Li-Ion Battery Charger

Discussion in 'The Projects Forum' started by RodneyB, May 6, 2015.

  1. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
    633
    13
    I am getting some Battery packs, Li-Ion 14.8 Volt. 800mAh

    They are supplied with over voltage charging protection, Short Circuit Protection and Under voltage.

    The battery manufacture suggests that it is charged at 200mA.

    I have attached a drawing of the circuit I want to build to charge the battery, However I am not sure if this will limit the current.

    I am going to use a 220 to 24 Volt transformer, rectified and then the voltage adjusted to the 14.8 volts. I need to confirm the suggested charging voltage.

    How do I make sure the current is limited to charge at no more than the 200mA.
     
  2. ScottWang

    Moderator

    Aug 23, 2012
    4,855
    767
    You have to setup the charger voltage of battery then you need one LM317, and you want to limited the current to 200 mA, so you need another LM317 to limits the current as Precision Current Limiter at the right middle on page 9, you could put the current limiter in front of battery charger.
     
    RodneyB likes this.
  3. Søren

    Senior Member

    Sep 2, 2006
    472
    28
    Hi,

    Protection circuits are for... Surprise... Protection!
    They're not made for charge voltage disconnect and they may go permanent open circuit if tripped!

    Do you know if the protection is per cell or one for the entire battery?
    Since you don't mention balancer lines, I'll assume that none is to be found - then you really want the protection to be per cell!

    Please provide a datasheet or at least a link for the batteries.

    Do you have a fire insurance that don't mention self inflicted damage? ;)


    Just answering the (max) current limit question, you need a couple of resistors and a transistor.

    What purpose do you think that C1 have?

    Connect a diode (eg. 1N400x) from out to in, to protect the regulator, in case the power is ever cut, while a battery is charging.

    A 14.8V Li-Ion is 4 cells in series and will have a full charge voltage of 16.4V. Add voltage drop of regulator, of contact resistance etc. and you're looking at say 20..22V.
    A 24V secondary is too high, as it will give you peaks of ~32..33V, so quite a loss. An 18V secondary would be much better.


    But... Don't start charging Li-Ions without having a proper charge termination and without knowing the proper way to charge them - life's to short for lots of stuff, why make it shorter :)
    Go buy a proper charger - sorry, but I don't let toddlers play with steak knives either.

    If, on the other hand, you're prepared to study the subject and to build a sensible charger and adjust it properly, then we can talk.
     
    RamaD likes this.
  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,998
    745
    you could put a transistor on the negative rail as a current limiter, like this ,http://danyk.cz/li-ion.png


    set the resistor RX to 7 ohms for a current limit of 200mA.
     
  5. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
    633
    13

    Thank you so very much !!
     
  6. Søren

    Senior Member

    Sep 2, 2006
    472
    28
    The diode is a waste - has got no function in there and the 1000µF cap is a waste too, charging a battey, but otherwise, it was the circuit I had in mind for his suicide mission :)
     
    Dodgydave likes this.
  7. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
    633
    13

    Little nervous about the "suicide mission" constant referral. I posted my circuit and query as I was unsure. I want to build the right charger, I want to understand why it works the way it does. I don't want to just buy one, I want to build it so I learn something new.

    This battery has cost a lot of money and time to purchase, I cant just walk into a shop and buy one. If what I propose to build is not going to work or is not safe, then I don't want to build it. That's why I was asking for advice
     
  8. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,801
    1,105
    Make sure you understand the hazards of charging lithium-based batteries. Check out Battery University.
     
    RodneyB likes this.
  9. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
    633
    13
    Thank you
     
  10. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,998
    745
  11. Søren

    Senior Member

    Sep 2, 2006
    472
    28
    +1 to using a chip made for the purpose!

    I didn't check this particular one yet (as I've been with the wider circles of in-laws all day, need I say more? ;))
     
  12. Søren

    Senior Member

    Sep 2, 2006
    472
    28
    Well, not trying to scare your socks off - just want you to be on your toes.
    Lithiums are potential fire bombs, but handled correctly, they're quite peacefully - hence the need for a chip that handles the charging protocol and terminates correctly - for instance, a lithium should never be floated (which the LM317 charger will do) it's charge once and then disconnect. A low initial voltage calls for a low initial charge current - not so simple a circuit with an LM317 either.


