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BQ24450 Charging with load on battery

Jeff Starkell
Intellectual 505 points
Other Parts Discussed in Thread: BQ24450

I'm using a BQ24450 to charge a 12V 2AHR battery, that acts as backup when the AC fails. Everything works as it should. Float voltage is set to 13.5V

This circuit has a load connected to battery at all times, that draws about 150mA, occasional drawing up to 350mA for shorty bursts. The DC power supply that powers the BQ24450 can deliver 1A@15V

Will having the load connected to the battery cause any problems for the charge algorithm? The sense resistor (pins 2,3) will obviously see the charge current as well as the load current.

  • 0
    Intellectual 505 points

    Question # 2

    We have an external +15Vdc power supply driving BQ24450 that charges a sealed lead acid battery. Load is connected to the battery terminals so while the +15Vdc is present the battery is maintained at float voltage, and the load is supplied from the +15Vdc supply via the charge path of the BQ24450. If the +15Vdc supply is turned off the battery continues to power the load. All this works as expected.

    End user expects (wants) the output voltage to be available if the battery is not present assuming the 15V supply is present, in other words he wants the ability to operate without the battery i.e use this device as a power supply with no battery backup. If I disconnect the battery (but have the +15Vdc present) the output voltage is maintained at Vfloat as long as there is no load, as soon as I connect a load to the terminals that normally connect to the battery the charge voltage drops to about 2Vdc. I tried connecting a capacitor in place of the absent battery (I tried 1000uF) and this tricks the BQ24450 allowing me to drive the load with no battery (the capacitor seems a suitable proxy for the battery). Is there any reason not to add a capacitor in my circuit in parallel with the battery terminals to allow operation without the battery?

    Is there any reason not to operate with a capacitor acting as the battery?

    FYI I repeated this test with a 100uF capacitor instead of 1000uF and observed that the 1000uF will support a 500mA load, while the 100uF will only support a 300mA load

  • 0 in reply to Jeff Starkell
    TI__Guru 56650 points
    1. bq24450 is a linear charger. Adding 350mA or 100mA load on charge current could increase the charge FET temperature. Also, this 350mA or 100mA system load could make the charging rate change. I don't see any other issue with charger.
    2. On datasheet 3rd page, CURRENT SENSE COMPARATOR gives the common mode voltage range from 2V up. If there is no output capacitor or a low output capacitor, the output voltage quickly drops to 2V or below, the current sense comparator may be wrong. So, it won’t give enough output current. This is just my guessing. This is an old part. I don't have more information.
  • 0 in reply to Wang5577
    Intellectual 505 points

    Wang,

    Perhaps I have a different data sheet, (Feb 2012), I don't see a specification for common mode voltage range for the current sense comparator?

    I'm not sure why the voltage would quickly dip when the Input power supply (15Vdc) is present, but I suppose there is some delay before the Drive pass transistor responds to the load and the large capacitor can cover that period.

    Thank you.