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BQ24650: Switch off behaviour,

Olrik Maier
Expert 4880 points
Part Number: BQ24650
Other Parts Discussed in Thread: BQ24610, BQ25713

Hi,

we are evaluating BQ24650 on the evaluation board, modified it for a 5 cell application ny changing the feedback resistors R1 and R2 ans some questions came up:

1: We realized if the part comes to full of charge it disables charging but then after 5s it switches on charging again for ~10s, switching off and then on again. This happens several times till it stops charging.

What is the reason for this ?

2: what is the reason for D1 in the typical Application on the first page of the DS ?

- we want to switch off the part completely when no charging is need - that meen Vdd switching off. Can therefore a transistor be used instead of D1 ?

- is a second transistor recommend on the output of the BQ24650 to prevent some "backcurren" from the battery ?

Do we have a alternative part like BQ24610 where the battery can be disconnect from the input and system but like BQ24650 for solar panel application ?

3: is there a more detailed description of the topics 8.3.2 and 8.3.5 available ?

- for the MPPSET is this only happen with solar panels that if they reach their max power level the voltage drops ?

- is there a more detailed description when the part has stopped charging how long it takes till new charging starts again ?

What exact is the voltage of the battery when recharge will start againg. What does it mean with respect to Vreg ?

Many Thanks

Best wishes,

Olrik

  • 0
    TI__Guru**** 158961 points
    Olrik,

    Regarding 1, can you send a schematic and some scope plots showing VBUS, VBAT, IBAT (which could be voltage across the sense resistor)? What is the MPP threshold? If VBUS drops to the MPP threshold, the charge stops charging.

    Regarding 2, D1 is to prevent the battery voltage from appearing at VBUS, through the body diode of Q1, when no VBUS is applied. D1 can be replaced by a transistor. Two transistors with opposing body diodes provide complete isolation between input power, the charger and the system. I am not aware of another standalone charger with MPPSET (VINDPM) and AC FET gate drive. The BQ25713 is host controlled with VINDPM so software could implement MPP function. You can add external PFETs with Zener clamps and some resistor on their gates to implement input power isolation.

    Regarding 3, MPPSET is the same as VINDPM for other chargers. The charger reduces its charge current output, and therefore input current, to prevent the voltage sensed at MPPSET pin from dropping below the MPPSET regulation voltage. This effectively means that the charger regulates its input voltage to the MPPSET point by lowering its input current.

    The charger stops charging when the voltage at VFB pin, sensed from the battery through the FB resistors, reaches the regulation voltage VFB - VRECH (2.1V +/-0.7% - VRECH=35-65mV over temp) AND the charge is below 1/10 of the max current set per the RSNS resistor (+/-25% over temp) for 100ms deglitch+250ms qualification = 350ms.

    Recharge starts when the voltage drops below VFB-VRECH. The "VRECH threshold" in the datasheet is VFB-VRECH.
  • 0 in reply to Jeff F
    Prodigy 10 points

    Hi Jeff,

    thank you for the answers.

    I used the EVM639 board, the only change I made R15 changed from 100k to 60k setup the output voltage to 19.565V ( charging an 18V LI pack ) . For Vin I used PSU ( instead of solar panel) setup 19.7V/1A.

    charge was OK but checking your description once recharge happens if VFB decrease by 50mV it is really sensitive ( if i understand well) once the BQ24650 charging stop battery voltage immediately starts to drop and 50mV converted to the full battery voltage level is 0.467V voltage drop ( might be this is the reason to start to charge again, then stop ( voltage drops ) etc. ). It looks to me too sensitive setup , since it causes unnecessary switch OFF/ON. what do you think?

    do you have any more comment, suggestion

    thankyou

    Mike

  • 0 in reply to Miklos Zoldi
    TI__Mastermind 25800 points
    Hey Mike,

    This is fairly common across battery chargers in the industry. A 450mV drop in the battery voltage (roughly 100mV per cell of a 4S configuration) is not uncommon. Realistically, the end user experience is improved if the battery remains topped off while the power source is applied. And in reality, a good performing battery will not relax that drastically once it has been fully charged. This recharge effect really comes into play when the system and battery self-leakage have drained the battery a fair amount.


    Regards,
    Joel H
  • 0 in reply to Joel Hernandez II
    TI__Guru 54651 points
    Hi,

    The thread is considered solved and closed.

    Thanks!
    Please click "This Resolved my issue" button if this post answer your question.