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BQ24079: Paralleling the OUT system output pin of multiple chargers

Karoly Pados
Prodigy 45 points
Part Number: BQ24079

I have an application with multiple batteries (2pcs of 500mAh each if matters). The application load can be permanently high so that the two batteries must share the load. Also, if plugged in to an external power source, the application should use that.

Since the application has otherwise low voltage (<3.0V), ideally I would connect the batteries in parallel. To make this kind of load sharing work including battery charging my idea is as follows:

  1. Add two charger ICs (BQ24079) to the system. Each of them charges one battery. This way the batteries are not only charged safely, but each time the system is plugged in, the batteries are automatically balanced (since they are each charged to full).
  2. The IN pin of the charger ICs are connected together (to the same external power source connecter). The input current limit (ILIM pin) of each charger is set to half of the total allowable current limit.
  3. The OUT pin of the charger ICs are also connected together (to the application load).

My questions:

  1. Is doing 2-3. above okay?
  2. Is it safe even if the batteries have different charge status?
  3. Will the batteries correctly share load if system output demands it?
  4. Will the batteries, assuming they are identical, discharge at about the same rate?
  5. If plugged in to an external adapter, will each BQ24079 pull about half the system load from the common power source?

My guess is: Connecting the OUT pins of multiple chargers is okay, because the BQ24079 has reverse current protection, so 2. is ok. Then theoretically they will share load because whichever's output is higher will supply the power, so 3. is ok. Then it follows that 4. is also ok, because each battery is always discharged only to the level of the other one. As for 5, I have no idea.

Can you please verify if the above suggestions are correct? I am unsure because the datasheet only mentions reverse current protection shortly, but does not describe it in detail, so I don't know in what conditions and on which pins does it work. I am also unable to tell the dynamic behavior of the chargers so I don't know if for example large oscillations will occur at he inputs/outputs.

Thank you,

Karoly

  • 0
    TI__Guru 68146 points
    Hello
    You are OK on item 1 and 2.
    Item 3, connecting OUT / system load was not planned and will need some closer study. The output does have reverse current protection OUT to battery and in. This would be the area that needs the most study.
    Will contact you.
  • 0 in reply to Bill Johns
    Prodigy 45 points
    Hello Bill,

    My reasoning is, If OUT has reverse current protection to at least BAT, then theoretically you should be able to connect the OUTs of multiple chargers in parallel, unless unit-to-unit variations (for example differences in internal resistances of the BQ24079) make load sharing too much non-perfect and thus unpractical.

    I am very interested in the results of your study, and thank you very much for looking into this.