You don't know til you try . . .

Constant Current Charger/Discharger.

, posted: 16-Jan-2010 10:23

I needed to assess the condition of a lot of Lithium-Ion cells.
While I could have used my regular charger it would have taken 1-4hrs (charge, discharge) per cell.
With 50+ cells to test I needed something better.

Tried various designs for Charger and Discharger.
The discharger was a little more tricky as I wanted relative(to consumer devices) high amps.

There was the LM350(higher amperage version of the LM317) based one.
Straight out of the datasheet.
Copnstant current source = constant current load for the battery.
The dropout was too great at the low voltages I was using.
4.2-3v in, Dropout up to 3v.

Then I tried this one.

It didn't work initially as I was trying to drive it with the battery that was being discharged.
This did not provide enough gate voltage to open up the mosfet

Once I increased the supply voltage it work as expected.
For 2A the gate voltage required 5+ volts.

This is the biggest PCB yet I have made on my mill.
Some issues as I have not sorted out the Z Axis backlash yet.
Depth of cut was all over the place and some tracks didn't make the cut.

A slight change was made to the circuit to allow for micro controller cut off.

8 Channels were made, the number of ADC inputs on most micros.
1 was faulty but I have not been able to find the fault yet.
Nothing fancy, discharge and count the time it takes to reach 3v.
As long as the cells are +-3mins of each other, they are safe to be packed together.

The charger runs much cooler now that I have reduced the input to 6.6v (3.3+3.3 linked PSU) from 12v.
It doesn't have to burn off so much energy.

Note: To link multiple PC PSU's together you have to keep the ground of each separate.
You can do this by making sure the case is not touching or disconnecting the internal PCB from the case.
The case is still earthed so no safety is compromised.

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