BQ25303J: Temperature Resistor calculations

Hi,

If RT1 or RT2 is calculated as negative value, then the particular thermistor does not work and please try another thermistor. 

Thanks,

Ning.

Using the same thermistor as in the datasheet, I need to change the temperature values such that  T1 is -7 degrees, ( or - 5 degrees if -7 does not work)  not as  per the datasheet design.
Please do the calculation for me and advise if the thermistor does not work as I am not convinced that is the problem..
I get a negative value even using the data sheet values so I think the equation is incorrect.
Appreciate help.

Hello,

Our responses will be delayed due to the holidays, sorry for the inconvenience.

Sincerely,

Wyatt Keller

Hello,

Can you provide the part number of the thermistor you are trying to use? As Ning said, if you are only getting negative values for RT1 and RT2 the thermistor may not work for the temperature range you are trying to set.

Sincerely,

Wyatt Keller

Hi Wyatt.
I've done the lower temperature calculation based on the thermistor (103AT-2) used in the data sheet example and I can't get the calculation to work at the lower temperature.

I also cant get the datasheet equation to work based on the actual data sheet numbers so maybe my math is in error and I've tried several ways in excel.

Are you saying that this particular thermistor (103AT-2) won't allow the lower temperature setting?

However, the *Actual* Thermistor fitted to my battery  is R25=10KΩ±5% B25/85=3435±3%
The following are from its table of values:

Standard  battery  temperature setting R T1  (0 °)  27.96KΩ     RT5 (60 °)  3.00KΩ
Required battery Temperature setting R T1  (-5 ° ) 32,09KΩ   RT5 ( 60 °) 3.00KΩ

Ideally a working excel spreadsheet would be of immense value for me to try various temperature settings as this is holding up my project.

Using the spreadsheet for the other device isn't a satisfactory solution as the equations are different.

Have you actually done the calculation to verify the equation? Do you have a spreadsheet that works?

Thanks
Rob

Please see the two equations for the devices. Note how one has VT1 - VT5 and the other has VT5 - VT1

I have tried to use the EXACT equation for the BQ25303 and always get a negative resistance.

Is the equation for the BQ25303 Correct?  Why, if they are 'similar' are the equations constructed so differently. I would expect some uniformity to avoid total confusion.

There is already an uncorrected error on the datasheet regarding the Resistors so I have no confidence in the calculations either way.

Also, when you say the spreadsheet is  'similar' and needs 'minor changes' it is not clear what the changes need to be.

As you are the experts, is it too much to request you make the appropriate amendments to the spreadsheet and make it available so I can be assured the resistor calculations are correct?

Regards

Rob

Hi Rob,

Are you able to get the RT1 and RT2 calculated values ( RT1 = 4.32 kΩ, RT1 = 21 kΩ) on 9.3.2.6 Thermistor Temperature Monitoring using the excel calculator?

Thanks,

Ning.