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Battery Management

Shibin Varghese
Expert 1130 points
Other Parts Discussed in Thread: BQ77915, BQ24610, BQ25792, TPS63000, CSD17577Q3A, CSD25402Q3A, BQ24630

Hi Team,
I am looking for a chip-level solution for a 4 cell Li-ion 18650 battery pack.
The device must include overcharging protection and over-discharge protection and the following features
The number of series cells 4.
Effective voltage 4*4.2V=16.8V
Cell balancing
Undervoltage protection
Overvoltage protection
Stand-alone devices will be more appreciable.
I think a single chip alone cannot make this all possible.
But I believe TI do have some solutions for the same
We are okay if the cell monitoring chip is able to make and break the connection to the charger as well as to the load if over voltage or under voltage is detected, and another chip to charge the battery pack.
Looking forward to your valuable reply

  • 0
    TI__Genius 10830 points

    hi Shibin,
    BQ77915 is a 3-5s primary protector with cell balancing. It only works in standalone. Please have a look at it.

     

  • 0 in reply to David Hien
    Expert 1130 points

    Hi David,

    Thank you for your reply.
    Your reply is really helpful.
    But still, I've some doubts.
    Based on a quick reading of BQ77915 is a good battery balancer but the voltage needed to charge a 4 cell battery pack is 4*4.2=16.8V or just above it. So I need a supply of 16.8V but there is no standard charger to give a 16.8V output.

    This circuit will work fine for a single Li-Ion battery. The TP4056 is a charger IC whose output is 4.2V for a range of input from 4.2V to 8V.
    I'm planning for a charger IC whose output is connected to the battery pack through BQ77915  and directly to the load.
    The charging is done through the BQ77915 when input is connected and the battery delivers charge when the input is disconnected so the BQ77915  can control the charging and discharging.

    I think BQ24165RGET alone can do this entire fiction for a single cell 

  • 0 in reply to Shibin Varghese
    TI__Guru*** 146975 points

    Hi Shibin,

    For standalone charger, BQ24610 with external power FETs is an option.

    For I2C host control charge, BQ25792 is an option.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Expert 1130 points

    Hi Jeff,
    Sorry for replying late.
    I've read the datasheet of BQ24610.
    I've some changes to my requirement.

    My doubt is my load is a 12V load so the battery pack must be of 12V means (3*4.2=12.6V).
    So can I use a 12V adapter for the same purpose?
    Also, 

    I believe the charging path for the battery is Adapter-Q1-Q2-RAC-Q4-L1-RSR-Battery.

    The discharging path for the battery is Battery-Q3-System.
    The BQ24610 can also be operated by disabling the power path (figure 24).
    So can I connect the output labeled as pack+&pack- directly as the input of BQ77915 IC so that I can charge all the cells connected in series evenly? 

  • 0 in reply to Shibin Varghese
    TI__Guru*** 146975 points

    Hi Shibin,

    If your load is 12V then you will need a 12V adapter but you cannot fully charge the battery to 12.6V with a 12V adapter.  You would need at least a >12.6V adapter when charging from a buck only charger like BQ24610.  You could then add a buck converter to provide 12V to the system or, a buck-boost from the TPS63000 family that could provide 12V even if when the battery is below 12V.

    The charge path is correct, except Q5 is needed to form the synchronous FET of the buck converter with Q4 as the power FET.  They switch on and off at Duty cycle D= Vbatreg/Vac.

    Regarding BQ77915, I don't support that device so you may want to create a new post about that IC.  Based on my review of the datasheet, it appears that you would need 3 BQ77915, one for each cell.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Expert 1130 points

    Hi Jeff,
    Thank you for your reply.
    I have a doubt which may sound crazy for you.
    Is there any problem if this battery pack is charged to 12V instead of 12.6V?.
    Also, what is the equation for the output voltage for a buck converter?.
    What values do I need to substitute for L1 to get a 12V output out of buck?.
    Also, I believe the current path to the system when the adapter is plugged in is Adapter- Q1-Q2 RAC-System. If this is true if my adapter is of 20V then the voltage across my load will be 20V and if the will be dropped to battery voltage is the adapter is removed.

  • 0 in reply to Shibin Varghese
    TI__Guru*** 146975 points

    Hi Shibin,

    Regarding 12V verus 12.6V, by charging only to 12V, the battery is not charged to full capacity.  The battery will not be damaged but it will likely not have longer life time either as charging only to 12V still counts as a full charge cycle.

