplaneflyer, is the LVC (Low Voltage Cutout) on your ESC cutting power to your motor to soon?

When the performance of that pack is getting lousy, time to retire the battery.

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Old 03-24-2014, 07:43 PM   #6
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Originally Posted by Merlotmaker View Post
I am flying my 450 helicopter with a set of 6 2.2A 3 cell 11.1V 30C Lipo Battery Packs which I monitor as carefully as possible. Post every flight I measure output voltage, IR of each cell, internal temperature and then I measure these same parameters pre and post charge.

This possibly seems a bit of an overkill but I am able to carefully see if any trends are occurring in any of my battery packs and I believe I have saved my helicopter exactly for this reason as one of my pack failed as I was spooling up the motor - as expected according to my recorded parameters.

I was expecting this failure because the one pack's IR different between the 3 cells was getting progressively further apart and progressively more. IR for the 3 new cells started of at 4,4,1 failed at 18,15,12. Knowing perilously little about battery electronics, I saw this as a sign of pending failure and I was, possibly by coincidence, correct.

But here is the real question.
If this parameter is as important as I think it is, How much difference between the 3 cells is too much? I am currently getting concerned when the max and minimum cell are 50% of each other. Is this too much or too little or am I barking up the totally incorrect tree here?
Another option is one of those top of the line chargers such as the Cellpro Powerlab 8 units.

This charger not only can charge your batteries, but when connected to your computer, do a fair job of battery analysis on your batteries.

Take a look:
http://www.wattflyer.com/forums/showthread.php?t=70621

I've also got a West Mountain Radio CBAII battery analyzer that works very well. But this unit does not monitor individual cells during a discharge test.

DennyV
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Old 03-26-2014, 03:29 PM   #7
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Thanks chaps,
That is some very useful information.

Sometimes, just listening to the performance and feeling how the machine flies does really override all of this measurement stuff.

Won't stop me from doing the monitoring though
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Old 03-26-2014, 09:30 PM   #8
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Originally Posted by Merlotmaker View Post
Thanks chaps,
That is some very useful information.

Sometimes, just listening to the performance and feeling how the machine flies does really override all of this measurement stuff.

Won't stop me from doing the monitoring though
Yup, that's the best way to do it!

FYI, if you're ESC allows it, program your ESC for a "Soft" power down on LVC. With this setting, rather than the ESC just shutting off the motor when your battey goes low, the ESC simply reduces power to the motor, while maintaining the LVC minimum voltage.

You might still damage your LiPo, but better the LiPo than the heli.

The Castle Creations ESC's have this feature, along with many other brands of ESC's.

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Old 03-26-2014, 10:32 PM   #9
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I just use either a stand alone Quanum telemetry unit or Frsky rec. with the FLVS sensor. Both give real time cell voltages while you fly. Batteries get "derated" when they sag too much under load or when a single cell starts to sag much lower than the others under load. A battery that used to be good in my EDFs, then gets used only less demanding applications when it longer can take the stress.

Batteries that are hard to balance and/or end up very unbalanced, get recycled.

It's never too late to have a happy childhood.
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