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BQ25601: battery charger IC BQ25601RTWR TS pin resistor divider calculations for setting the charging cutoff temperature.

Aagneya C
Prodigy 80 points
Part Number: BQ25601

We are currently using the battery charger IC BQ25601RTWR in our design that is used to control the charging and discharging of a 1500mA Lithium ion battery.

we wanted to control the temperature at which the charging is cutoff from 5degC to 50degC but the datasheet only provides the range details from 0degC to 60degC.

the TS pin resistor divider calculations where done for 5degC to 50degC we got the resistor values as 4.7K and 30k

the values for the calculation have been taken are as follows.

VREGN=4.7V

VT1-73.3% of VREGN
T1 (0°C) threshold, Charge suspended T1 below this temperature. Charger suspends charge. As Percentage to VREGN

VT3- 44.7% of VREGN
T3 (45°C) threshold, charge back to ICHG and 4.05V above this temperature. Charger suspends charge. As Percentage to VREGN

BATTERY DETAILS
RTCOLD(5C)- 22.37
RTHOT(50C)- 4.27

RT1-4.7K (APPROXIMATE VALUE)
RT2-30K (APPROXIMATE VALUE)

CALCULATIONS DONE:-

are the above values correct for this calculations of setting the charging temperature range from 5degC to 50degC or have we missed anything?

Also wanted to clarify some value details in the below table.
1)is the below table a standard that we have to follow or will the values vary for different temperature.
2)what does falling in the second column mean?
3)to change to 5degC to 50degC from 0degC to 60degC what are the percentage values that have to be used.
4)what does VT3 state mean for the battery(whether it will charge till the voltage is upto 4.05 and later stop charging or will the charging cutoff when it reaches the temperature,independent of the voltage ?)

PLEASE CONFIRM THE CACUALTION VALUES AND SUGGEST IF ANY CHANGES.

  • 0
    TI__Mastermind 31050 points

    Hello, 

    Thank you for reaching out via E2E. The resistor values you calculated are correct to make the TS cold threshold (T1) =  5degC and the TS warm threshold (T3) = 50degC.

    Based off the description provided in regards to cutting off charge above 50degC it appears you are instead trying to make the TS hot threshold (T5) = 50degC. To make this adjustment please change the VT2 value in the calculation tool to 2.10V (34.2%). 

    Unfortunately, Given the thermistor values associated with 5degC and 50degC you provided there does not exist a resistor divider network that meets the thresholds you are looking for. The closest solution I found is 0degC to 50degC, which can be achieved with RT1 = 7.896 Kohms and RT2 = 105.5 Kohms. Feel free to check if other thresholds close to 5degC to 50degC can be achieved. 

    Please see answers to your other questions below. 

    1) The table attached from the datasheet is the standard the BQ25601 will always follow. The temperatures associated with each threshold (0degC, 10degC, 45degC, and 60degC) are valid assuming the recommended 103AT-2 thermistor and RT1 = 5.23 Kohms and RT2 = 30.9 Kohms are used.  Using different resistor/thermistor components will adjust the temperature value, but the threshold values in the table will not change. 

    2) The rows with the description falling are provided to show that there is hysteresis associated with each threshold. For example, when the TS pin voltage is rising, signifying temperature is dropping, charge will be suspended when TS voltage crosses Vt1 threshold (73.3%). Then when the battery temperature rises charge will be resumed when TS voltage falls below Vt1 falling threshold (71.5%). 

    3)Answer to this question is mentioned above. If you desire charge to be suspended outside of the range of 5degC to 50degC then the percentage values used in calculation should be 73.3% (VT1) and 34.2% (VT5). 

    4)TS voltage in the range between VT3 and VT5 (nominally 45degC to 60degC) is considered a warm battery. JEITA guideline recommends the battery only be charged up to 4.1V instead of the normal regulation voltage set via VREG register (default 4.2V). 

    Let me know if you have any further questions. 

    Best Regards,

    Garrett 

  • 0 in reply to Garrett Kreger
    Prodigy 80 points

    had some quires and observations to share.

    1)I have actually calculated with the VT1 and VT5 percentage values that you have stated above and got the below resistor values for the range of 5C to 50C could you please confirm whether this is accurate.

    2)also calculated for the range that you have suggested and I have seen a mismatch in the resistor values, would like to know the reason for the mismatch?(below table shows the calculation)I think the thermistor which you have taken as reference is different from the one which we have taken.

    New observation!

    =>we have tested with the resistor values RT1=7.68k and RT2=309k (closest values)Which is supposed to cutoff for the temperature range from 5degC to 51degC as per the calculations.

    we see that the charging get cutoff at 5degC lower side but higher side does not cutoff even when the temperature is above 60degC(battery case temp around 54degC). and also after some time it starts to charge again even when the temperature is constant(time duration kept for charging is around 4 hours)

    do you have any inputs on this observation since we expect it to cutoff at 51C.

    Regards,

    Aagneya C

  • 0 in reply to Aagneya C
    TI__Mastermind 31050 points

    Hello Aagneya, 

    After doing additional research into the TS resistor network on this device I found that the range from Cold to Hot needs to be a minimum of 50degC for the device to function as expected. Therefore the various resistor networks for 5degC to 50degC are not viable options. 

    1) I have confirmed that the resistor values you calculated mathematically work. My initial calculations showed a solution did not exist when NTC value at cold and hot = 22.37 Kohms & 4.27 Kohms respectively to match the table you initially provided. Although, this solution will not function as expected. 

