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USB Battery Charger question

Discussion in 'Power Electronics' started by aaronmarsh632, Jun 28, 2013.

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  1. aaronmarsh632


    Jun 28, 2013

    I'm quite new to electronics but I want to learn as much as I can and make some handy gadgets so I wanted to start with a USB battery charger.

    I have a 9v 250mAh rechargable battery - 1 of the square ones. I want to charge it over USB. I figured 5v that the usb is putting out will take a bit longer to charge but i'm in no hurry. I was reading about the charge rate on wikipedia here about the time taken based on the mAh.

    If I understand correctly it would take a 9v 250mAh output to charge my battery in 1 hour, is this correct?

    If a USB 2.0 port puts out 5v 200mAh how long would this take to charge?

    Just a couple more questions, If I was using a wall plug to charge the battery that outputs more than 9v 250mAh would the battery be unable to handle the voltage and amps? I know I can 'knock down' the voltage using a 9v regulator but not sure about the amps.

    I hope I have used the correct termonology here, but I think you'll get what i'm trying to do.


    EDIT: Actually I just thought I think i'm going to need a diode in here to stop the charge going back from the battery into the USB port right? am I correct?
    Last edited: Jun 28, 2013
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Welcome aaron.

    Charging a 9V battery from a 5V port will take forever. In order to charge any kind of rechargeable battery you need to force a charge current into the battery. This requires that the output voltage of the charger is higher than the battery's voltage. Otherwise the battery will discharge into the charger.

    Another point is that most batteries are not charged by a voltage but by a regulated current source.

    First of all I recommend you learn about batteries, their chemistry and charging methods in general. The battery university is a good point for starting.
    After that, you will know the charging needs of your battery, Use this knowledge to find a suitable charger circuit. Since you want to charge a 9V battery from a 5V port, you will need a circuit with an integrated step-up regulator. You can find suitable charger IC e.g. on LTC's website.
  3. aaronmarsh632


    Jun 28, 2013
    Hi, thanks a lot for this information, I will check out the links you suggested as I have lots to learn :)

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