Hi,
I am using bq2954 IC from TI in my charger circuit to charge 2-cells (LiFePO4) in series. I have used the circuit in the following application notes.
I have modified the above circuit to charge 2-cells in series. I have also replaced Q1 to Q4 using TPS2829 high-speed mosfet driver as they have advised in the notes. My inductor value is 15uH for 2 cells, rest everything is similar to the above circuit. My cell characteristics are:
Nominal Voltage per cell: 3.2V
Capacity per cell: 20Ah
When I use it to charge 2 cells in series (the cells are at 3.3V each at this moment, so total is 6.6V), I apply an input of 9V, and I am able to charge it properly as is shown by the correct waveforms.
However, when i reduce each cell voltage to arnd 3V each (6V total), and I apply the 9V to charge them, it seems that it is too much as now the waveforms become distorted showing that charging is not done properly. Then, when I reduce the Vin to arnd 7.8V the waveforms are correct again.
Could someone please explain as to why this is taking place? Why must I change the Vin when the battery voltage changes? Since I am designing a battery charger I must always supply a constant Vin irrespective of the battery pack voltage. How could I solve this problem?
Thanks a lot.
Sumit Bhowmick (sumitbhowmick.7@gmail.com)
