How to detect the State of Charge of an iPhone's battery?

Discussion in 'The Projects Forum' started by Arzan, Nov 18, 2012.

  1. Arzan

    Thread Starter New Member

    Nov 18, 2012
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    0
    I am building a battery pack charger (Li-Po) for the iPhone, and need to know a way to detect the state of charge of the iPhone's internal battery, so that I could stop charging it when it's completely charged.

    The battery pack charger will connect to the USB. While connected to the USB, there are two batteries that need to get charged. The iPhone's internal battery, and the battery pack inside the charger.
    When plugged in, I want the current from the USB to first charge the iPhone, and then, once the iPhone is completely charged, direct the current to the battery pack.

    Essentially I need a circuit or component that can help me detect whether the iPhone is fully charged or not, and always direct current to the iPhone if it is not a 100% charged
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,003
    3,232
    It's likely that the current drops when the iPhone battery is charged so you would need to detect the current going into the iPhone. You could use a small shunt resistor in series with the iPhone charger and use a comparator to detect the voltage across it to determine when the current drops to the lower level.
     
  3. DickCappels

    Moderator

    Aug 21, 2008
    2,651
    632
    The iPhone has a charger built in and it will manage the charging process to get the best and safest charge and stop charging at the right time. Just supply 5 volts until after the phone says the battery is full. You can supply the five volts from USB, but you might want to have a look at the USB cell battery charging standard just to be safe.

    http://www.usb.org/developers/devclass_docs/USB_Battery_Charging_1.2.pdf
     
  4. Arzan

    Thread Starter New Member

    Nov 18, 2012
    2
    0
    That seems like a good approach. Would it be easier to use a current comparator directly?

    Once the switch has taken place, the current will be redirected away from the iPhone and towards the charger's internal battery.

    What would happen if the iPhone's state of charge drop from 100-99%, would the current get redirected to the iPhone? It doesn't matter if it does or doesn't. My reason for asking this question is to understand how and when the flip flops in the comparator would reset.
     
  5. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Are you trying to charge the phone's battery when it is away from the charger circuit in the phone? Then use a charger IC that is made to charge a lithium battery.
     
  6. crutschow

    Expert

    Mar 14, 2008
    13,003
    3,232
    What do you mean "use a current comparator directly". What type of current comparator? Using a standard voltage comparator as I proposed is a current comparator.

    The comparator doesn't have a flip-flop. It will just change output state when the current goes from above the current threshold to below. If you disconnect the charge to the iPhone, once it is charged, then the state of the iPhone battery will not be detected since the iPhone can't charge, even if it wants to.

    Note that you will need some method to initially bypass he current comparator to get the current flowing when starting the charge.
     
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