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BQ2002T Vin Range

wanjeri kirubi
Prodigy 160 points
Other Parts Discussed in Thread: BQ2002T, LM317, BQ2002

I have a problem with bq2002T NiMH fast-charge management IC. It does provide enough voltage but not the amp. It started at 120mA at 12 V once I connect it to a 12V power supply, and it will keep dropping to less than 10mA. Is this normal? Obviously, the battery was not charged due to very little juice. I am wondering if I should l charge 12V NiMH battery with more than 12V. I saw the spec saying up to 25 V or 18+Vbat? I did try 24V power supply, the amp started at 1200mA and then kept dropping down. Is this normal?

  • 0
    TI__Guru*** 142860 points

    The bq2002T is a charge controller.  It controls a current source by turning it on or off.  The bq2002T does not control the current but merely turns a current source on or off. 

    You must design your current source to supply the required current.  This requires your Vin to be greater than your maximum pack voltage plus the dropout of your current source (an LM317 for example as on the EVM).

    The EVM has a Vin spec of 10V to 25V.  Another constraint that Vin must meet is at least 4V greater than Vbat but no more than 18V above Vbat.  So, yes, you must use a Vin of at least 16V for your 12V pack.

  • 0 in reply to Chris Glaser
    Prodigy 160 points

    I already use LM317 as the current source, but it looks like the current kept dropping. Right now, the Vin is 24V, and l can see the current kept dropping from 700mA when it was first connected to an empty 12 battery pack, and dropped to around 500mA when it started to trickle charge. Am I correct that l should see constant current tin fast charging?

  • 0 in reply to wanjeri kirubi
    TI__Guru*** 142860 points

    Can you post a schematic?  Is it a 12 cell pack (i.e. ~18V in fast charge)?

    Also, I would measure the Vin and Vpack on the board to account for any voltage drop along the wires connecting everything.

  • 0 in reply to Chris Glaser
    Prodigy 160 points

    I use the same schematic as the EVM for BQ2002 device. The cell pack is 12 V, 17.8 in fast charge. The Vin is 24V, the Vpack in fast charge is 15V. 

    The question is why don't I get a constant current in fast charge? The current just keeps dropping from 700mA in fast charge to 450 mA in trickle charge.

  • 0 in reply to wanjeri kirubi
    TI__Guru*** 142860 points

    What current is the LM317 set to provide?  Could you look at the current with a scope?

    The bq2002 operates by always applying the fast charge current level to the pack.  To get the top off and trickle charge rates, it varies the duty cycle of the applied fast charge current.  It could be that your meter is reading wrong since the current is fluctuating between none and full charge current.  A scope picture will verify this.

    Also, the LM317 does have some thermal protection.  With a 24V to 18V differential at 700 mA, you are dissipating 4.2W, most of which is in the regulator.  This may be tripping its thermal protection and causing it to reduce the charge current.  Reduce your Vin to 21V and see if you still see the reduced current.