Mechanical Engineer + Software Developer (Project Leader)
· MangaluruIndia
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# Active load/resistor for cell capacity tester

My lithium cell capacity tester uses two 7 ohm, 5W ceramic resistors in parallel, effectively 3.5 ohm.
It should ideally consume 1A of current to measure capacity.

But as the voltage of the cell decreases, the current passing through the cell also decreases, leading to incorrect capacity being measured.

Is there a way to make an active load that consumes 1A irrespective of the cell voltage? I don’t need anything fancy with adjustable knobs and other things. Just need a direct replacement for the two resistors in parallel.

Lithium cell capacity tester

Electrical Engineering
Contributions
Abhi Verma

Abhi explained how rechargeable lithium battery capacity is measured and also why constant current load is not practically possible.

Josh Pinkley

Josh gave an idea about using a home brew constant current circuit.

Ravi Pujar

Ravi gave the idea of using motorized rheostats and MOSFET.

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ROBOToTHON 2017 Mentor, Electronics Engineer
· DelhiIndia

hahaha… then it’s keeping track of time. it will be 0.85 amp at 3v. I = 3 v/3.5 R , 0.85Amp, my calculations was correct 😉.
See even if it tracks time or not, it will be a approx value only, due to constant change in battery’s parameters.
I’ll test my idea above whenever get back to my home.

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Mechanical Engineer + Software Developer (Project Leader)
· MangaluruIndia

Thanks Abhi Verma

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Embedded systems engineer, Founder @Valetron Systems, Blogger @www.raviyp.com

Yes Harsha, as Abhi said, You need to build a Smart Load for it to reduce resistance when current goes down. You could use motorized potentiometer or rheostat for variation(http://www.onlinecontrols.com/mpots.htm). Again you will need to do some programming for this to control the motor.
A P Channel MOSFET could also be used if you are able to control its Internal resistance programatically.

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Ravi gave the idea of using motorized rheostats and MOSFET.

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Mechanical Engineer + Software Developer (Project Leader)
· MangaluruIndia

Thanks Ravi, but I know nothing about them. :P

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· FredericktownU.S.

Would a constant current load work?

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Josh gave an idea about using a home brew constant current circuit.

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Mechanical Engineer + Software Developer (Project Leader)
· MangaluruIndia

Thanks Josh Pinkley :)

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