+ Reply to Thread
Page 1 of 2
1 2 Last
Results 1 to 15 of 18

Thread: Batter Charging with MAX712 IC

  1. #1
    Jack.Straw
    Jack.Straw is offline
    Jack.Straw Newbie
    Join Date
    Dec 2008
    Posts
    55

    Batter Charging with MAX712 IC

    Hello. I am wanting to build a battery charger for a project i'm working on. First, the details:

    Charger IC: MAX712 Data Sheet
    Battery Pack: 6 AA Rayovac Hybrid NiMH 1.2v, 2100mAh
    Power Supply: Regulated Laptop Power Supply - 19vDC 3.42A
    Intended Charge Rate: 1C using voltage slope detection

    There are a number of calculations on the MAX712 datasheet that i'm not 100% sure about. Can anyone confirm these values for me?


    R1:
    R1=(minimum wall-cube voltage - 5v) / 5mA
    Is 19v my "minimum" wall-cube voltage since it's a regulated power supply? Assuming the answer is yes: (19v - 5v)/5mA = 2.8k ohm resistor?

    Rsense (current sensing resistor):
    Ifast=(capacity of battery in mAh)/(charge time in hours)
    Ifast=2100mAh/1 hour = 2100

    Rsense=.25v/Ifast
    Rsense= .25v/2100 = .000119 = .119 ohm resistor?

    PNP (Q1)
    PDpnp = (maximum wall-cube voltage under load - minimum battery voltage) x (charge current in amps)
    The 2N6109 on the datasheet shows a max of 40, but i'm worried about heat. Will a normal TO-220 heatsink like the one below be enough??

    PGM Settings:
    PGM0/PGM1 - Open/Open (6 Cells)
    PGM2/PGM3 - Ref/Ref (90 min timeout w/ voltage slope enabled)

    Other values (defaults):
    Vlimit = Ref
    TLO = Batt-
    C1 = 1uF
    C2 = 0.01uF
    C3 = 10uF
    R2 = 150 ohm
    R3 = 68k ohm
    R4 = 22k ohm
    D1 = 1N4001


    Thank you for any advice you can offer!
    -Scott
    Reply With Quote Reply With Quote  

  2. #2
    Chippie
    Chippie is offline
    Chippie Excellent Chippie Excellent Chippie Excellent Chippie Excellent Chippie Excellent
    Join Date
    Mar 2003
    Location
    Good 'ole England
    Posts
    939

    I would have said that 19v input is a little on the high side for charging 6 AA batteries......

    That said reducing the input voltage is going to be good for the power dissipation in the 2N6109....which is not operating in a linear mode anyway...

    Assuming a fully charged Nimh cell is around 1.2v,then 6 of these will equate to 7.2v..

    Having an extra 4volts headroom for the circuit to function correctly I'd say 12v would be sufficient..

    In which case R1 would be 1.4k (12-5)/0.005

    Rsense stays the same although a preferred value would 0.12 ohms
    Last edited by Chippie; 16th November 2009 at 08:12 PM.
    Eccentric millionaire financed by 'er indoors
    Reply With Quote Reply With Quote  
  3. #3
    Jack.Straw
    Jack.Straw is offline
    Jack.Straw Newbie
    Join Date
    Dec 2008
    Posts
    55

    Chippie, thanks for the response. I was goign with that 19v supply because I was having problems finding an inexpensive 12v, 2100ma+ power supply. I don't really need the 1C charge rate though. I found this 12vDC 1A switching power supply on ebay.

    If i used that one, would I need to do a C/3 charge rate?
    Last edited by Jack.Straw; 16th November 2009 at 10:15 PM.
    Reply With Quote Reply With Quote  
  4. #4
    Boncuk
    Boncuk is offline
    Boncuk Excellent Boncuk Excellent Boncuk Excellent Boncuk Excellent Boncuk Excellent Boncuk Excellent Boncuk Excellent
    Join Date
    Nov 2007
    Location
    Phetchabun,Thailand
    Posts
    3,302
    Send a message via MSN to Boncuk Send a message via Skype™ to Boncuk

    Sounds better to me.

    Leave R1 at 1.4KΩ and increase Rsense to 410mΩ for a charge current of C/2. (600mA).

    That way you'll have enough headroom for both, supply voltage and current.

