Hi,
I'm currently working on a project that uses the fuel gauge IC mentioned in the subject for our battery management circuit.
The problem that I noticed in our current setup is that every time the bq24040 battery charger IC terminates charging (CHG pin goes to HI-Z), the fuel gauge seem to always fall short in terms of reporting a 100% SOC on the battery and only reports 93% 95% on a full charge.
I made sure that I have properly configured in firmware the bq27421-G1A's basic battery parameters according to the battery that I am using which has a capacity of 180mAh:
Design Capacity = 180mAh
Design Energy = 666
Terminate Voltage = 3V
Taper Rate = 202 (based on a taper current of 8.9mA)
The weird thing though is that if I use the same firmware that handles the configuration of the bq27421-G1A using the BQ27421EVM-G1A and BQ24040EVM evaluation boards and duplicate the battery management circuit of our system by adjusting the trimmer potentiometers and configuring jumpers of the evaluation boards to match our setup, the bq27421-G1A chip of the BQ27421EVM-G1A will always report a correct 100% SOC (using the same re-chargeable battery) between the time when the bq24040 chip of the BQ24040EVM is tapering up to the time it terminates charging which is what I was hoping for our setup to also behave.
The only difference that I noticed with our setup is that the bq27421-G1A's VDD pin was left floating (no 0.47uF capacitor connected between it and the VSS).
I don't have a good background in hardware design (i.e. I am only mostly involved in embedded software design) so I don't know if the missing decoupling capacitor in the fuel gauge's VDD pin has something to do with this problem or if there is some other factor causing the bq27421-G1A's SOC in our setup to always fall short of reporting a 100% charge during a full charge condition.
I would appreciate it if anyone will be able to help me figure out what's causing this problem.
Thanks.
--Louise