    All the more reason to charge it correctly. This will render it safe as long as you don't pierce it with sharp objects.

    Lithiums that develops a bulge (depending on the degree) shouldn't be charged, unless you're absolutely sure of what you're doing, as this is an early warning that it's planning to go - and not with a wimper.
     
  13. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
    633
    13
    Thank you for this I looked at the suppliers I use in South Africa. The LT1512 is available for US$15.00 each, I need to look for an alternative supplier. Or an alternative chip
     
  14. MrAl

    Well-Known Member

    Jun 17, 2014
    2,438
    492

    Hello there,

    People here have expressed concern because Li-ion batteries can be dangerous. That doesnt mean that they are dangerous if properly handled though.

    The most important mode of operation, the most dangerous, when these cells have been known to smoke and burn, is when, yes you guessed it, when charging. That's the reason for concern. Statistically charging is the most dangerous time for the Li-ion cell. So caution should be applied in large doses.

    But of course we still want to be able to use these batteries and not shy away from them either. They are very useful and all we have to do is follow a few rules.

    First, dont let an individual cell get lower than 2.5 volts.
    Second, dont charge with too high of a current.
    Third, dont let the individual cell voltage get above 4.20 volts.
    Fourth, never leave a charging cell unattended.
    Im sure people here can add to this list too.

    Again the most important is the charging phase, where we have to limit current and voltage.

    For a pack of 14.8 volts, that means that they have FOUR cells wired in series, with 4.2 volts for each cell. This can be dangerous because they are wired in series. More so than if they were just single cells.

    The only way to know for sure how to handle this is to read the manufacturers recommendations, in full detail though. So kindly provide myself and others here a link to your battery pack and we can add more accurate information so you dont have to wonder or be afraid of charging it.

    BTW, many people like to charge theirs in a steel garbage can with no leads touching the metal. That way if anything blows, at least it is in a safe container.

    So provide a link to the pack or maybe the make and model number and im sure we can help more on this.
     
    RodneyB likes this.
  15. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
    633
    13

    Thanks for your reply, I am extremely nervous but at the same time now very curious and determined to master this
     
    Last edited: May 9, 2015
  16. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
    633
    13
    Please see attached the battery data sheet
     
  17. Søren

    Senior Member

    Sep 2, 2006
    472
    28
    Datasheet says 400mA charge current.

    A couple of questions to potentially save time...

    Do you mind working with inductors?
    Do you mind working with surface mount components?
     
  18. MrAl

    Well-Known Member

    Jun 17, 2014
    2,438
    492
    Hello again,

    It looks like they have a charge monitor chip on board in case anything goes wrong.

    The key points would be:
    1. Charge at a lower than recommended charge current. 200ma should be ok.
    2. Charge up to a lower than maximum charge voltage. They quote 16.8v, and since there are four cells if we decrease by 0.1v per cell it would be 16.4 volts max. Decreasing by 0.05v per cell would be 16.6 volts, so stay between 16.4 and 16.6 volts. The benefits of slightly lower top voltage is greater cell longevity. The drawback is a slight decrease in capacity, but not that much.

    You should also test whatever charger you build first, and when charging for the first few times keep measuring the temperature of the pack to make sure it does not overheat. If you put it into a steel garbage can for the first few times that would help a little too if something went wrong. Once you are more confident you can stop doing it that way if you like.

    So your charger should be manually adjusted for 16.4 to 16.6 volts max, and a max current of about 200ma. The voltage set point is very critical, but the current set point can be anything close to 200ma, even 240ma would be ok. Btu that voltage setting must be very close to the actual voltage, so use a volt meter that is known to be accurate.

    I always charge single cells myself as i dont like series setups. But in addition to the normal charge circuit i also set up a secondary unrelated circuit that MONITORS the first circuit and constantly reports the charge status by constantly measuring the voltage and current getting to the cell. That means there is always a secondary system always checking up on the first system. This can be easily done with a cheap, low end micro controller, or just as easy a set of comparators and voltage reference.
     
    Last edited: May 9, 2015
    RodneyB likes this.
  19. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,998
    745
  20. Søren

    Senior Member

    Sep 2, 2006
    472
    28
    They don't it's their protection circuit - please don't give advice on semi-dangerous stuff if you don't really understand it yourself.
     
Loading...