Hi,
I am currently doing a project where I am developing a series 2-cell (LiFePO4) charger using the bq2954 IC. I am using the circuit available in the bq2954 application notes,
I have modified the above circuit slightly to use it for 2-cells in series. I have replaced Q1 to Q4 with a TPS2829 high-speed mosfet driver. Then, I have modified voltage divider design to make it work for 2-cells in series (in the notes, they use 6 cells) The following are my battery specs:-
Nominal voltage (per cell): 3.2V
Capacity (per cell): 20Ah
Thus, for this 2-cell charger circuit, i have set my charge current as 5A via the current sense resistor, frequency as 200kHz and Vin as 9V. When I bought the cells, they were all at arnd 3.3V. When I initially used my 2-cell charger circuit to charge 2 of these cells (total battery pack voltage for the 2 cells arnd 3.3+3.3=6.6V), i supplied 9V input and i was able to charge them properly (as the MOD waveform was correct at the MOD pin and at the drain of the power mosfet).
However, later i discharged these 2 cells to arnd 3V using an electrical load, and then I applied the same 9V input to charge them. You would expect them to charge normally, but with the reduced battery pack voltage for these 2-cells (now at arnd 3+3=6V total), the Vin applied (9V) was too much, as the MOD waveform became distorted when this 9V was applied at the Vin. So, when I reduced the Vin to arnd 7.8V the MOD waveform became correct again.
Could someone please explain as to what is happening? Why does the input voltage to the charger circuit need to be changed proportionally with the change in battery pack voltage. Becasue when the cells are discharged, i will still have to supply a constant 9V and I cannot keep changing the Vin as the cells get charged.
I will be greatly obliged if someone could help me. Thanks a lot. Do let me know if u have any queries abt my problem.
Regards,
Sumit Bhowmick (sumitbhowmick.7@gmail.com)
