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Original question:

BQ24650: Designing for Lead Acid Battery without using a Thermistor - Testing Stage

BQ24650: slva437a

Prebin Babu
Prodigy 160 points
Part Number: BQ24650

Hi, I have designed a charge controller using BQ24650 for my lead acid battery using details from slva437a. The controller is working fine, but the battery is getting over charged at 17.1 V. I am using a 12V lead acid battery whose fully charged voltage is 14.7. So my questions are 

1) how to bring down the charging voltage ? 

2) the i made the connections as in application note for LED indicators, stat1 and stat2. but its not working

Prebin Babu B

  • 0
    TI__Mastermind 24075 points

    Hi Prebin,

       Charge regulation voltage is VBAT = 2.1*(1+ R2/R1)  section 8.3.1 of BQ24650 datasheet. For lead acid you have to set 2 thresholds, 1 for max charge and 1 for float charge, so you have a FET driven by STAT2, which changes the resistance of the bottom resistor of the resistor feedback to get a different regulation voltage. When FET is driven, your R2 and R1 (here R1 would be equivalent resistance of bottom resistor||with Rp-b) should be configured such that the above equation equates to a VBAT of 14.7V, and R2 and R1 should be configured such that the above equation for VBAT equates to your float charge voltage.

  • 0 in reply to Kedar Manishankar
    Prodigy 160 points

    Thank you sir for the reply. 

    I have designed it to work without battery thermistor to change the CV point. For that i leave out Rp-t. 

    Rt is kept at the same 887K . Rp-b is taken as 4.3M and Rb is 162k. All these are same as app note as in the app note it is designed for a 14.8 v. 

    So, if we are are not using Rp-t,  what all other changes we have to make ??? 

  • 0 in reply to Prebin Babu
    TI__Mastermind 24075 points

    Hi Prebin,

        Explanation would still be same as above. Not using the thermistor to adjust charge voltage will simplify the calculations for the above equations. The app note does its calculations based off of having a parallel NTC (battery thermistor) with Rp-t and takes this into account when calculating Rt, Rp-b and RB. You would not be able to use the same values for the other resistors if you don't want temperature to change the regulation voltage. Please use the above equation and recalculate the correct values you require to accordingly set your float charge voltage and charge regulation voltage.

  • 0 in reply to Kedar Manishankar
    Prodigy 160 points

    Thank you so much sir,

    the circuit is working great with the new changes.

    But there is one more issue, the stat1 and stat2 leds are not glowing. again, the connections are made as per the data sheets. What changes i have to make?

    Other than that the battery is getting charged and the controller is able to keep it on float conditions also. 

  • 0 in reply to Prebin Babu
    TI__Mastermind 24075 points

    Hi Prebin,

        What voltage do you observe on STAT1 and STAT2 when charge is in progress and when charge is complete?

  • 0 in reply to Kedar Manishankar
    Prodigy 160 points

    hai sir,

    At STAT1 the voltage is found to be -2.98V and STAT2 is -1.25V when battery is in floating condition . during charging condition its found to be +.51 across both the leds. 

    is it necessary to keep the leds close to the IC?

  • 0 in reply to Kedar Manishankar
    Prodigy 160 points

    hai sir,

    At STAT1 the voltage is found to be -2.98V and STAT2 is -1.25V when battery is in floating condition . during charging condition its found to be +.51 across both the leds. 

    is it necessary to keep the leds close to the IC?

  • 0 in reply to Prebin Babu
    TI__Mastermind 24075 points

    Hi Prebin,

       When battery floating, STAT1 STAT2 voltages aren't an issue, as 

  • 0 in reply to Kedar Manishankar
    TI__Mastermind 24075 points

    Sorry continuing my earlier statement: When battery floating, STAT1 STAT2 voltages aren't an issue, as the output low saturation voltage from datasheet is 0.5V, so STAT1 STAT2 high in this scenario follows expected operation during battery absent detection. STAT2 voltage when charging is not correct (unless you are somehow in float charge condition during this test condition) and STAT2 should be high (LED not lit) from the datasheet. Most likely there is something wrong with measurement or LED, if charger is able to charge correctly and enter float charge condiiton, as these are based off of STAT1, STAT2 voltages. ALso measure STAT1, STAT2 voltage close to IC w.r.t GND, not across LED. 

  • 0 in reply to Kedar Manishankar
    Prodigy 160 points

    Thank you sir,

    i will test this and update asap. 

  • 0 in reply to Prebin Babu
    TI__Mastermind 24075 points

    Hi Prebin,

        Closing thread for now. Feel free to re-open if issue with BQ24650.

  • 0 in reply to Kedar Manishankar
    Prodigy 160 points

    Thank you sir,

    the issue is been resolved . 

    Prebin Babu B