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BQ24650: Designing for Lead Acid Battery without using a Thermistor

Part Number: BQ24650

I was designing a charge controller for the lead-acid battery using BQ24650. As the ambient (atmospheric) temperature here is around 25 to 30 Deg Celsius we want to use the IC without using the thermistor as mentioned in slva437a.

1) What all changes we need to make to the EVM 

2) Is it ok to use without thermistor as the temperature changes slightly around the 25 degrees?

3) Can we give a constant Voltage corresponding to 25 degrees Celsius (from the Thermistor datasheet) across Rp-t?

  • Hi Prebin,

    Thanks for reading my app note!

    Regarding 3, if you do not need the thermistor that changes the CV point, then you can leave out Rp-t.

    Regarding 2, if you also do not need an external thermistor to monitor the battery temperature, then change R9 to 30.1k in order to match R12 and do not install JP1.

    Regarding 1, from the app note, I still recommend adding Q3, R22 and R24, in order to change the CV voltage to the float voltage after termination. There are no footprints for these components on the EVM and so you will need to do some work to make them fit. R10+R13 need to sum to the appnote resistors R23+R13. EVM R15 needs to change to app note R15 value. R6 needs to change the maximum charge current value that you desire. R17 and R19 will also need to change the MPP of your solar panel.

    You might also want to read www.ti.com/.../slva829.pdf. This presents high leakage by the charger in low light conditions when powering from a solar panel.
  • Thank you, sir, for the replay.

    Regarding Answer 2, in slva437a it is mentioned that R9 to be 5.23K and R12 to be 7.5K if we don't want to use the thermistor. Please guide me on what values should be used as R9 and R12.  

  • Hi,

    I mistyped earlier. The appnote is correct regarding sizing R12 to 7.5kohm. You want to size the resistor so that V(TS) is held half way between VHTF and VLTF per d/s page 7.