Help Designing Charging Controller

Discussion in 'The Projects Forum' started by Xyberz, Dec 30, 2011.

  1. Xyberz

    Thread Starter New Member

    Dec 30, 2011
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    0
    Hi,

    I'm new to this forum and I'm hoping if someone or some members can assist me on designing a charging controller for a battery pack I'm making. Obviously I'm no expert in electronics but can understand at least the basics.

    My goal is to create a charge controller that will charge a 12V battery from an AC power supply. The battery's capacity will be anywhere from 10,000 - 20,000 mAh. All the controller has to do is to allow an AC adapter to properly charge the 12V battery and maintain the charge once it's full without overcharging and possibly damaging the battery. Not sure if this stuff is already built into typical A/C adapters or not as I'm not sure what's inside a typical A/C power supply.

    Also another wanted feature of the controller is for it to be as small and thin as possible. I'm looking to keep the height of the charge controller to a minimum.

    Any tips and help to achieving this would be greatly appreciated! :D

    Oops, forgot to ask one more thing. I'm looking to possibly integrate solar cells into the system to alternatively charge the battery but I don't know if it's possible to use 6 1.75W 0.5V 3x6 solar cells to charge a 12V battery. If possible I'm assuming there would have to be some type of voltage stepper as the total amount of volts achievable by that amount of cells would only amount to 3V. Please correct me if I'm wrong on that or if it's even possible. Thanks!
     
  2. k7elp60

    Senior Member

    Nov 4, 2008
    478
    69
    Welcome to this forum. In order to provide more help you need to tell us what kind of battery you are talking about. Different types of batteries use different charging techniques, which may include limit on charge current, float charge and etc. So is it a lithium, lead acid, nickle metal hydride?
     
  3. Xyberz

    Thread Starter New Member

    Dec 30, 2011
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    Didn't think about that. It will be lithium-ion battery type. Thanks for the heads up on that.
     
  4. k7elp60

    Senior Member

    Nov 4, 2008
    478
    69
    I have very little experience on Lithium-ion batteries so perhaps another member of this forum can help you.
     
  5. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    A 12V Li-Ion battery is not available. Each cell is nominally 3.7V so three cells make 11.1V, not 12V. The battery cells are charged to 4.2V each so the battery will be 12.6V when fresh out of the charger. Your circuit must have a method to sense when the battery has dropped to about 9V then disconnect its load.

    The charging circuit must sense the battery voltage and reduce the charging current if the voltage is lower than 9V. It must sense the temperature of the battery then reduce the charging current to prevent thermal runaway. The charging circuit must limit the charging current to what the battery manufacturer recommends. The voltage must be limited to 4.20V. When the voltage reaches 4.20V then the charging circuit must sense when the current drops to about 1/40th then disconnect the charger.

    Usually a battery charger IC is used to do all that stuff.
     
  6. Xyberz

    Thread Starter New Member

    Dec 30, 2011
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    0
    If that's the case can't you wire batteries in series to achieve near 12V? Also is there very affordable options for battery IC chargers on the market? I do have access to China manufacturer that can help stack a custom battery pack together to get the 12V.
     
    Last edited: Jan 1, 2012
  7. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    A series lithium battery is charged with a balanced charger circuit where each cell is sensed and charged separately.

    If you connect three 3.7V Lithium rechargeable battery cells in series and charge them to 4.2V x 3= 12.6V then the cell with the lowest capacity (they are all different) will get fully charged first and become overcharged which causes it to catch on fire (a very hot fire where water makes it burn hotter).

    If you buy a certified battery and charger from a local hobby store then it is a good investment. It is worth it.
    But if you buy a cheap Chinese battery and charger (on E-Bay) from a no-namebrand manufacturer then you are gambling and hoping that it does not catch on fire.

    Cheap (affordable?) usually means CHEEP CHEEP, cluck cluck, cluck.
    Gullable people buy cheap stuff. I buy high quality things when they are on sale.
     
  8. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    If you are dead set on "12V", there are some new Li-Fe type "12V" batteries available and being sold for motorcycle use. Of course, they are actually about 14V at full charge which is similar to a 12V lead acid battery at full charge. You can find the new Li "12V" batteries on the net sold for replacement for motorcycle batteries.

    http://www.cyclebuy.com/shopping/shorai/index.htm
     
  9. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    I have seen some new electric scooters creeping slowly along the street and at night their lights are extremely dim. Which type of battery do they use? Carbon zinc?? Lemon??
     
  10. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    I was going to guess "Chinese".....

    could be lead-acid gel cells.
     
  11. CraigHB

    Member

    Aug 12, 2011
    127
    15
    As already mentioned, Li-Ion cells have a very specific charging profile which has to be adhered to strictly. There are lots of charger controller chips out there for that purpose. Most are designed for a single cell, but you can stack them using a virtual ground to "balance charge" a series pack. The Microchip MCP73833 is an example.

    It's possible to build your own charger from the most basic components, but you need very high accuracy. Terminal charging voltage has to be exactly 4.2V for a 3.7V Li-Ion cell and cannot deviate more than 20mV either way. You also need need to regulate current.

    In any case, Li-Ion cells are dangerous if you don't know what you are doing with them. They can burst into flames due to over-loading or over-charging. You need to do some research on them before proceeding any further. I would suggest you use an SLA battery unless you absolutely need the size and weight savings.
     
  12. Xyberz

    Thread Starter New Member

    Dec 30, 2011
    4
    0
    Well the most cost effective manufacturer for mass quantities of a product would be Chinese sadly. So that's the choice I've decided to go with. Doesn't mean that everything "Chinese" is "cheap" or "cheaply made".

    I just found this on the internet using a voltage step-up converter.

    http://www.national.com/pf/LM/LM2621.html#Overview

    The only thing I'm wondering is how this affects power drain on a 10Ah battery?

    Also the battery I need needs to be flat, like .25" thick so sadly a motorcycle battery would be too big.
     
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