    Boncuk
    Proper Planning Prevents Piss Poor Performance
    Reply With Quote Reply With Quote  
  5. #5
    Jack.Straw
    Jack.Straw is offline
    Jack.Straw Newbie
    Join Date
    Dec 2008
    Posts
    55

    Boncuk, thanks for the response. I'm confused by the 600mA part of your response. I was under the impression that C/2 would require 1050mA? (2100mA/2)
    Reply With Quote Reply With Quote  
  6. #6
    Jack.Straw
    Jack.Straw is offline
    Jack.Straw Newbie
    Join Date
    Dec 2008
    Posts
    55

    The 600mA thing has thrown me for a loop. I was thinking C/3 would be my max rate with 2100mAh batteries and a 1A power supply. Am I not understanding something?
    Reply With Quote Reply With Quote  
  7. #7
    Chippie
    Chippie is offline
    Chippie Excellent Chippie Excellent Chippie Excellent Chippie Excellent Chippie Excellent
    Join Date
    Mar 2003
    Location
    Good 'ole England
    Posts
    939

    Charging at 600ma just means the cells wil take longer to charge....3hrs.....


    Remember, the cells are 2100mah capacity, charging at 2100ma means they will be charged in an hour(thereabouts)

    The 1 amp psu is ok because the load current = the charge current ....600ma in this case
    Eccentric millionaire financed by 'er indoors
    Reply With Quote Reply With Quote  
  8. #8
    Jack.Straw
    Jack.Straw is offline
    Jack.Straw Newbie
    Join Date
    Dec 2008
    Posts
    55

    Leave R1 at 1.4KΩ and increase Rsense to 410mΩ for a charge current of C/2. (600mA).
    Ok, thanks. He meant 600mA for C/3, not C/2. Thanks again Chippie!
    Reply With Quote Reply With Quote  
  9. #9
    audioguru
    audioguru is offline
    audioguru Excellent audioguru Excellent audioguru Excellent audioguru Excellent audioguru Excellent audioguru Excellent audioguru Excellent audioguru Excellent audioguru Excellent audioguru Excellent
    Join Date
    Mar 2004
    Location
    Canada, of course!
    Posts
    19,288

    Quote Originally Posted by Chippie View Post
    Assuming a fully charged Nimh cell is around 1.2v,then 6 of these will equate to 7.2v.
    Please do not assume things that are wrong. Look it up.

    In Energizer's Ni-MH battery applications manual they show that their AA cell's voltage peaks at 1.55V when it is charged at 1C. Then the total for 6 cells is 9.3V.
    Attached Images
    Uncle $crooge
    Reply With Quote Reply With Quote  
  10. #10
    Chippie
    Chippie is offline
    Chippie Excellent Chippie Excellent Chippie Excellent Chippie Excellent Chippie Excellent
    Join Date
    Mar 2003
    Location
    Good 'ole England
    Posts
    939

    Quote Originally Posted by audioguru View Post
    Please do not assume things that are wrong. Look it up.

    In Energizer's Ni-MH battery applications manual they show that their AA cell's voltage peaks at 1.55V when it is charged at 1C. Then the total for 6 cells is 9.3V.
    Fair comment...

    But I then didnt specify any particular brand of cell.....


    Nothing wrong with erring on the side of caution......
    Eccentric millionaire financed by 'er indoors
    Reply With Quote Reply With Quote  
  11. #11
    audioguru
    audioguru is offline
    audioguru Excellent audioguru Excellent audioguru Excellent audioguru Excellent audioguru Excellent audioguru Excellent audioguru Excellent audioguru Excellent audioguru Excellent audioguru Excellent
    Join Date
    Mar 2004
    Location
    Canada, of course!
    Posts
    19,288

    Quote Originally Posted by Chippie View Post
    But I then didnt specify any particular brand of cell.....
    The brand doesn't matter. Ni-MH is Ni-MH. They are all the same.

