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Dual battery power supply with automatic fail-safe / fail-over

Other Parts Discussed in Thread: TPS25200

Hello dear Ti comunnity :)

I'm looking for solution to switching between two batteries which are used to power up whole device (Boost converter followed by LDO's).

What I want to achieve is little uncommon solution so :

BAT1 will be primary power source and BAT2 will be secondary which will be active when BAT1 will be depleted/removed/damaged.

I know that I can use PowerMUX or ideal diodes with enable input to control voltage of BAT1 via comparator or battery monitoring IC to make ideal cut off / source switch.
But I'm looking for something more useful which will give me more freedom in terms of charging.

So I want to achieve charging of BAT1 when BAT2 will be used and vice versa to mantain proper discharging of both batteries - this is my main goal here.
I want to avoid as much as possible any mainentrance due to battery replacement or switching them in place (reconnecting them at PCB) due to nature of Li-Ion bat.

I'm not looking for charging both batteries in the same time or discharging them equal (because I found some solutions that working this way).
My charging current is ~ 2A which is rather typical (wall adapter 5V/2A), I'm using dedicated IC for charging Li-Ion battery (both are 18650 type 3.7V).

Cut off voltage will be 2.7-3V then second power supply will be turned on to maintain proper operation and 1st bat should be charged until it will be charged at 95%
When BAT1 will be fully recharged it should be switched back to power up device, then BAT2 is going to recharge.

Wall adapter will be connected all the time but only to maintain charging (not as a 3rd power supply option).
Is there any IC solution for that ? I assume I will need some MOSFET's but how to control that charging sequence?

What if both batteries are depleted (wall adapter disconnected/damaged etc.) which battery will charge then?
Let's assume both 18650 will be at ~ 2.7V - we are not talking about removed /damaged cell - both will be connected all the time.

Thank you for time and help :)

PS: sorry if my post does not belong to this forum, if there is more suitable place for my question then please move my thread to proper place.

  • Hi,
    Thanks for reaching out on E2E.
    I am looking into this and get back to you soon.

    Best regards,
    Rakesh
  • Hi Mateusz,

    Thanks for the detailed explanation on your requirement. I have understood what you are trying to achieve.
    Can you please share the end product details?
    As you mentioned, one way is to use Powermux devices and then configure to switch to the desired battery source based on the battery voltage threshold. Similar way on the charger side for sequential charging. If there is a micro-controller on your system, the path controllability (ON/OFF) can be achieved using it. Please refer app note www.ti.com/.../slva948.pdf I think you are trying the same thing (?)..

    If you are looking for "Charger IC solution" for sequential charging, Please let me know so that I can divert this post to the corresponding forum.


    Best regards,
    Rakesh
  • I already have charging IC :)

    That app note is more or less corelated to what I'm going to achieve.
    I do not use any microcontolers in my design, everything is straight forward.

    I'm not using hi current chips BTW :)

  • Hi Mateusz,

    We don't have straight forward solution/device to fulfill your changeover logic.
    We need to do workaround with additional circuitry (supervisory, comparators) to make decision on which path to enable/ disable. However, in some situations (when both the batteries are charged), we may need additional intelligence/logic circuit to prioritize the battery discharge path. Please let me know if you need any help in this regard.

    Best regards,
    Rakesh
  • I will definitely need your help and advice.
    I never done something like that before.

  • Sure.
    I will get back in a couple of days.

    Regards,
    Rakesh
  • Hi Mateusz,

    You will need a device which can block reverse current flow when it is OFF. I suggest to use TPS25200 as per below configuration for your 5V system. One charging path and opposite discharging path can be made active depending on the status of the battery voltage.

    I feel, the implementation gets much simpler through a micro-controller to help the decision making.

    When BAT1>3V & BAT2<3V -> make EN2=HIGH

    when BAT1<3V & BAT2>3V -> make EN1=HIGH

    if both BAT1>3V & BAT2>3V -> make EN1=HIGH (you can choose)  

    Let me know your thoughts..

    Best Regards,

    Rakesh

  • Your solution look superb.
    But my knowledge about uC is pretty non-existent.

    I will still need to use voltage comparators and other logic IC to make priority charge / discharge sequence and maybe additional IC's to avoid unproper operation when both batteries are charged/discharged to some level.

    But if I'm going to stay with uC what kind of IC are you propose to use in this design?

  • Hi Mateusz,

    For uC part suggestion - please open a new post with the subject of "uC part suggestion" at e2e.ti.com/.../ and your post will be directed to the right engineer.

    Best Regards
    Rakesh
  • Hi Rakesh.
    Lets assume that I want to stick with "uncommon" method excluding uC.

    What will be needed to make those circuit working? I know that both batteries needs to be measured by comparators (switching when voltage fall below ~ 2.7V).
    And I know how to set proper voltage on those comparator or add some LED's to open drain output of those.

    But what else will be needed to make everything working?
    I'm pretty sure about that logic IC  or comparators will need some hysteresis to avoid unnecessary switching between batteries.

    Or maybe there should be some built in timer? If I know charging current and charger will be always connected + I also know battery capacity (charging time could be measured as well).
    How about that simple solution? Charge and discharge path will be controled via timer.

    At the beginning I was thinking about something similiar to your design but with diffrent IC (from Linear Tech - #LTC4415).
    But I still didn't figure out how to make it more automatic in terms of switching and making priority when both batteries are charged/discharged or if one of those is removed/damaged.

    The biggest problem is that I'm not using always connected power supply (as a main or aux) only two batteries to achieve best performance.
    With always connected power supply it would be much easier to adapt everything to my preferences.


  • Hi Mateusz,

    I have asked battery supervisor team to suggest solution to realize this change over logic. They will get back to you soon.

    Best Regards
    Rakesh
  • Hi Mateusz,

    Most of the battery supervisory solutions are centered around single battery system and are not able to carry your system logic requirement. Unfortunately, I cannot help much on the discrete implementation for your system design. Let me know if you have any questions on the devices.

    Best Regards
    Rakesh
  • Ok.
    Thank you for time and help.

    Then I will need to learn how to use uC and finish this project later :)