Microcontroller Controlled Variable Current / Voltage Supply – LiPo Charger

Discussion in 'The Projects Forum' started by Jakezilla3, Dec 10, 2010.

  1. Jakezilla3

    Thread Starter New Member

    Dec 10, 2010
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    Let me first say to the major contributors to this site THANKS! This site was a great help when we covered something in school and I needed some additional info to make it stick. Sometimes it made me wonder why I was spending so much to learn the same things....

    Anyway, I am currently designing a Lithium Ion / Polymer battery charger. LiPo chargers at their hearts are just simple Constant Current supplies until each cell is at the max voltage (usually 4.2V) and after that they are Constant Voltage supplies until the current has reached a negligible amount. I am very confident with the microcontroller/code side of things, but I am weak transistor/power design department.


    Key Design Points -

    Voltage Measurements - An array of SSR’s connected to a single accurate/calibrated buffer+ADC will switch through each cell and measure the voltage. A single buffer+ADC line is preferable to have high accuracy/repeatability, with a lower cost than having moderate accuracy for all lines.

    Current Measurements - Allegro Microsystems Inc Hall Effect sensor for high accuracy, low loss, safe isolated design.

    Input 12-19VDC (commonly used power supply in the hobby, most have 20A+ supplies)

    Charging Output 1S-6S Lithium Based Battery (3.0V minimum 25.2V max). I want at least 10amps, but if a higher current is only limited by a more expensive FET/bigger heat sink, that would be preferable.

    A lot of designs for chargers of this type use some sort of pre-built voltage buck/boost chip or schematic with discrete components. If I am already taking so much care to measure the voltage and current with a high level of accuracy, why not just have the microcontroller modulate the converter? The only cons to this type of design I can think of is relying on software for power control can be dangerous, but an external “watch dog” circuit can be devised. What would the current supply circuit look like? I am assuming some sort of Boost, Flyback, Cuk, or Sepic supply, but at first glance it looks like those are primarily design to take a constant input voltage and supply a constant output voltage. Can I just switch a FET? Any input on this would be great.
     
  2. blueroomelectronics

    AAC Fanatic!

    Jul 22, 2007
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    Oh it's a question.
     
  3. Jakezilla3

    Thread Starter New Member

    Dec 10, 2010
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    Is this not the right place to post project ideas/ questions?
     
  4. blueroomelectronics

    AAC Fanatic!

    Jul 22, 2007
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    Yes it is but...
    would normally be considered a statement (you were showing us how it's done). The addition of a question mark "?" would make it a question.

    It is one of the better descriptions of a project though.

    Food for thought:
    1. microcontrollers require a stable often 3 to 5V supply.
    2. there are many LiPo charging ICs on the market.

    Is this a school project?
     
    Last edited: Dec 10, 2010
  5. t06afre

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    May 11, 2009
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    Come on give the new member some slack here :rolleyes:
     
  6. retched

    AAC Fanatic!

    Dec 5, 2009
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    You NEED to use temperature monitoring in your charging circuit.

    It has a lot to do with charging Li(*) batteries.

    Many batteries have the charging circuit IN the cell or in the pack.

    You do not only want voltage/current monitoring. These change as the temperature changes. If it is on fire and at 3v, what good is your charger?
     
  7. Jakezilla3

    Thread Starter New Member

    Dec 10, 2010
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    Not a school project (graduated last May), not a work project, just a project centered around my hobby. Thanks for your input on temperature monitoring, I will have the option to monitor temperature on log it, but that is not a feature most people in the hobby want or would use.

    I fully understand that micro's need stable voltage supply which can be accomplished easily with a voltage regulator.

    Many CC/CV charger IC's only handle a small number of cells, and small current. I want to be completely variable with number of cells and current.

    I included the details of the project, because I have seen some people get hounded when they don't. My overarching questions are -

    Is there a method of controlling a power source with a microcontroller rather than a collection of discrete components that is commonly used?

    Do these method include a boost circuit, or should I boost the voltage before the current supply phase?

    Do any of these method include a current loop rather than using GND as current return path?


    I have just done a lot of digging and it seems like this is either new ground or it is not well documented. Thanks for all of your input!
     
  8. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    http://ww1.microchip.com/downloads/en/DeviceDoc/21823c.pdf

    Is this close to what you are looking for? There are a lot of Li-Po charge controllers around, the universal cell ones are typically a variant of the single cell for charge balancing.

    Temp, voltage, and current must be monitored for all cells closely. They don't act like NiMH when overcharged, they explode or start on fire.

    Search for "Charge Management" in ICs at DigiKey and you'll find about everything, including All In One ICs, but many of those are BGA packages that aren't useful for a prototype.
     
  9. Jakezilla3

    Thread Starter New Member

    Dec 10, 2010
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    Thanks for the reply TOG, but that is one of the chips that I was reffering to. It will only charge a single or dual cell pack at 1A max. Also, plugging one of these into each cell of a 10 cell pack will not work because it assumes the ground path as the current loop, causing shorts across ground. There are certain features lacking in commanally available chargers on the market that I would like to remedy, but designing the current source is my weak spot. (sorry if there are typos phone=no spell check :) )
     
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