Sep 02, 2012, 06:37 PM
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Joined Feb 2003
157 Posts
Mini-HowTo
Measuring IR (Internal Resistance) with a CB II

You can measure the internal resistance of a battery in the following manner.
Connect a battery to the CB II (computer battery analyzer) and start the test, the current will level off after a short period of time.
Record the voltage and hit pause in the test menu.
The voltage will rise and level off.
Record this voltage.
Subtract the first voltage from the second, this is the voltage drop.
Divide this voltage drop by the current, that is the IR of the battery.
Multiply this by 1000 and you have the IR in Milli ohms.

You can use the balancing port to check each cell, but the reading will be high because of the small diameter wire adds to the resistance.

E=IR or R=E/I
That is resistance = voltage drop divided by current.

Use a higher current to get a more accurate reading when using the power leads.
The first graph is A123 batteries both 1100 and 2200 mah.
The second graph is a 1300 mah lipo through the balancing leads

# Images

Last edited by lquick; Sep 03, 2012 at 08:50 AM. Reason: Make it more clear.
 Sep 03, 2012, 08:02 AM Registered User Joined Feb 2003 157 Posts I welcome any comments, good or bad. Lloyd
 Sep 03, 2012, 09:34 AM That's a funny word NE Ohio Joined Apr 2003 3,720 Posts How do you account for the varying amount of time that the voltage takes to "level off"? Good thinking to use this method. I never would've thought of it. We can conceivably use the duty cycle to plot the IR throughout the whole range of capacity , but not accounting for the varying temps.
Sep 03, 2012, 09:50 AM
Registered User
Joined Feb 2003
157 Posts
Quote:
 Originally Posted by gulio How do you account for the varying amount of time that the voltage takes to "level off"? Good thinking to use this method. I never would've thought of it. We can conceivably use the duty cycle to plot the IR throughout the whole range of capacity , but not accounting for the varying temps.
When you first start the test, the voltage and current change rapidly, so it is hard to take a consistent reading
When the current levels off, it is easy to write down the reading and hit pause.
The voltage then returns to the resting voltage after a while and is easy to write down.

I found out that the IR is quite consistent through out the discharge cycle.
The IR lowers slightly as the battery heats up.

Lloyd
 Sep 03, 2012, 09:54 AM That's a funny word NE Ohio Joined Apr 2003 3,720 Posts What about logging the voltage and time for all cells in a pack while the CBA pulls on the main leads? Maybe could be done with celldiag? and then lined up later? I'd try this , but not enough time. Too busy flying. Maybe when in off-season.
Sep 03, 2012, 09:55 AM
That's a funny word
NE Ohio
Joined Apr 2003
3,720 Posts
Quote:
 Originally Posted by lquick When you first start the test, the voltage and current change rapidly, so it is hard to take a consistent reading When the current levels off, it is easy to write down the reading and hit pause. The voltage then returns to the resting voltage after a while and is easy to write down. I found out that the IR is quite consistent through out the discharge cycle. The IR lowers slightly as the battery heats up. Lloyd
It is the "after a while" that I was asking about because there are times when the voltage rises for 200+ seconds depending upon load and cell IR and temps. I'm not sure if that would throw off the readings.
 Sep 03, 2012, 09:56 AM Southern Pride Haralson County GA. USA Joined Oct 2004 35,074 Posts Years ago I measure IR by first applying a 1C load for 5 seconds and then applying a 10C load for 5 seconds and the use Ohm's Law to calculate IR. One of the problems with Lipolys especially is that IR is very dynamic and charges rapidly with internal heating. There are now many chargers which can supply IR readings but non of the different brands use the same means (loads) or durations. All in all it is much the same as balancing leads,we like this way and that is that. Charles
Sep 03, 2012, 10:02 AM
Registered User
Joined Feb 2003
157 Posts
Quote:
 Originally Posted by gulio How do you account for the varying amount of time that the voltage takes to "level off"? Good thinking to use this method. I never would've thought of it. We can conceivably use the duty cycle to plot the IR throughout the whole range of capacity , but not accounting for the varying temps.

When you hit pause, the discharge stops and you are not measuring IR any more, you are just waiting until the voltage reaches resting voltage to use in the calculation.

Lloyd
Sep 03, 2012, 10:07 AM
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Joined Feb 2003
157 Posts
Quote:
 Originally Posted by gulio It is the "after a while" that I was asking about because there are times when the voltage rises for 200+ seconds depending upon load and cell IR and temps. I'm not sure if that would throw off the readings.
I wait until the voltage stops rising, it seems to level off quite soon,on my batteries.

Lloyd
Sep 03, 2012, 10:22 AM
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Joined Feb 2003
157 Posts
Quote:
 Originally Posted by everydayflyer Years ago I measure IR by first applying a 1C load for 5 seconds and then applying a 10C load for 5 seconds and the use Ohm's Law to calculate IR. One of the problems with Lipolys especially is that IR is very dynamic and charges rapidly with internal heating. There are now many chargers which can supply IR readings but non of the different brands use the same means (loads) or durations. All in all it is much the same as balancing leads,we like this way and that is that. Charles
On the two cell A123 batteries I got 38 mohms,39 mohm,and 41 mohms,as the battery warmed up at 10 amps.

The two cell VPX battery measured 92 mohms at 10 amps.

Lloyd
 Sep 03, 2012, 12:11 PM Southern Pride Haralson County GA. USA Joined Oct 2004 35,074 Posts A123 Systems 2300 cells should be between 7 and 9 mOhms. Either your cells are terrible or the leads / connectors are.
Sep 03, 2012, 12:42 PM
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Joined Feb 2003
157 Posts
Quote:
 Originally Posted by everydayflyer A123 Systems 2300 cells should be between 7 and 9 mOhms. Either your cells are terrible or the leads / connectors are.
Yes they are about 10years old, that is why I was testing them. I noticed that my cheap lipos had much lower IR.

Lloyd
Sep 03, 2012, 04:45 PM
Registered User
Joined Feb 2003
157 Posts
Quote:
 Originally Posted by everydayflyer Years ago I measure IR by first applying a 1C load for 5 seconds and then applying a 10C load for 5 seconds and the use Ohm's Law to calculate IR. One of the problems with Lipolys especially is that IR is very dynamic and charges rapidly with internal heating. There are now many chargers which can supply IR readings but non of the different brands use the same means (loads) or durations. All in all it is much the same as balancing leads,we like this way and that is that. Charles
I used the method above and I get 38 mOhms total for the 2 cell A123 pack.
I put a watt meter in series with the battery and speed control and varied the throttle to get 2 amps and 20 amps.
Works great.

Lloyd