    Regarding the equation, VBATREG = (1+Rfbtop/Rfbbottom)*2.1V.  Regarding the inductor, please use the recommended inductor range per the datasheet Table 4 page 31.  The design spreadsheet at the link below should help with those calculations.

    Yes, that VSYS=VAC less the drop across Q1, Q2 and RAC or VBAT when no VAC is applied.

    www.ti.com/.../sluc175

    Regards,

    Jeff 

  • 0 in reply to Jeff F
    Expert 1130 points

    Hi Jeff,
    Thank you for your reply.
    Your reply was really helpful.

  • 0 in reply to Shibin Varghese
    Expert 1130 points

    Hi Jeff,

    In the table inductor, capacitor, and sense resistor as a function of charge current, inductor value is in the range of mH and capacitors are in the range of mF. Is it a clerical mistake or do we really need components of that much value? If needed, when calculating the value of f0, it is in the range of 0.04KHz but the recommended frequency range is between 12 and 17KHz.
    Also, the dimension of the MOSFET Si7617DN seems very small, is that is the apt MOSFET for a current of 10A?.

  • 0 in reply to Shibin Varghese
    TI__Guru*** 146975 points

    Hi Shibin,

    So sorry.  The datasheet pdf conversion sometimes prints mH and mF instead of the correct uF and uH.  Si7617DN is a 35A FET in PowerPAK package.  It should easily handle 10A.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Expert 1130 points

    Hi Jeff,

    Thank you for your reply

  • 0 in reply to Shibin Varghese
    Expert 1130 points

    Hi Jeff,
    Sorry to bother you again.
    Can I use BQ24610RGER for charging a 3.6V or 3.2V Li-Ion battery other than a 4.2V battery?.
    in the datasheet as well as in the excel sheet you are provided the battery voltage is fixed at 4.2V.
    BQ24610RGER is a charger so I believe there won't be any issue if I replace the 4.2V battery with a 3.6V battery.

  • 0 in reply to Shibin Varghese
    TI__Guru*** 146975 points

    Hi Shibin,

    Yes, you set the battery regulation voltage (>2.1V) using the external resistor divider that connects from battery, to VFB pin to ground.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Expert 1130 points

    Hi Jeff,
    I just completed designing with BQ24610RGER.
    The adapter current is set to 9A and the battery charge current is set to 8A.
    I am using CSD25402Q3A P-channel MOSFET on the adapter side and CSD17577Q3A N-channel MOSFET on the battery side. both have a continuous current greater than 20A,
    But I am a little confused about the thermal effects, so I am planning to use two MOSFETs in parallel on the adapter side and on the battery side.
    Will this affect the performance of the system.
    So here will be four P-channel MOSFETs at the adapter side.
    Do I need to use an external gate driver if the above-mentioned is the only hope?.
    Also, what is the need for a charger IC if most of the BMS has built-in over-voltage protection which will disconnect the battery/load from the adapter if a predefined overvoltage is reached?.

  • 0 in reply to Shibin Varghese
    TI__Guru*** 146975 points

    Hi Shibin,

    You can estimate the power dissipation for the PFETs using Pd=Irms^2*RDson~=(IOUT*D)^2*Rdson for high side and =(IOUT*(1-D))^2*Rdson for low side.  With its thermal pad soldered to PCB copper of at least 3x area of the pad (even if via'd down to bottom layer), I predict single FETs will be okay.  The gate capacitance and charge for these FETs is relatively low so the charger's internal gate driver is likely okay as this charger was developed several years ago when FETs had higher gate capacitance and charger.  Definitely, no additional gate driver needed for the battery FET.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Expert 1130 points

    Hi Jeff,
    We have another requirement for a battery charger but the battery is LiFePO4.
    Can I use the same BQ24610 chip.
    There is a chip BQ24630 specifically for LiFePO4.
    The pinout, package, and reference design are exactly the same as the BQ24610.
    Can I use BQ24610 for both cases? or do I need to change the charger for LiFePO4?. 

  • 0 in reply to Shibin Varghese
    TI__Guru*** 146975 points

    Hi Shibin,

    Both chargers use the same profile but BQ24630 has higher VRCH and lower VFB to support the lower charge voltage of LiFePO4 batteries.  The BQ24610 will still charge LiFePO4 batteries but the recharge cycles may start much sooner than desired. 

    Regards,

    Jeff