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BQ24773EVM-540: Gradual decrease in charging current when battery voltage is less than MaxChargeVoltage()

syt93
Prodigy 70 points
Part Number: BQ24773EVM-540

Hi,

I am testing a single cell Li-ion battery with BQ24773EVM. I am using the I2C interface to program the registers. Battery Voltage is 3.685 V and CELL pin is connected to GND. Multimeters are used monitor charging current and battery voltage.

Following are the configuration parameters: 

ChargeOption0_MSB (0x01) = 0x81

ChargeOption0_LSB (0x00) = 0x4E

// ChargeVoltage = 4.224 V

ChargeVoltage_LSB (0x0C) = 0x80

ChargeVoltage_MSB (0x0D) = 0x10

// MinVoltage = 3.584 V

MinVoltage_reg (0x0E) = 0x0C

// ChargeCurrent = 960 mA

ChargeCurrent_LSB (0x0A) = 0xC0

ChargeCurrent_MSB (0x0B) = 0x03

I am reading back every register to make sure that all the parameters are set correctly.

When I start the charging process (0x00[0] = 0), multimeter reading for current is 949 mA. However, as battery voltage increases, current starts to drop slowly (1 mA drop per 3 mV gain approx.). Am I missing something here in terms of configuration or setup? All the power-up parameters (CELLS, REGN etc from EVM user's guide) are at expected voltage levels.

  • 0
    TI__Guru 56650 points
    "ChargeVoltage = 4.224 V" What is Vsrn voltage?
    If the output resistance is too high, the charger may run into CV mode early. You may see the current drop.
  • 0 in reply to Wang5577
    Prodigy 70 points
    At this moment,

    Vsrn = 3.976 V
    Current Reading = 730 mA

    There is only battery directly connected at the output of EVM (between BAT and GND pins of J3) so what else could cause a high output resistance?
    Anyway, you can see that Vsrn is below set ChargeVoltage.
  • 0 in reply to syt93
    TI__Guru 56650 points

    Now, the voltage loop doesn't limit the output current. We can step by step check other loops:

    1. check the charge current loop: Would you verify the SRP and SRN  series resistor value and measure Vsrp-srn voltage on IC side? If the Vsrp-srn = 7.3mV, the issue is not on charge current loop.

    2. check the input current loop: check Vacp-acn value to see if it matches the setting value.