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Battery Charger for 9V NiCD and NiMH.

Kim Rogers
Prodigy 70 points
Other Parts Discussed in Thread: BQ25171-Q1

I need suggestions for a single chip solution for charging 9V batteries.  The temperature could be as high as 100C.
Looking for a low cost solution.  Not interested in Li ion due to flamability issues.  Please advise your best suggestions.

  • 0
    TI__Genius 10240 points

    Hi Kim,

    Please check out BQ25171-Q1.

    www.ti.com/.../BQ25171-Q1

    Best regards,

    Eric

  • 0
    Prodigy 70 points

    Thanks for the reply, but Did you read my post??  I need to charge 9V batteries.  Not Li batteries.  not 7V NiMH batteries.  I need something that supports 8 cells not 6.  Something that supports NiCD as well.  Also that chip isn't any good at 100C.

  • 0 in reply to Kim Rogers
    TI__Genius 10240 points

    Kim,

    NiMH battery nominal voltage is 1.2V and the fully charged voltage is 1.5V per cell. 9V is not the typical value to correlate 8-cell configuration. 9V is the typical 6-cell NiMH battery fully charged voltage. The cell configuration was not mentioned in your first post. We don’t have a solution that is good for 8-cell NiMH battery at this moment.

    Regards,

    Eric 

  • 0
    Prodigy 70 points

    On the chip it has a pin referenced as /CE  then externally it says host.  What does this mean?  Can it be used without a connection to /CE?

  • 0 in reply to Kim Rogers
    TI__Mastermind 19195 points

    Hello Kim,

    If you do not intend to toggle /CE with a host you can connect the pin to GND. This will leave charging always enabled.

    Best regards,

    Nick

  • 0
    Prodigy 70 points

    Looking for how I determine the values for Iset, TS voltage divider. using a 6 cell NiMH configuration.  I still wished I could find a chip suitable for charging 9V NiCD

  • 0 in reply to Kim Rogers
    TI__Mastermind 19195 points

    Hello Kim,

    You can refer to section 8.1 of the datasheet there is a design example that explains how to select the values. The bq25171-q1 can also work for NiCD.

    Best regards,

    Nick

  • 0 in reply to Nick Brylski
    Prodigy 70 points

    Thanks.  Unless I missed something here is what 8.1 says.
    "8.1 Application Information

    A typical application consists of the device configured as a standalone battery charger for Lithium-Ion, Li-Polymer, LiFePO4, or NiMH batteries. The battery charge profile and the safety charge timer are configured using a pulldown resistor on the CHM_TMR pin. The charge voltage and the number of cells are configured using a pulldown resistor on the VSET pin. Charge current is configured using a pulldown resistor on the ISET pin. A battery thermistor may be connected to the TS pin to allow the device to monitor battery temperature and control charging. Pulling the CE pin high disables the charging function. Charger status is reported via the STAT1 and STAT2 status pins."

    While that explains the why and the how it doesn't explain how the values are derived.  That is what I am looking for.  How to calculate the values.  Nothing in 8.1 explains that.  I have looked over the sample applications, but nothing speaks to how the values are set.

  • 0
    Prodigy 70 points

    Also how do I set Rs and Rp to set the temp value higher as the charge will take place at about 87C.  I appreciate all the input. 

  • 0 in reply to Kim Rogers
    TI__Mastermind 19195 points

    Hello Kim,

    Sorry for the delayed response, for ISET you can refer to this image:

    VSET please use table 7-2 to select the appropriate resistors.

    For Rs and Rp let me look into this more, we have a calculator available but it needs some tweaks to be OK at 87C. What is the charging range you were looking to use here?  

    Thanks,

    Nick

  • 0 in reply to Nick Brylski
    Prodigy 70 points

    The charger would be idle for most of the time.  However when the main circuit fires, it's because the local temp in the box has exceeded 87C.  That would transition the main circuit to operating off the battery instead of a power supply for some period of time.  Once the temp is reduced to some level, say 75-80C then the main circuit would go back to being powered from the supply and the charger would start charging the battery.

  • +1 in reply to Kim Rogers
    TI__Mastermind 19195 points

    Hello Kim,

    See attached the calculator you can refer to. First page contains calculator using the 103AT thermistor that we recommend using. With this thermistor it is not possible to shift the threshold high enough for 86C charging. Adding in Rs/Rp only allow around 10-15C shifting of the thresholds.

    For your use case you may consider a 100k NTC but this would also shift the cold threshold up to over 50C. I added the second page where you can put in custom thermistor values and a typical 100k NTC is plugged in. This would get the charging range between ~50 - 100C.

    Another option is connect an external 10-kΩ resistor from TS to GND, disabling the thermal monitoring.

    TS_design_bq2517x.xlsx

    Best regards,

    Nick

  • +1
    Prodigy 70 points

    it looks like the 10K resistor disabling thermal monitoring is going to need to be the option  thanks for the assist.