lithium Ion Battery Charging from Multiple Sources.

Discussion in 'Power Electronics' started by haythamhakla, Apr 25, 2017.

  1. haythamhakla

    Thread Starter New Member

    Apr 10, 2017
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    Hi,
    I have a NCR18650 LI battery and I'm designing a circuit that can charge it from a low power source ( solar, TEG, piezzo), as well as USB when the low power source is not available. The circuits will always be connected to a load that is switched on/off as needed.
    For the low power circuit part, I am using a BQ25570RGRT nano power booster and have the circuit designed as per Texas instruments' application notes.
    For the USB part, I am using an MCP73833 charge management controller from microchip that is recommended for USB charging.
    Both chips would supply around 4.2V to charge the battery.
    The questions that i need help with are:
    1- is it ok to have both charging circuits connected to the battery as shown below? Each circuit eventually has a VBAT output that connects to the +side of the battery. Is there any risk connecting the two charging circuits in parallel like that? Both circuits will be charging at the same voltage level. If the USB power is connected, and the low power energy source is supplying power as usual, will the battery be charged from both sources? will it be charged by the source that can deliver more current?
    2- When the load is connected to the circuit while one or both charging circuits are active, my assumption is that the charging circuits will still try to charge the battery, but the latter will be drained because the current consumption of the load is higher than what the charging circuits can supply. Is that a correct assumption?
    3- Should I put some diodes between the VBAT pins and BAttery to make sure no circuit forces current into the other? each VBAT is already connected to a FET transistor as show in the picture below.
    Thanks a lot for your support.


    upload_2017-4-25_9-42-35.png

    upload_2017-4-25_9-53-36.png
     
  2. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    One of the chargers will eventually be the dominant one, and will try to take over, diodes will prevent each charger feeding one another.
     
  3. haythamhakla

    Thread Starter New Member

    Apr 10, 2017
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    I am affraid that if i put diodes, they will drop the charging voltage down by their fwd voltage and the battery wouldnt be charging at the appropriate voltage value.
     
  4. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    So just use one charger then, those cells use a monitoring chip from start to finish.
     
  5. haythamhakla

    Thread Starter New Member

    Apr 10, 2017
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    I need the two chargers.
     
  6. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    So put them in parallel as said, one will be dominant.
     
    haythamhakla likes this.
  7. Caps18

    New Member

    Jun 17, 2013
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    Is that a problem? And I agree with the diodes, but is it possible to connect them in parallel to combine the amperage of the charge?

    I am wanting to create a circuit to take a solar charge (present or not due to sun) and combine it with a bicycle dynamo hub generator which only outputs when the bike is going fast enough, and even then doesn't produce the correct amount of amps to charge a USB battery pack (5V @ 2A is needed) The dynohub can put out 1A, and the solar panel can put out more amps than the battery pack can handle but is very variable (and is regulated). Sometimes I ride at night, sometimes I ride slow in the mountains and can't generate the amount of power needed from one source.
    https://www.amazon.com/dp/B01EXWCPLC/ref=twister_B01LYZCQM6?_encoding=UTF8&psc=1

    I would like to use one of these to monitor the amount of power being generated.
    https://www.amazon.com/Eversame-Mul...rd_wg=FIetU&psc=1&refRID=4C7RRYNS6M3Y7FRC9NK2
     
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