Circuit ideas- vary load applied to a battery based on DAQ device output?

[davey31415]

I would like to load a battery such that a constant current is being drawn from it.

Can anyone suggest circuits/techniques to do this? 

I've considered using a digital pot controlled by LV/DAQ but would prefer something a bit less expensive. 

It has been some time since I studied solid state devices but I am hoping some sort of MOSFET or BJT circuit would do the trick.  If you know the name of a certain circuit which may do the job, or have an alternative suggestion, please let me know.

Accuracy is not critical.  I'm interested in graphing the discharge characteristics of various NiMH batteries under different conditions.  At the moment I'm switching between fixed resistances using a bank of relays. 

I'm looking for a low-cost solution as this project is more to advance my LV skills and satisfy my curiosity than anything else.

Thanks.

[Wayne.C]

Sounds like you need to build a constant current source with a voltage control.   This kind of circuit could also be called a current to voltage converter.

This app note from Analog Devices should give you some ideas.

http://www.analog.com/static/imported-files/application_notes/AN-968.pdf#xml=http://search.analog.co...

[GovBob]

Sounds like an interesting project an I hope you will post what you come up with.

We've done this same sort of testing using a HP 6050A DC electronic load controlled via GPIB and used the associated LabVIEW drivers. This is not a low cost option.

[Doug_L._Bear]

The PXI-4130 can act as a current sink, you may want to look into that.