    2) The difference between your calculation and the one I provided is the values used for NTC hot and cold. I calculated RT1 = 7.896 Kohms and RT2 = 105.5 Kohms using R_COLD = 27.28 Kohms (0degC value from 103AT datasheet) and R_HOT = 4.27 Kohms (49degC value from your original calculation table). 

    I have checked both sets of resistor values and the behavior on the device should be similar using either solution. 

    3) The range from Cold to Hot needs to be a minimum of 50degC for the device to function as expected. 

    I recommend switching to either your solution or mine for 0degC to 50degC range. You can also try a solution for 5degC to 55degC temperature range if that fits your requirements better. 

    Best Regards,

    Garrett 

  • 0 in reply to Garrett Kreger
    Prodigy 80 points

    Hi Garret,

    thank you very much for the valuable information!

    just wanted some more explanation regarding the minimum 50C range between the RCOLD and RHOT.

    we will be proceeding with 0C to 50C resistor Values for our device.

    Also wanted to know what register bit do we expect the battery charger IC to show when the battery charging is cutoff.(we see that when cutoff happens it shows the bit 64(hex value)(binary 0110 0100 ).

    when charging it shows the hex bit value of 74(binary 0111 0100)

    please provide your inputs on the same.

    Regards,

    Aagneya C

  • 0 in reply to Aagneya C
    TI__Mastermind 31050 points

    Hi Aagneya, 

    Aagneya C said:
    just wanted some more explanation regarding the minimum 50C range between the RCOLD and RHOT.

    This is a general guideline we use for setting the TS resistor divider network. The reason is higher resistor values create higher error in the circuit. RT1 and RT2 solutions for temperature ranges less than 50C generally require a resistor in the range of hundreds of Kohms or are not mathematically possible. Using such large resistors lead to error, such as your earlier observation of HOT temperature cutoff not accurately occurring at 51C.

    Aagneya C said:
    Also wanted to know what register bit do we expect the battery charger IC to show when the battery charging is cutoff.(we see that when cutoff happens it shows the bit 64(hex value)(binary 0110 0100 ).

    I am not sure which specific register you are referring to when you say it changes from 74 hex to 64 hex. On the BQ25601 when charge is cutoff due to battery temperature outside the acceptable range REG09 bits 2:0 will provide the status of the NTC_FAULT (110b = HOT, 101b = COLD) and REG08 bits 4:3 = 00b to indicate IC not charging. 

    Best Regards,

    Garrett

  • 0 in reply to Garrett Kreger
    Prodigy 80 points

    Hi Garret.

    thanks a lot for the inputs!

    we have tested the new resistors at different temperature and below are our observations. kindly go through and provide your valuable feedback.

    OBSERVATION

    =>Tested Battery charging cut-off with RT1 : 7.68k & RT2 : 82.4k (Expected Cut-off at 0°C and 50°C ).

    Battery charging cut-off at -2°C and +51°C as expected ,

    Test summary is below;

    REG09 READING ( JEITA_VALUES ) :

    Condition Expected REG reading09 Observed REG Reading09 REG Reading observed
    Normal 0x00 0x00 5 degree to 45 degree
    Cool 0x03 0x03 Below 5 degree( 5°C to -2°C )
    Warm 0x02 0x02 above 45 degree( 45°C to 51°C )
    Charging STOP at COLD TEMP 0x05 0x05 below  -2 degree
    Charging STOP at HOT TEMP 0x06 0x06 above 51 degree

    REG08 READING ( Charging status ) :

    Condition Expected REG reading0 Observed REG Reading at
    		 Remarks
    Charging 0x74 ( ( 0111 0100 ) -2°C to +51°C Expected behavior
    Not charging 0x00 Observed at 12V supply removed condition only ,
    not observed after charging cut-off

    Expected behavior
    Fully charged 0x7c ( 0111 1100 ) Battery fully charged Expected behavior
    Battery not connected 0x65 ( 0110 0101 ) After charging cut-off :
    below  -2 degree & above 51 degree( most of the times)
    Needed clarification from TI
    Unknown state 0x64( ( 0110 0100 ) below  -2 degree & above 51 degree
    Needed clarification from TI
    Note :
    After cut-off, battery status ( REG08 ) reading : 0x65( Battery Not connected ) and 0x64( unknown state ).

    =>In the above table REG08 reading the "battery not connected" and "unknown state" REG08 bits 4:3 is 00b in both the cases. which one is the expected      one?.

    =>could you please clarify the expected COOL and WARM temperature for the current TS pin settings( 0°C to 50°C )?.

    Regards,

    Aagneya C

  • 0 in reply to Aagneya C
    TI__Mastermind 31050 points

    Hi Aagneya, 

    REG08 = 0x65 hex is expected with charge cut-off due to NTC_FAULT and battery not connected. The NTC_FAULT causes REG08 bits 4:3 to = 00b and the battery being disconnected causes the device to be in VSYSMIN regulation because VBAT < VSYSMIN. 

    The only difference in the unknown state is that now device is not in VSYSMIN regulation (REG08 bit 0 = 0b). This is the expected REG08 reading when charge is cut-off due to NTC_FAULT, but battery voltage is above the SYS_MIN setting. 

    Aagneya C said:
    could you please clarify the expected COOL and WARM temperature for the current TS pin settings( 0°C to 50°C )?.

    The expected temperature will depend on your NTC. The COOL threshold will correspond to when NTC resistance approximately  = 20.35 KOhms (based on 103AT datasheet a temperature between 5°C and 10°C). Then the WARM threshold will correspond to when NTC resistance approximately = 6.71 KOhms (based on 103AT datasheet a temperature between 35°C and 40°C). 

    Best Regards,

    Garrett