Charge completion LED for Li-ion charger

Discussion in 'The Projects Forum' started by browner, Feb 1, 2014.

  1. browner

    Thread Starter New Member

    Feb 1, 2014
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    Hello everyone - I have a Li-ion charge controller PCB that I'm hoping to modify to include a charge completion LED. I believe I need to use a simple FET based circuit to achieve this but don't know where to start choosing parts and working out what resistors I need etc... I have the desire but not the knowledge sadly!

    [​IMG]

    The charger is based on the S8254A charge controller IC. When charging is complete pin 1 is pulled high to stop the charge via this P channel charging MOSFET.

    I hoped to use an N channel MOSFET on that same pin to switch on a charge completion LED when the pin is pulled high. However reading the datasheet for the charge controller (links above) tells me this would mean the LED remained illuminated when the battery and charge controller were disconnected from the power supply. It would stay illuminated as long as the battery is in a fully charged state and over time this would obviously drain the battery while in storage until the voltage fell enough to release the IC from the overcharged state.

    [​IMG]

    The battery is a 4 x 18650 Li-ion pack with a fully charged voltage of 16.8v across the input/output pins of the pcb. The wall plug power supply I use supplies the charge controller board with 18v 500mA to those same input/output pins.

    I hope to use some simple logic to illuminate the charge completion LED only when both the following conditions are met:
    1) charge controller IC pin 1 is high (18v) indicating the cell voltages have all reached 4.2v
    2) pcb is connected to power supply (not sure how to detect this. there is only a slight voltage difference when charger is connected 17.9v vs 16.8v!)

    The whole setup of cells and charge controller has to fit inside a small canister of the bike light that it drives. There is some room for additional circuitry to control the charging led. Wall charger connects via a standard centre positive round DC jack - I could maybe modify the socket to detect when the charger is attached?

    Any suggestions to help me out?
     
    Last edited: Feb 1, 2014
  2. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    It's not real simple. To know when an Li-Ion is fully charged, you have to know it is at final charge voltage (charger is in CV mode) and the charge current has dropped to a very low level, but that exact level depends on the cells A-HR rating.
     
  3. browner

    Thread Starter New Member

    Feb 1, 2014
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    The IC on the board does all of the hard work and pulls up pin 1 when the cells are fully charged. All I need to do is monitor that pin for the status LED.

    I'm thinking I should have a "test" button that can be momentarily depressed to show the status LED. That's probably easiest.

    A circuit something like this presumably with a momentary switch in series with the LED load? [​IMG]
     
  4. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Seems like you can use your FET idea, but hook the led and it's current limiting resistor to the input voltage to the charge controller instead of the battery.
     
  5. browner

    Thread Starter New Member

    Feb 1, 2014
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    Ok so I have come up with this circuit that I think will do the job...

    [​IMG]

    The circuit diagram shows the basic setup of the unit at present plus my proposed modifications (boxed).

    The battery is managed by the charge controller PCB which has two pins (P+/P-) that serve as both charging pins and also the output pins from the battery to the LED driver board that it powers. An 18V DC wall adapter can be connected to charge the battery as shown.

    I have added diode D1 (low drop, 0.35V) so that my charge status LED (LED1) should only light when the 18V DC adapter is present - do you think this will work?

    The P-FET gate is attached to the control pin of the charge controller (pin high (20V) = fully charged) to control the status LED.

    Can someone check that I have the source and drain of the FET connected the right way around? I plan to have the load on the drain and the source connected to +18V.

    Do I need a zener diode for protection of the MOSFET? How would I fit one if I do?

    Thanks, Chris
     
  6. ronv

    AAC Fanatic!

    Nov 12, 2008
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    I don't think you need D1.
    No zeners required as your FET has 30 volt gate to source voltage rating.
    Gate and source look good to me.
    Wait... Needs to be an NFET.
     
    Last edited: Feb 4, 2014
  7. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Forgot it needs to be on when the charger is off, so it needs to be an NFET.
    The chip doesn't drive much current so it may turn on slow depending on what the resistor is from gate to source on the PFET, but the led current is very low so no problem I think.
     
  8. trader007

    Active Member

    Feb 27, 2010
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    wait a minute... isnt this a protection pcb? its not a charger... While it can provide a function that will charge the batteries, I don't think its quite proper to use this alone to charge li-ion batteries.
     
  9. ronv

    AAC Fanatic!

    Nov 12, 2008
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    You know what? I do believe your right.
     
  10. trader007

    Active Member

    Feb 27, 2010
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    Well i only noticed because I spent months looking for a charging pcb like the one you show. Which is actually a nice board because of the balance function... and I don't see why they couldnt make pcb's like this that also have full charging support...
     
  11. burger2227

    Member

    Feb 3, 2014
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    All you need is a steady voltage after it is charged most of the way. According to this, you can leave it trickle charge forever.

    [​IMG]
     
  12. browner

    Thread Starter New Member

    Feb 1, 2014
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    Thanks for your replies people.

    I added D1 so that the charge completion LED was only illuminated when the wall charger was present and should hopefully not remain on when it is removed.

    I did wonder about the charging functionality of that PCB. I guess I should really be using a dedicated Li-ion charger to drive it. I believe the power supply I am using has basic constant current capability (~500 mA) but that's about it. We'll see how long the cells last although it gets them up to 4.20v and then stops the supply so really I can't see why it should matter too much. But then I really do know very little :p
     
  13. trader007

    Active Member

    Feb 27, 2010
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    honestly its probably fine. manufacturers actually do it this way with cheap electronics that have small batteries so if they do catch on fire it wont be that dangerous :p

    but with larger batters, like what you have, should be charged and monitored more closely. The PCB you have is what you need, but you really should add a dedicated charger on top of it for a 2nd layer of protection. If you over charge, you can easily start a fire.
     
  14. browner

    Thread Starter New Member

    Feb 1, 2014
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    Taking this advice on board can anyone recommend a dedicated charger for a 4 cell 16.8V Li-ion pack?

    I have found this one but it's expensive! http://cpc.farnell.com/mascot/2241000060/charger-li-ion-3-stp-4-cell/dp/BT03481
     
  15. trader007

    Active Member

    Feb 27, 2010
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  16. browner

    Thread Starter New Member

    Feb 1, 2014
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