3 cell Li-ion charging

Discussion in 'General Electronics Chat' started by RedsDuty, May 28, 2016.

  1. RedsDuty

    Thread Starter New Member

    May 28, 2016
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    Hello

    This is my very first post on AAC...
    I am just beginning with electronics and I'm doing so in making a few projects I think are useful.
    I've already learned a lot from experience.

    So here's my question
    I want to charge a 3 cell li-ion battery pack (4 parallel, 3 series). I would just charge them to 12.2V (using a standard 12v 1A power adapter)
    And I would like to di the following things with them:
    Power a 12V (15W) amplifier circuit, and a 5V bluetooth audio chip + if possible a USB port to use as a battery bank.
    I'm using 2000 mAh Li-ion batteries (saved from laptop batteriess + capacity tested).
    Now I've read that for charging the packs, it's better not to put 12V at both ends and hope for the best :) .

    So... Is it really needed to buy a BMS for such a small system, or can I get away by just charging them?
    Or is there a possibility to make a complicated voltage divider to split the 12v in 3 4v supplies?
    Also, since the voltage of the batteries diminish over time, my step-down converter will not be able to give constant 5v...
    (I've tested this with my power supply, there's quite a big difference between full 12v and empty (around 8v))
    Should I step down untill right under 5v and use a constant boost converter (and deal with the extra inefficiency) or is there another solution.

    Sorry for my many (and possibly stupid) beginner questions
    but thanks for those who decide to help!

    Greetings
    Reds Duty
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Only if you can tolerate the resulting explosion and your house on fire :eek::eek:! Li-ion cells need a special charging profile for safety. Have a read of this, then buy a proper charger.
    Welcome to AAC!
     
  3. RedsDuty

    Thread Starter New Member

    May 28, 2016
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    This guy seems to be doing it... He makes camera things (and made his own EV) all without a BMS...
    What's so dangerous dangerous about it?
    I see in the article it's better not to fully charge the batteries (because voltage stress).
    And certainly not to charge it over the specified voltage (4.2v per cell * 3 cells = 12.6v --> I'm only at 12.2).
    They do explain how chargers limit current at the end, but I don't see where it's stated "Dangerous to explosion" when you don't...
    Could you tell me why? I'm probably going to buy a BMS for safety though...
    Not trying to be anoying, just curious...


    Oh and BTW...
    I would charge the 4p3s battery at 12v1A, which makes it about 250mA per cell...
    People charging with a variable bench power supply are charging them at 1A per cell, which is way more... And you can see the current automatically decreases over time...

    Thanks for the help!
     
  4. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    That's fine if the cells start off in an identical state of charge and have identical capacities. Chances are they won't, so the cell with most charge or least capacity will reach the full charge state before the others. Then, while the others play catch-up the first cell is being over-charged! It's important to 'balance' the cells: which is something a proper charger with individual cell monitoring will do but a "standard 12v 1A power adapter" can't. The standard adapter may not have over-current protection. That "1A" figure is likely just the rated maximum current it will stand before suffering; not a controlled current.
     
  5. RedsDuty

    Thread Starter New Member

    May 28, 2016
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    I've read a lot about those dangers indeed... The afformentioned youtuber used some cells of the same capacity (I know they still have different internal resistance and such) and tested it. The conclusion was that it was important if you use a lot of power.
    Since my amp is only 15W at max, that means at 12V I will have a current draw of about 1.25A, divided by 4 in parellel, which is about 310mA.
    In combination with the bluetooth module it probably won't go over 500mA per cell... Which he has shown in his video isn't such a big problem...
    Here's the video:


    That is possible... I will check it! But won't this also mean I can't use a max 2A rated BMS, because it will try to take more than 1A?

    Sorry for being so difficult.
     
  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    A proper BMS keeps the cells balanced by monitoring and controlling the cell voltages and their charging current. It should have some way for the user to set the maximum current. Check the datasheet before purchase.
     
  7. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
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    Imo I would go to hobby king and buy the right battery and charger for the job ... Also just cause it works for him doesn't mean it won't kill or hurt you.. Lipo are very dangerous because they can output a high current very quickly..
     
  8. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
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    Another option you could check out would be to use a dc-dc booster circuit with all the cells in parrell with a standard 4.2v 1amp cheap ebay charger with battery protection.. Imo this would be the safest and best option unless I am wrong..
     
  9. RedsDuty

    Thread Starter New Member

    May 28, 2016
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    That is probably true... I will just buy a BMS or a charger...
    Okay, thanks for all the info!
     
  10. JMac3108

    Active Member

    Aug 16, 2010
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    @RedsDuty
    Listen to the advice from these guys. LiIon can be very dangerous - not something to mess around with. A professional LiIon charger has many important safety features other than the obvious ...

    (1) Safety timer to stop charging if the cells don't reach charge termination within a certain time.
    (2) A trickle charge mode that charges at a much reduced current is the cell voltage is below a certain low level (over-discharged).
    (2) A circuit to monitor a thermistor which is typically embedded in the pack. The cells should not be charged if the temperature is above a value, usually 40-45C.

    Its not worth a fire in your house! Buy a real charger. Or if you really must design your own charger, use a charge controller chip from Linear Tech or TI that incorporates all of these features.
     
  11. RedsDuty

    Thread Starter New Member

    May 28, 2016
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    That's true, I will buy a real charger!
    I was just curious about why it is so dangerous, now I know :)
     
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