I have to make a Automated battery test bench for my final year project in the university.To perform the battery capacity test i have to make a DC load in order to take constant current from the battery.Anyone have an idea of how to create a DC load to take constant current??
I'm glad if some one can help me..
Thank you..

 

Hello sachcha and welcome to All About Circuits.

Such a project is definitely viable, but we do not hand out answers to academic problems.
We do, however offer some help along the way.

So where are you with the project? What are your thoughts on the matter?

Why, for instance do you want a DC load? This would be unconventional; normally a pulsed system is used to reduce heat dissipation.

You haven't told us if you project is mainly about the automation or the electronics hardware.
Constant current loads are commercially available, have you looked?

 

There are a million different ways to build a constant current load. It depends on how much current and how much voltage across it since that defines power dissipation.

The most important lesson to learn here is that NOBODY can design something until it's characteristics are properly defined. That is always the most important step of any project. Get the information you need to start.

 

Since you used the term "automated", and since you're a senior(?), I think it's reasonable to expect a computer controlled rig allowing infinite control over discharge rate and profile along with unattended data acquisition, eg. voltage recorded every few seconds throughout a test. If you can't use a computer, "automated" gets a lot trickier.

 

Fist of all thank you all for your valuble comments and ideas..

Hello sachcha and welcome to All About Circuits.

Such a project is definitely viable, but we do not hand out answers to academic problems.
We do, however offer some help along the way.

So where are you with the project? What are your thoughts on the matter?

Why, for instance do you want a DC load? This would be unconventional; normally a pulsed system is used to reduce heat dissipation.

You haven't told us if you project is mainly about the automation or the electronics hardware.
Constant current loads are commercially available, have you looked?

I'm just at the beginning of this project the test bench should be capable of performing several test.I have decided to start with the battery capacity test.To perform the capacity test the battery should be discharged with the constant current which is equal to it's nominal capacity/20.For that purpose I want to design a circuit to take constant current from the battery even when the battery voltage changes.

Thank U

There are a million different ways to build a constant current load. It depends on how much current and how much voltage across it since that defines power dissipation.

The most important lesson to learn here is that NOBODY can design something until it's characteristics are properly defined. That is always the most important step of any project. Get the information you need to start.

The current will depend on the battery nominal capacity but the current is between the range of 1 A - 8 A, and the voltage is within the range of 8V-16V.

Maximum current - 8 A
Maximum voltage -16 V

Thank U

Since you used the term "automated", and since you're a senior(?), I think it's reasonable to expect a computer controlled rig allowing infinite control over discharge rate and profile along with unattended data acquisition, eg. voltage recorded every few seconds throughout a test. If you can't use a computer, "automated" gets a lot trickier.

Yes this is computer controlled one

Thank U

 

I have to make a Automated battery test bench for my final year project in the university.To perform the battery capacity test i have to make a DC load in order to take constant current from the battery.Anyone have an idea of how to create a DC load to take constant current??
I'm glad if some one can help me..
Thank you..

Attached is a circuit that I use. The transistor needs to be mounted on a heatsink. The transistor shown is a NTE251, but a 2N6284 is a cheaper substitue. I have also used N channel mosfets. The pot adjusts the current. The value of the resistors in the emitter of the transistor are selected for thecurrent range. With the values shown I can adjust the current from about 75mA to will over 10A.

 

It's worth noting that you probably need a MOSFET to get to 10A? Otherwise you might need as much as 1A or so at the base of Q1, and the op-amp can't get anywhere near that much. Also, with computer control, one could just control the reference voltage applied to the op-amp, replacing the resistor voltage divider and pot.

 

It's worth noting that you probably need a MOSFET to get to 10A? Otherwise you might need as much as 1A or so at the base of Q1, and the op-amp can't get anywhere near that much. Also, with computer control, one could just control the reference voltage applied to the op-amp, replacing the resistor voltage divider and pot.

Thats not true, the transistor is an NPN darlington, and this one has a hfe of 750@ IC of 10A, so the base current would be about 13mA.
I have the circuit working on my bench now and has been working for about 15years.

 

...the transistor is an NPN darlington...

Missed that the first time thru. It's similar to what I use, an old 2N3035 on a chunk of aluminum with a smaller transistor together in a darlington pair.