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32367 Views 12 Replies Latest reply: Feb 28, 2012 11:50 AM by MauPham RSS
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Feb 13, 2012 1:26 PM

Charging a 6 Volt battery with a 5V usb conection

I have a device that uses an usb connector to talk to a computor.  Lookng for a method on charging a 6 Volt lithium battery set.  Obviously I need a switch power supply that can change a 5 V 1.5 A source to a 7 volt with enough current to charge the battery in a reasonable time.  I can find power supplies on the digikey web page that will do this but at 50 mA which would take for ever to charge.  Is there a method or a joule thief power supply part that can do this at a good rate. 

  • Ben designspecialist-sensors 35 posts since
    Mar 21, 2011
    Currently Being Moderated
    Feb 13, 2012 4:22 PM (in response to AllieCat)
    Charging a 6 Volt battery with a 5V usb conection

    Are you looking to get 5V @ 1.5A out of a normal USB port on a computer? Typically, USB's max output is 2.5W (500mA) and that target is realized only with device/host negotiation (external chargers designed for cell phones often can supply that much).  Also, there are some major issues with just throwing a voltage on lithium rechargeable; they die a fiery death if not done properly.  Is the output of this boost converter going into an appropriate charger (constant current then constant voltage) that balances the cells if needed?  If this is for a hobby project I'd highly recommend just buying an off the shelf charger for your battery.  If this is for a production unit, it might warrant a bit more research before deciding what option to go with.

  • tarheit Novice 3 posts since
    Feb 13, 2012
    Currently Being Moderated
    Feb 15, 2012 8:13 AM (in response to AllieCat)
    Charging a 6 Volt battery with a 5V usb conection

    Why not use something like VWRBS2-D5-S9-SIP.   dc-dc converter, 4.5-9VDC input, 9VDC output, 222mA maximum output

     

    http://search.digikey.com/us/en/products/VWRBS2-D5-S9-SIP/102-1506-ND/1558901

     

    You will of course need to limit the current and reduce the voltage to meet your charging requirements, but that's the easy part.

    • Ben designspecialist-sensors 35 posts since
      Mar 21, 2011
      Currently Being Moderated
      Feb 15, 2012 10:17 AM (in response to AllieCat)
      Charging a 6 Volt battery with a 5V usb conection

      So to be 100% clear. You need 9V in your system for something other than charging batteries.

        • Ben designspecialist-sensors 35 posts since
          Mar 21, 2011
          Currently Being Moderated
          Feb 15, 2012 3:27 PM (in response to AllieCat)
          Charging a 6 Volt battery with a 5V usb conection

          If all you are trying to do is power 5V components and charge a lithium battery off you should be able to get away with a single cell lithium technology battery and two chips(charger w/OVP and boost converter w/ over discharge protection).  There's likely no need for a linear regulator when a switcher can just generate the 5V you need and offer over and under voltage protection (both critical in this case) for the Li battery.  If the linear regulator is off in an untouchable part of the design or there are other needs like noise or ripple problems that the linear regulator can help solve then there may be a case for it. 

           

          Are you sure you have a 6V set of batteries (I'll assume set of 2).  Most Li based product are nominally around 3.7V (7.4nom or ~8.4 charging), and most chargers are designed around that range of voltages and specific type of battery (there are many types of Li batteries).  Designing incorrectly with Li rechargeables can have extreme consequences, especially if going into a product.  You may want to consider a chemistry such as NiMH that has it's own trade-offs, but doesn't have the same issues if done incorrectly.  If working with lithiums, quite a more research and design requirements need to go in the process and talking to your lithium battery supplier for recommendations might be warranted.  Here's another approachable article on some of the pitfalls of using Li technology.  It is geared more toward hobbyists, but the information is relevant all the same.  There are lots of references designs out there  (more on than just in that link) for this and similar applications, and even premade boards that include both the MCP37831T and TPS61200 http://www.google.com/search?q=MCP73831T+and+TPS61200.

  • Sripathy Novice 2 posts since
    Feb 28, 2012
    Currently Being Moderated
    Feb 28, 2012 11:32 AM (in response to AllieCat)
    Charging a 6 Volt battery with a 5V usb conection

    Possibly the best solution (for charging a 6 V battery(Chargeable ones only) is to possibly use a doubler and try a zener to clip it to what evr level required and try chrging. Please note that you the chrging current is limited at source(USB PORT) to 500 ma, ideally you should not draw more than 250 ma, internal circuit capacities could be different with different computers/devices, That's why!

  • MauPham Novice 7 posts since
    Feb 28, 2012
    Currently Being Moderated
    Feb 28, 2012 11:50 AM (in response to AllieCat)
    Charging a 6 Volt battery with a 5V usb conection

    Alliecat:

     

    The Constant Current and Constant Voltage specs of a Li Rechargeable Battery serve as limits not to exceed. Charging the Battery under those limits are safe.

     

    If you use USB as the power source, you're unlikely to exceed the Constant Current limit. Just observe the Constant Voltage limit.

     

    Measuring the voltage of a battery doesn't comsume power through the current-sense resistor like measuring the charging current, and is more easily accurate.

     

    For only 2 cells, and within a limited number of charge/recharge cycles, cell balancing is not necessary if you start out with matched cells.

     

    Also, at the low charging current provided by USB, over-temperature protection might not be necessary.

     

    If your moniker AllieCat suggest your cheap modus operandi, you can cut lots of corners to save money and save power waste (in order to pump charge into the battery faster) when designing the battery charger for USB only.

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