NiCd Charger with BQ2002* Series Chip (Texas Instruments)

This is a discussion on the Texas Instruments B2002 series NiCd charger IC.

This is just a brief idea on how to use this family of IC's with little hassle and is by far not conclusive to be complete by any means.

I have simplified the use of this IC to a minimal set of components, also arranging PCB layout to allow maximum number of duplicate circuits on the one board. Each individual circuit charges ONE NiCd battery of 1300ma capacity.

Below is the circuit I created for use with any of the BQ2002 family.

Points to consider are,

1. VCC must be 5VDC, +/- 0.5V. The "ground/negative" for VCC is separate from the 5VDC 33A "ground/negative" supply. You ideally need two supplies for clean power.
2. Two ATX power supplies from old computers is excellent choice for powering both input terminals. That is what I am using. I use the 5VDC 33A supply to charge battery and the 5VDC 5A supply to run the circuit. Obviously the details will vary from supply to supply.
3. LED is orientated the way it is (contrary to datasheet), to turn on when charging and to turn off when a problem occurs or charging has completed. Other LED's can of course be easily incorporated anywhere else in the circuit for easier run-time diagnostics.
4. Resistor R3 and R4 is the voltage divider used to set the type of charge termination. Assuming VCC is 5VDC, the divider is set LOW (35mv) for charge rate 1C and Peak Voltage Detection termination. HIGH is (4.5VDC to 5VDC) for Negative delta V termination and MID 2.5VDC (+/- 0.5VDC) for charge rate of C/2 with Peak Voltage Detection. This clears up some contradictions between the evaluation board and the BQ2002 family datasheet.
5. Resistor R5 is set to terminate charge as a safety "switch" once voltage has reached 2VDC for the NiCd battery.
6. Optoisolator U2 is used to isolate the BQ2002 control circuit with that of the power charging circuit. Without this, interference does occur with the switching off of the charge process at the collector of Q2. U2 can be any opto with R8 resistor changed accordingly to limit current to the internal LED.
7. U4 is any power transistor able to handle your desired charge rate of MID, LOW or HIGH. Here it is a TIP120 darlington operating at 1300ma, the 1C charge rate. Change the R7 resistor value feeding the Base if you have a different transistor.
8. D2 is just stopping battery "power" from surging back into the circuit. Remember to calibrate the correct resistor value at U3 voltage/current regulator, after taking into account the drop across this diode. U3 can be any current regulator...just change R1, C2 and C3 accordingly.

Below is the PCB layout ideal for my purpose of maximum compact layout on a rectangular PC board. I will save more room by vertically laying out the resistors relative to the PCB board. The T0222 cases to the far right are located for easy attachment of low wattage heat sinks. The lighter grey copper traces are either insulated wire or trace bottom side of board. This is the view from the TOP side.

Control circuit is left of U2 and the power side is to the right of it.

The NiCd jumper goes to your battery to be charged. The positive output label did not show up nicely, so use the electrolytic capacitor C3 to determine polarity at the 5VDC 33A jumper and the output of Diode D2 feeds to the positive teminal.

This concludes my quick description of a cheap and versatile IC. I made it brief with many assumptions made about the prospective audience, so any questions...do let me have it

I wrote this short writeup as there is little detail on correct schematics available for this BQ2002 charger IC on the WWW, especially on ones for a single battery.