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BQ24650 charging at half rated current

TIM FOSTER
Prodigy 60 points
Other Parts Discussed in Thread: BQ24650, BQ2031

I have designed a prototype board using the BQ24650 to charge an SLA battery using the SLUSA75 datasheet and basing the design on the SLVA437A Application note.  I used the circuit in the App note, except that I removed D1 and D3 and eliminated the connection between the upper node of D3 to C2 (We don't want Vin to feed through to our system load).  Vin is 24V.  I also changed Rsr (R6) to be 0.2Ohms so that during the Bulk Charge phase (aka Fast Charge phase) the current will be 40 mV / 0.2 Ohm = 200 mA.  However, when we charged the battery, the voltage across the sense resistor (SRP to SRN) never got above 21 mV.  

So we purchased the BQ24650EVM and per the SLUU444A User's Guide, I changed the sense resistor (R6) to be our 0.2 Ohm resistor.  I partially discharged the battery and recharged it using the EVM and the voltage across the sense resistor was steady at 27 mA for the full charge until it started to taper off to finish charging.  (I also used a clamp type Ammeter around the wire to verify that it was only charging at about 130 mA).  We thought that perhaps it was because I didn't fully discharge the battery, so I discharged the battery all the way, then went through the recharge cycle with the same results, the voltage across the sense resistor never goes to 40mV.  All three datasheets indicate that it should charge at 40mV so what is going on?

The battery under test is a 12V 1.3Ah Panasonic LC-R1213P.

  • 0
    TI__Mastermind 32400 points

    Tim,

    decrease the sense resisitor and see if it makes any difference.

    -Tahar

  • 0 in reply to Tahar Allag
    Prodigy 60 points

    Tahar,

    I decreased the sense resistor from 0.2 Ohm by placing another one in parallel and then I had 40 mV across the sense resistor when charging.  Then I removed one of the resistors (so the sense resistor is 0.2 Ohm) and the voltage across the sense resistor is 27 mV during bulk charge.  It does this both on the eval board and on my board (that I designed and built based on the reference design).

    It does appear that the chip will go through the appropriate charge cycles when charging my 1.3Ah battery with the 0.2 Ohm sense resistor, but at half current.  The Inductor is 10uH, and the output capacitor is 10uF.

    I have also designed my board so that by changing the R, L, and C values I can charge a 100 Ah battery, but this doesn't work at all.  I change the inductor to be 3.3uH, sense resistor to 0.003 Ohms, and the output capacitor to 33uF, but it behaves as if there is not battery connected at all.  Voltage across the sense resistor is 0.  The FETs in the design are AOD4184A, rated at 50A, so that isn't the problem as I am expecting 40mV/0.003 Ohm = 13.3A during bulk charging.

    Any clues as to why this design doesn't work - not even on the EVM?? 

    At this point, does anyone have an alternate chip / design that I could use?

  • 0 in reply to TIM FOSTER
    Intellectual 290 points

    Tim, I am struggling through some things with this same part but I know if the operating range for the TS pin is not observed you will get low to no charging from the BQ24650 because the controller thinks the battery is out of temperature range. Maybe that is a place to start.

  • 0 in reply to James Stafford
    Prodigy 60 points

    I used a resistive divider circuit to make the TS pin happy, per the app note.  I have 5.23k_1% from VREF & TERM_EN to 7.5k_1% to ground.  The midpoint of the divider goes through 100 Ohms into the TS pin.  This is an exact copy of the app note circuit.

    I have given up on this chip and have moved on to the BQ2031.  So far the BQ2031 is behaving as the datasheets and app notes say.