Dear AAC friends-

I'm attempting to build a simple circuit that will apply a constant voltage to a Lithium-Ion battery. If the applied voltage is greater than the battery, it should source current at a constant voltage, if the applied voltage is less than the battery, it should sink current at a constant voltage.

The circuit I've tried so far is attached in the image(s) below. I like this circuit because it allows me to control the voltage applied just by dialing a pot. It works well for sourcing current (image 1), however, as soon as the applied voltage drops below that of the battery, it flattens out (image 2). That is to say, the circuit cannot sink current at constant voltage.

I would like to build a circuit that can do both, as indicated by the cartoon graph in image 3. I would like to get behavior indicated by the dashed curve.

Does anyone have any suggestions for this?

Thanks!

 

Does anyone have any suggestions for this?

Yes: STOP.

Applying a constant voltage, unlimited current supply to a lithium battery is the best way to make it explode in flames.

ak

 

Sorry- I should have explained in greater detail what I'm trying to do.

You're absolutely right. Applying voltages that are substantially different from the cell voltage would cause a huge spike in current. The goal is actually to first charge (or discharge) a battery at a constant current until a particular voltage limit is reached, and then hold at that voltage. I have accomplished the constant current part with two other circuits that I haven't included in this post. Once the battery has hit the voltage limit, the goal is to hold that voltage until the current decays to a particular value. That is the goal of the circuit I am trying to build here.

In the case of a discharge, for example, we could think of discharging a battery from 3.7V to 3.2V at 50 mA constant current, then holding at 3.2V until the current decays from 50 mA to 10 mA, and then stop. The problem is that with the circuit as designed, I can't sink this current (50 mA down to 10 mA).

 

Found a solution, turns out if you add a pnp transistor to make a half-bridge this circuit can be made bi-directional. (see attached image)

Thanks everyone!