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BQ20Z655-R1: Increasing the number of parallel cells

Ugur Bak
Prodigy 160 points
Part Number: BQ20Z655-R1

I am using BQ20Z655-R1 for a lithium ion battery which is in the 4Serial-3Parallel cell configuration. In addition, I don't use LED or LCD in this lithium-ion battery. My cells have 3500mAh capacity.

I am planning to increase the number of parallel cells but I am not sure about maximum number of parallel cells(for example 4Serial - 8 Parallel Configuration). 

Especially, my concern is about cell balancing. Becasue internal balancing current is 5mA.  What are your recommendations?

Note: I have to use this ready-made design

What are your recommendations? Can I use this PCB/circuit for 4serial-8parallel configuration?

  • 0
    TI__Genius 14985 points
    8x3500mAh = 28Ah
    You are likely right that 5mA of internal cell balancing current will not be likely to achieve balanced cells if they start to diverge. Your options are either to ignore cell balancing or to use external cell balancing.
  • 0 in reply to dMax
    Prodigy 160 points

    As I said before, I have PCB(with bq20z655-r1) and it is working very well for 4Serial-3Parallel configuration. Now I am planning to use 2 parallel PCB. I mean 4S-3P parallel with 4S-3P and seperate 2 PCB.

    Please check the image(sorry for easy/simple image): 

    Maybe It can be solve something for balancing. What is your idea for that?

     

    Please check this document for cell balancing: http://www.ti.com/lit/an/slua340c/slua340c.pdf

  • 0 in reply to Ugur Bak
    TI__Expert 6575 points
    Since the cells are on different bq20z655-R1, it may require an external balancing circuit that can read the capacity and SOC of each packs cell and implement a cell balancing method.
    Unfortunately, the bq20z655-R1 will not be able to handle this case by itself.
    Regards,
    Swami
  • 0 in reply to Swaminathan Ramanathan75031
    Prodigy 160 points

    Can I use 2 parallel circuit like the picture. Thank you.

  • 0 in reply to Ugur Bak
    TI__Genius 14985 points
    It is not a good idea to directly connect them like that. It will cause many problems. Over time the cells in one pack may age differently than the other pack. Here is one example of what could happen. When charging both packs from a single charger, one will hit full first and the charger will stop while the other one is not full. Then the voltages will relax down and the higher pack voltage will try to charge the lower pack voltage. Likewise during discharge, one will hit empty earlier than the other and discharge must stop. The other one will not be empty and will then try to charge the other one. Currently will also not be balanced between the two cells and many unpredictable things could happen.
  • 0 in reply to dMax
    Prodigy 160 points
    Thank you for your answer. This resolved my issue but I have to ask, during discharge if the one pack will be empty earlier than the other, I hope PCB1 will not provide output voltage and current isn't it? In addition, can I ask what is your advice for this system?
  • 0 in reply to Ugur Bak
    TI__Genius 14985 points
    Hi Ugur,
    If one pack hits empty and you stop the discharge then nothing will keep the other pack from providing output voltage to the other pack.

    The safest way to use two separate batteries in parallel is to have them both be separate and complete systems. That would mean separate chargers and separate DC/DC converters to charge and use each battery. You could also use ideal diodes to OR the two battery outputs together, but you would still need separate chargers. Then you need some controller and logic to decide which to charge and which to discharge.
  • 0 in reply to dMax
    Prodigy 160 points
    Hi ,

    1. I understand, I will not use this method.

    2. I have to use one charger so I have to provide capacity with 6 parallel. I will design a battery management system (for this system) for 14.4V and about 7-10 A. Is there an IC you recommend?

    Thank you.