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bq24725a: Charge current to battery lower than requested

John Treubig
Prodigy 220 points
Other Parts Discussed in Thread: BQ24725, BQ24725A

Hi,

Our current design works, yet when our 2P2S LiPO pack is below 20% charge (uses bq20z95 internal), the bq24725 output current falls to 120mA when commanded to 2Amps.  We've got another system that will charge the battery with out issue at this % charge (uses bq24751), so we know it's not the battery design.  The bq20z95 will limit current due to temperature and %charge when the battery is below 5%.

I did find that our designers had placed a 0.1 uF cap between SRP and SRN that I removed.  This change doubled the output current from 65mA to 120mA.  It now mimics the bq24725EVM (0.1 uF on SRN and 0.1 uF between SRN and SRP.  We also have the 10 and 7.5 ohm series resistors in  SRP/SRN.

Once the battery hits about 22%, charging takes off at the SBS commanded current.  I'm puzzled at what could be the issue.

I've also lowered the SRP to SRN cap to 0.01uF with out effect. 

Thanks in advance,

John

  • 0
    TI__Mastermind 20570 points

    bq24751 charge current is set by external resistor not SMBus. Thus it will be no be affected by the presettings in the gauge.

    Please provide the register readings on bq24725A when the issue occurs.

  • 0 in reply to Jing Zou
    Prodigy 220 points
    I understand that the bq24751 is controlled via a resistor divider. This outputs 8.5 volts in that configuration and we've implemented this in the SBS with the register set below:

    ChargerOption (0x12) is not adjusted from TI default
    ChargerCurrent (0x14) is set to 0x08F0 //Equals 2.288 amps
    ChargerVoltage (0x15) is set to 0x2134 //Equals 8.500 volts
  • 0 in reply to John Treubig
    TI__Mastermind 20570 points
    Is this a read from the register when the charge current is 120mA or is this what you write to the register? Some gauge settings can overwrite the register contents when charging based on certain charging condition.