I've been developing a charger for a pack of NiMH batteries using the BQ2002T and a switching constant current boost converter. The charge current is 600mA (~C/4 for 2300mAH AA Batteries).
I'm having issues with the charge terminating prematurely. I have the TM pin set to VCC/2, so it should be a 320 minute charge cycle at C/4 with a top off and trickle cycle.
The problem is, after the batteries have been charging for <1 hour, the charge will terminate. I've monitored the TS (temperature) input of the BQ2002, and it stays right around 1.69V for the entire time so I'm pretty sure I can eliminate this as the problem
My initial thought was that the switching converter was causing enough noise on the battery line for the voltage to intermittently spike past 2V and terminate the charge once the cell voltage climbs up a bit, however the charger is going in to Top-Off mode after terminating - verified by seeing that the CC output is high for 286us and low for 4.2ms. Looking at the state diagram for the BQ2002T, it appears that a battery voltage over the MCV of 2V should send the charger to the Trickle State (on for 286us and off for 17.9ms). Looking at the voltage on the BAT pin with the oscilloscope shows intermittent spikes passing MCV (2.0V), but the state diagram seems to indicate that this isn't my problem?
Any help is appreciated! Thanks!