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Battery Characterization

Model 2651A
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shoverbj
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Joined: February 26th, 2014, 1:25 pm
Country: United States

Battery Characterization

Post by shoverbj » October 14th, 2015, 11:45 am

Hello,

I am trying to develop a battery characterization program using a 2651a and LabView. I read through Application Note 3221 as kind of a baseline and have gone forward with some programming and testing based on that paper. I am a bit confused though. During charging, I set the source to above the battery voltage (12.6V on a 3-cell Li-Ion pack) and compliance limit for the current to a set amount and the battery charges at that current until it hits the voltage and my code shuts it down. The problem lies in discharging at a set current. I set the source voltage to below the battery level (9V) and the compliance limit to a set value (say 1.5A), but that compliance limit doesn't seem to be behaving the way I would expect. The current varies approximately linearly with the limit value, but the absolute value of the incoming current is always larger than the limit. For example, with 1.5A my current is reading approximately -2.2A, if I take the current up to 2A the reading is about -2.6 or so. Am I misunderstanding something, or how do I limit this current value?

Thanks
Ben

brian.d.smith
Keithley Applications
Keithley Applications
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Re: Battery Characterization

Post by brian.d.smith » October 15th, 2015, 4:15 am

Did you create a working script in Test Script Builder before you started your LabView VI?

shoverbj
Posts: 8
Joined: February 26th, 2014, 1:25 pm
Country: United States

Re: Battery Characterization

Post by shoverbj » October 15th, 2015, 6:18 am

No, I did not develop a TSP script, but currently I am just trying to do it with the front panel controls (the LabView will come later).

So here are my settings for "charging":
Battery = 3-cell LiIon
Source Voltage = +12.6V
Limit Current = 3.5A
4-wire
Hi-Z off mode (to keep battery from draining)

Under these settings the current measured at 3.4999A and the voltage was rising.

For discharging, my settings were:
Same battery
Source Voltage = +9.0V
Limit Current = 1.0A
4-wire
Hi-Z off mode

Under these setting the voltage was dropping like I would expect, but the measured current was -1.13A. As I raise the limit current, the measured current dropped (further negative) somewhat linearly.

So I guess the crux of the question is, is there a way to set the Limit to a negative value? Is this even the issue or is there a setting that I am missing that will limit this value to a tighter tolerance?

Andrea C
Keithley Applications
Keithley Applications
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Re: Battery Characterization

Post by Andrea C » October 15th, 2015, 7:28 am

Try the sink mode. This will improve the accuracy of the source limiting when operating in Quadrants 2 and 4 (accepting power from external device). The inaccuracy, however, may shift to Quadrants 1 and 3. So between charging and discharging, you may need to turn the sink mode on/off.

Front panel: Config SRC: V-Source: Sink-Mode: Enable

Remote: smua.source.sink = smua.ENABLE

shoverbj
Posts: 8
Joined: February 26th, 2014, 1:25 pm
Country: United States

Re: Battery Characterization

Post by shoverbj » October 15th, 2015, 8:52 am

Thanks Andrea!

That was the missing step. Automating that change when going from charging to discharging shouldn't be very difficult at all.

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