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BQ24650EVM-639: EVM modified for SLA per your app note does not reach final battery voltage??

MikeH
Guru 15580 points
Part Number: BQ24650EVM-639
Other Parts Discussed in Thread: BQ24650

So, as I feared, I spent a lot of time and money on a new prototype design of the SLA-modified EVM circuit as you publish in SLVA437A (Using the bq24650 to Charge a Sealed, Lead-Acid Battery). Since you do not offer this modified design in an EVM, I had to "roll my own".

The design basically works (charger charges the battery), but there is something going on that has me stumped. When the charger is in Constant Current mode, the battery voltage begins to rise as expected. BUT IT NEVER REACHES THE 14.85V BULK CHARGE VOLTAGE. When the voltage does peak (~14.6) THE CHARGE CURRENT BEGINS TO DECLINE. The BQ24650 never switches out of CC mode, but the current begins to decline.

Can someone offer some perspective into what the heck is going on? Why would be current decline while still in CC mode? Why would the battery voltage never reach the 14.85 target?

  • 0
    TI__Mastermind 24075 points

    Hi Mike,

       Can you share the portion of your schematic focusing on the resistors you use for the charge regulation voltage, at bulk and float charge? Above looks like normal operation of CC phase to CV phase, where in CV phase charger is operating by accurately regulating the output voltage, while allowing charge current to taper down till it hits termination. I want to verify what the target output regulation voltage is.

  • 0 in reply to MikeH
    TI__Mastermind 24075 points

    Hi Mike,

       I have verified your calculations for the resistor configuration to set your charge regulation voltage, and they are correct. Can you re-run this charge profile, but can you also plot VFB voltage? BQ24650 senses battery voltage based off of VFB, and charge profile is according to the voltage on this pin, so if there is any voltage drop (ex. impedance between battery and SRN * charge current, forward voltage drop of diode etc) between battery and where BQ24650 senses battery voltage, then BQ24650 could incorrectly sense early end of charge profile.

    From above graph, this is expected charge profile, so let us debug where the voltage discrepancy occurs between set charge regulation voltage, and actual "fully" charged battery voltage.

  • 0 in reply to Kedar Manishankar
    Guru 15580 points

    Kedar,

    I very  much appreciate your taking a closer look. But I recently discovered the LTC4162 which is vastly superior to the BQ2460. I am shelving this design in favor of the LTC chip. I sincerely hope that TI will take a very close look at the LTC chip and come up with a REAL SLA charger design. My impression is that the BQ2460 SLA design is a bit of a hack, trying to force a LiIon design to perform the correct charge profile for an SLA battery.

    Thanks again. Hope to be back in the future.