BQ25890: BQ25890

Thanks for your kind reply Jeff. 

I am working on a design which had been done by some other engineers. The charger chip is connected to two 1850mA and 2400mA  Li-ion (4.3v) batteries which connected in Parallel. I realized that the batteries don't get charged to their full capacity. Is there a formula which can be used for Battery Capacity vs Safety Timer setting ?

Regards

Ali

Hi Jeff;

One last question. In the design I have the output of  SW node after the inductor (VSYS4_2V) is connected to the STAT LED  (please see the attached Picture) and go nowhere else in the design. The rest of the circuitry such as processor, memories, sensors, etc are powered via V_BAT or the batteries.

There are many designs online which have been done the same way but I have also seen few designs that use the SW  node output to power the rest of their circuitry rather than using V_BAT.  Which would be the best design in terms of  battery charging optimization?

Thanks

Ali

Hi Jeff

I have attached my battery charging profile for your attention below.  From the profile I realized that the CV started few minutes ( about 12 minutes) after the CC had started to drop. 

The charge current is 850mA and Voltage is 4.272V and I have the BQ25890 with two parallel batteries (1850mA and 2400mA) connected to the BAT pins. The System ( Rest of the circuit) is also connected to the BAT pin.

My questions are , whether the two different battery capacity is causing this issue and could this delay have any effect on battery performance?

One of my colleges believes that the drop in battery temperature at CC would cause this delay in CV. Would this also be the cause? 

Thanks

Ali

 

Thanks again Jeff.
One more issue , My circuit uses RT1= 6.8K and RT2=22k which makes Vcold(0c) 64.2% and Vhot (60c)28.10% of VREGN. None of these percentages are listed in page 11 of Bq25890 datasheet. What state of the charge should I expect for these values?

Thanks
Ali

Hi Jeff

Let me explain it this way, The design was done couple of years ago and the Engineer who designed it ( and not with us anymore) chose 6.8K for RT1 and 22k for RT2. R(TH) is 103AT. I used slurax7  spreadsheet to reverse calculate the value of V(T1) and V(T5) for my design when VREGN is 4.8v. I got 64.2% of VREGN for V(T1) at 0C and 28.1% of VREGN for V(T5) at 60C.

Looking at the table in page 11 of BQ2589 datasheet, shows me that my charge should suspend at 73.25% ( 0C)  and also at 34.375% (60C), while my calculated percentage values are something around V(T2_HYS) for V(T!) and the percentage value for V(T5) is not even listed in the table.

My question is what charging behavior I  should expect from the charger when the battery temperature reaches 0c and 60c for my design based on the table in page 11.

Thanks

Ali

103AT-2,3

Ali,

The JEITA compliant temperatures are 0C and 60C and correspond to VT1 and VT5. Choosing different RTHhot and RTHcold thermistor values instead of those at 0C and 60C allows you to change to different charge cutoff temperatures, which is apparently what your predecessor did. You need to find RTHhot and RTHcold and then find the temperature that corresponds to that thermistor resistance. For example, solving the RT2||RTHxx/(RT1+RT2||RTHxx) for VT1% gives RTHcold = RT1 / (1/0.7325-1-RT1/RT2) = 122kohm, which corresponds to -32C cutoff temp for a 103AT thermistor. I solved for RTHhot and get 4.23kohm, which corresponds to 50 C cutoff temp. Please double check my math.

I have never heard of a LiIon battery that can be charged less than 0C so I suggest you revisit that value.

Regards,
Jeff