    Energizer AA Ni-MH cells are made in Japan by maybe Sanyo. Their 9V Ni-MH battery is made in Germany by whoever.
    Uncle $crooge
    Reply With Quote Reply With Quote  
  12. #12
    Boncuk
    Boncuk is offline
    Boncuk Excellent Boncuk Excellent Boncuk Excellent Boncuk Excellent Boncuk Excellent Boncuk Excellent Boncuk Excellent
    Join Date
    Nov 2007
    Location
    Phetchabun,Thailand
    Posts
    3,302
    Send a message via MSN to Boncuk Send a message via Skype™ to Boncuk

    Quote Originally Posted by Jack.Straw View Post
    Boncuk, thanks for the response. I'm confused by the 600mA part of your response. I was under the impression that C/2 would require 1050mA? (2100mA/2)
    Oops! Number twister. I used 1200mA.

    Anyway 1050mA will be too much for your small power supply. Go for 80 to 90% load of the rated power.
    Proper Planning Prevents Piss Poor Performance
    Reply With Quote Reply With Quote  
  13. #13
    Jack.Straw
    Jack.Straw is offline
    Jack.Straw Newbie
    Join Date
    Dec 2008
    Posts
    55

    Ok, thanks guys. I ordered a 12v 2A regulated power supply and am going with a C/2 charge rate. I designed and built this layout last night:



    I hope i got it right. The last thing is to add LED indicators. The datasheet gives this example:



    Does that mean i should put one LED between R1 and V+ without any other resistors? (i'm used to putting a 1k resistor before my LEDs, but think R1 handles it here?)

    Then a 2nd LED between V+ and FASTCHARGE with a 470 ohm resistor preceding it? Why does it say 470 ohm minimum? Should i just use the 1k resistors i have around?

    Thanks again for all your help!
    -Scott
    Reply With Quote Reply With Quote  
  14. #14
    szabi
    szabi is offline
    szabi Newbie
    Join Date
    May 2006
    Posts
    40

    Hello and Happy New Year!

    I have built the charger circuit based on the MAX712/MAX713 from this site: Fast, smart and universal NiCd/NiMh battery charger

    I've tried with both used and brand new NiMh batteries of 1100 and 1300mAh capacity.
    The circuit seems to work fine, except that it SWITCHES OFF after about 15-20 minutes of charging!
    (After no more than 5 pictuers, my camera turns off saying that batteries are depleted)
    Set to 4 hours max charge time according to datasheet of MAX712. (PGM2, PGM3 = BATT-)
    I also tried from 1 cell to 4 cells, (+ dip switches changed) Rsense 0.6 and 1 Ohm, Rshunt 0,6K and 1,4K Ohms with same results.
    Charging currents are fine. (250mA / 1 Ohm, 415mA / 0,6 Ohm)

    What could be wrong? What should I check?

    Could be that I am using a BD 434 on a decent heat sink? By touch it's slightly warm, I'd say around 45-50 deg C, so it handles that current easily.

    Thanks.
    Last edited by szabi; 31st December 2009 at 02:51 PM.
    Reply With Quote Reply With Quote  
  15. #15
    audioguru
    audioguru is offline
    audioguru Excellent audioguru Excellent audioguru Excellent audioguru Excellent audioguru Excellent audioguru Excellent audioguru Excellent audioguru Excellent audioguru Excellent audioguru Excellent
    Join Date
    Mar 2004
    Location
    Canada, of course!
    Posts
    19,288

    Maybe your power supply voltage is not high enough.
    Uncle $crooge
    Reply With Quote Reply With Quote  
+ Reply to Thread
Page 1 of 2
1 2 Last
« Previous Thread | 12V power supply »

Similar Threads

  1. MAX712 Charger
    By Connor in forum Electronic Projects Design/Ideas/Reviews
    Replies: 9
    Latest: 31st December 2009, 02:47 PM
  2. laptop batter light blinks when charging
    By Marks256 in forum Chit-Chat
    Replies: 9
    Latest: 8th July 2008, 08:19 PM
  3. MAX713/MAX712 in series
    By kein0r in forum General Electronics Chat
    Replies: 13
    Latest: 30th March 2007, 10:08 AM
  4. Help with pbc layout and schematic matching...MAX712...
    By eyevancsu in forum Electronic Projects Design/Ideas/Reviews
    Replies: 3
    Latest: 29th September 2003, 09:15 PM
  5. NiMh charger using MAX712/MAX713
    By tibor in forum Electronic Projects Design/Ideas/Reviews
    Replies: 3
    Latest: 26th September 2003, 11:36 PM

Tags for this Thread


eXTReMe Tracker