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Original question:

TPS25751: Issue with the communication between TPS25751 and BQ25798

BQ25798: BQ25798 suitable for LiFePO4? Or issue with defaults and possible consequences?

Hasse Muller
Prodigy 10 points
Part Number: BQ25798
Other Parts Discussed in Thread: BQ24172

Tool/software:

From the TI Application Note "LiFePO4 Design Considerations" the BQ25798 is said be a good choice for a LiFePO4 battery. However, looking at the BQ25798 datasheet( BQ25798 I2C Controlled, 1- to 4-Cell, 5-A Buck-Boost Battery Charger with Dual-Input Selector, MPPT for Solar Panels and Fast Backup Mode datasheet (Rev. B)) I am not so sure anymore.

Is it indeed suitable for LiFePO4 batteries?

So, What happens when the battery is completely empty? Registers are reset and by default it charges autonomously with Li-Ion voltages which are too high for LiFePO4? Charging will never reach the end voltage, does it stop charging at some point or is there a serious risk involved? If that happens incidentally, how does it affect the lifetime of the battery (can it be acceptable for very low occurrences)? The registers can only be set when the device is manually turned on, not while charging. 

It's intended purpose is for a portable medical device with a 2 cell 2200mAh LiFePO4 battery via USB-C. The device does not start automatically when charging, it needs to be turned on separately and it is also not supposed to be used while charging. So no BQ25798 registers can be set when charging from completely empty, it is used in autonomous mode.

Thanks for helping out!

  • 0
    TI__Guru**** 166666 points

    Hi Hasse,

    You are correct that the default VREG is 4.2V/cell and LiFePO4 needs only 3.6V/cell.  Since the charge profiles are the same between the 2 chemistries, BQ25798 can safely charge >1S LiFePO4 but only if the VREG is changed later in software.  For example, if 2S LiFePO4, the PROG pin can set VREG for 1S LiIon or 4.2V.  That way the default charge can still occur.  The software would then need to change VREG to 2x3.6V=7.2V.  

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 10 points

    Hi Jeff, 
    Thanks for that. However, our application/device does not start when charging, it requires manual turning on when not charging. That has to do with induced leakage current (where they wrap the device in tin foil) as per 60601 for medical devices. So when fully empty, a user charges the device with the default profile but set to 1S as per your suggestion. Then charging is complete at 4.2V which is not full at all, right? Do you have any idea at what percentage is would then be? So the device is then used, registers set to 7.2V at boot, and then has a severely limited use time as it wasn't fully charged. I'll have to check if this can be acceptable.


    Also what happens when a full LiFePO4 is full (or on more than 4.2V) without registers being set and on 1S config? High voltage error? 

    There is no other device from TI that is autonomously using LiFePO4 settings, configured through some hardware pin or something?

  • 0 in reply to Hasse Muller
    TI__Guru**** 166666 points

    Hi Hasse,

    There are no other I2C host controlled devices with default for #cells x 3.6V.  We have older standalone (resistor set) chargers like BQ24172.  Would that be of interest?

    Regards,

    Jeff  

  • 0 in reply to Jeff F
    Prodigy 10 points

    We feel the totally empty battery is a pretty rare situation as the device then has not been used for a long time. So we are considering that approach. 

    I am also wondering if a 2S resistor settings is an option if our battery pack has a protection circuit with overvoltage protection (we are checking that), do you agree? It probably charges our battery a lot more than with the 4.2V / 1S setting, taking away the problem of the device needed to recharge immediately after turning on for the first time.

    Any thoughts about that?

  • 0 in reply to Hasse Muller
    TI__Guru**** 166666 points

    Hi Hasse,

    All packs have a protector chip that protects against over voltage. A series FET opens and the chargers thinks there is no battery. It enters trickle charge and tries to close the protector FET by applying no more than 100mA and 2x2.5V at BAT for 1.5s.  If the protector FET doesn't close in 1.5s, the charger enters a normal charge cycle, charging the BAT pin capacitors to termination. The cycle repeats until the FET closes.  

    Regards,

    Jeff