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SLUA533: What is the diode for?

Other Parts Discussed in Thread: BQ24765

Hello,

I am implementing a non-host controlled charger (bq24765), with a bq20z75 fuel gauge. I am using SLUA533 directions.

 

On page 1 of SLUA533, under "Implementations", Step 1, item 3.:

Solder a BAT54 diode between test point Vddp and the BAT pin. The anode of BAT54 is connected to
Vddp.

What is this diode's purpose?

Do I need it in my implementation? My batteries and the charger are integrated (always connected).

 

Thanks!

Mike

  • It is for waking up a deeply discharged battery. Without host and a deeply discharged battery, the bq20z75 losses power and can not send out any command to bq24765. So, the charger system can not start up and is locked in this mode. We have to charge a deeply discharged battery up first. After the battery voltage is higher enough, the bq20z75 can send out commands and the charger system runs normally. 

    The VDDP is 6V/90mA source. It is used as a power source to charge a deeply discharged battery. The BAT54 from VDDP to battery gives a current path to wake up a deeply discharged battery. After battery voltage higher than the minimum voltage of the gas gauge circuit, bq20z75 can send out command to bq24765.

  • Thank you Wang, that does answer my question. I will not need this in my design.

  • Hi There,

    Why does the diode deliver current to the battery pack instead of VCC of the gauge?  If it was connected to the gauge, communications could be restored immediately, allowing the pre-charge current to flow, instead of waiting for the pack to charge up to 4.5V @ 90mA, or am I missing something?  What causes the gauge to shutdown?  Is it low voltage on VCC, or on the cells?  From my testing, it appears to be the cells.  Is this the reason for connection to the pack instead?

    Thank you,

    Jeremy.

  • Hello,

    I'm having trouble with this and would appreciate any suggestions people might have.  I am trying to simulate a pack that has been left on the shelf for years and has self discharged below the operating voltage of the BMS, to answer the question; will the pack start charging again if I connect a charger?

    If I bypass the BMS and discharge the battery down to 4V (below the 4.5V needed for the BMS supply), when I connect the charger, (with diode from VDDP to Pack+ as per SLUA533); communications are restored, but (pre) charging does not commence.  Am I perhaps missing some configuration?

    Thanks,

    Jeremy.

  • Is there anybody out there?

  • Do you read out charge's 0x14H and 0x15H register value?

    Do you modify the pack gas gauge IC to boardcast mode?

  • Yes, I have the gauge in broadcast mode and the charger is being programmed to the appropriate voltage, but with zero current.  Perhaps the problem relates to the gauge losing power when the pack voltage falls below 4.5V, the minimum supply.  Can I assume that the gauge will run properly from this state, or do I need to issue a reset?  I have noticed that the PMS_CHG flag is set, but I can't find any documentation describing it.  I have set ZCHG0 high, ZCHG1 low and have tied the PMS pin to PACK to enable zero-volt precharge, but again, there is very little documentation describing this function.

  • I have found that if I issue a reset (00, word 0041), charging will commence.  Is this to be expected, or is there some configuration missing that will allow charging to start without a reset?

    Thank you,

    Jeremy.