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BQ24725AEVM-710: No able to get 8A from the Charger

John Nowocin
Prodigy 30 points
Part Number: BQ24725AEVM-710
Other Parts Discussed in Thread: BQ24725A,

I have the BQ24725AEVM-710 and custom circuit board with the BQ24725A. On both circuits I am able to communicate to them via I2C, set and read the charge options, charge current, charge voltage, and input current. I am testing the charge current capability and want to verify that it can provide up to 8A at any point within the specified voltage range for a 1S battery. To determine and verify the charge current maximum I have the proxy for the battery cell as a programmable load that is able to sink up to 20A.

Test Setup:

  1. Attach power supply to charger input and programmable load to Pack + and Pack - 
  2. Power up the charger board and set the voltage and current limits.
  3. Charger Option 0x12 is set to 0x9932
  4. Increase the programmable load current in 0.010A or 0.100A increments

Other Notes:

  • Ilim on the BQ24725AEVM-710 is 0.75V therefore having an external limit of ~4A and Ilim on the custom board with BQ24725A is >1.6V therefore having the external limit disabled.
  • No load is connected to Vsys, therefore all the input current can be sent to the battery without limiting 

Results:

In both cases as the current is increased the voltage across Pack + and Pack - decreases (as would be expected because the internal circuitry probably just changes resistance). When the current on the programmable load gets to ~1.3A on the BQ24725AEVM-710 it appears the charger stops and voltage drops to 0V. The same concept is true with my BQ24725A custom board which gets to ~2.13A before this occurs.

Questions:

  1. What could be causing the charger to cutoff before the maximum defined/rated 8A?
  2. Could it be with the test setup?
    1. The programmable load acts like a resistor where an increase in current results in a decrease in voltage. Whereas, a battery would have different characteristics (i.e. initial increase in current would increase voltage and a following decrease in current would maintain voltage).
    2. If so, what is the best way to test the ability to achieve 8A without a test battery rated at that range?

  3. Could there be a protection setting that could be disabled?  

  • 0
    TI__Expert 5330 points

    Hi John,

    It is best to use constant voltage mode to simulate the battery instead of constant resistance.  This is a little bit tricky with an eload, but it can be done.  If by any chance you have a bidirectional power supply that can source or sink current, this is the best way to simulate the battery.  (We use Keithley Sourcemeter and Kepco Bipoloar Power Supplies.)  In that case you can just set the bidirectional power supply to your battery voltage.

    But assuming that you don't have a bidirectional power supply available, you can do something similar with the eload in constant voltage mode.  The trick is that you have to "kick start" the system by initially attaching a (single direction) power supply and the eload both to the battery terminal.  You will want to set the power supply voltage just slightly higher than the eload constant voltage, but set the current on the power supply to a low level, such as 100 mA.  Then the eload should pull the power supply down to its CV setting, pulling the 100 mA. 

    At this point you can enable the charging on the BA24725A.  You should see its charging current added to the 100 mA of the power supply, so if you set it to charge at 1A, you should see 1.1 A going into the eLoad.  At this point you can turn off or disconnect the power supply if you want to, and you should see the current into the eLoad matching the value set in the charge current register.  (Be sure if you turn of the PS it is set to enter HIZ and not drive voltage to GND.) 

    Regards,

    Steve