18650 Battery Design Voltage question

Discussion in 'Power Electronics' started by Spencer Hutton, Sep 1, 2016.

  1. Spencer Hutton

    Thread Starter New Member

    Sep 1, 2016
    17
    0
    I am an electronics newbie and am designing a bluetooth boombox that will run on 16.8 VDC powered by Lithium Ion batteries. I plan on wiring four 4.2 V (2000 to 3000 maH) Li-Ion cells in series for a total of 16.8 VDC. I was then going to connect that to another chain to have a 4S2P 16.8 Volt battery pack with a BMS controling it all. Image attached shows a single series of 4 cells.

    My goal is to be able to use the boombox either on batteries or while plugged in. the boombox is powered by an amp that requires 10-25 V. I would also like the batteries to charge while using/plugged in. I have read that you need to supply more than 16.8 VDC to the battery/bms. So I felt a 19 V laptop charger would suffice. With the diagram in the link, The charging/discharging are not separate; wouldn't that mean that I would be supplying 19 VDC to the load (amp and bluetooth module)? 19V is ok as the modules i am looking at are rated up to 25 VDC. How does the 16.8 V come into play if i have a 19V power supply? Meaning, am I supplying 16.8 V or 19V to the amp?
     
    • BMS.jpg
      BMS.jpg
      File size:
      60 KB
      Views:
      7
  2. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,290
    1,255
    If the BMS also manages the charge of the batteries they won't go above 4.2 volts per cell. It may just need some voltage in excess of the charge voltage to work.
     
  3. Spencer Hutton

    Thread Starter New Member

    Sep 1, 2016
    17
    0
    Will i get 16.8 volts to my amp or 19?
     
  4. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,290
    1,255
    16.8
     
    Spencer Hutton likes this.
  5. Spencer Hutton

    Thread Starter New Member

    Sep 1, 2016
    17
    0
    That's still 16.8 even if connected to the 19v power adapter?
     
  6. Spencer Hutton

    Thread Starter New Member

    Sep 1, 2016
    17
    0
    That's still 16.8 even if connected to the 19v power adapter?
     
  7. tcmtech

    Well-Known Member

    Nov 4, 2013
    2,034
    1,659
    As long as you have a power supply that can meet the minimum voltage and current requirement for the BMS circuitry and have an adequate reserve current above that to run the amplifiers while the batteries are charging things should work just fine.
     
  8. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,290
    1,255
    Maybe you should post a link to the spec for the BMS.
     
  9. Spencer Hutton

    Thread Starter New Member

    Sep 1, 2016
    17
    0
  10. Spencer Hutton

    Thread Starter New Member

    Sep 1, 2016
    17
    0
  11. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,290
    1,255
  12. Spencer Hutton

    Thread Starter New Member

    Sep 1, 2016
    17
    0
    Ok, or I could just use a voltage indicator LCD and unplug it when it gets to about 16 V right? That's how my kid's Power Wheels 6v lead-acid battery works. Just plug it in overnight and make sure to not leave it plugged in. Then charge over night about once a month even when not using. I'd rather not get a specialty charger as I am trying to keep it simple.
     
  13. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,986
    745
    Your cells will charge upto 4.2V each, that's 16.8V total out, they will discharge to 3.6V each, then the bms will cut off the batteries.
     
  14. Spencer Hutton

    Thread Starter New Member

    Sep 1, 2016
    17
    0
    Thanks Dodgydave
    That's nice. So the boombox will stop working as if the batteries were "dead". What about when charging? ronv indicates that this is just a balancer (charge all batteries evenly). Does that mean if i leave it plugged in, that it will continue to try to charge the batteries? I thought i read that these little boards (various models) will apply a "trickle charge" when the batteries reach capacity. Thanks in advance for your reply.
     
  15. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,986
    745
    I have use these bms pcbs on 18650 cells, the charger can be left on permanently, the cells charge upto the set level, then switch off, the output is available continously until the cells drop to 3.6v, then it switches off the output until they are charged again.

    If you try to use the cells below 3.5v there is a greater risk of ruining the cells.
     
    Spencer Hutton likes this.
  16. Spencer Hutton

    Thread Starter New Member

    Sep 1, 2016
    17
    0
    ...And with the (+) end of the power source (19V AC adapter) connected to the B+ of the BMS and also to the (+) of my amp, i would be able to run the boombox at the same time the batteries charge?
     
  17. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,986
    745
  18. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,290
    1,255
    But I don't think you can leave it hooked up to a 19 volt supply.
     
  19. Spencer Hutton

    Thread Starter New Member

    Sep 1, 2016
    17
    0
    what about a 16.8v power supply. Would that do the job?
     
  20. JWHassler

    Member

    Sep 25, 2013
    201
    33
    Do you have a charger for the batteries, or just a power-supply?
    It sounds like you are using a laptop supply, which would expect a charge-controller inside the laptop: in this case, you must furnish that.
    Apologies if I missed something, but I didn't see any mention of a proper lithium-battery charging circuit.
     
Loading...