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Safe USB battery extender

Discussion in 'Power Electronics' started by thezombiekat, Oct 6, 2014.

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  1. thezombiekat

    thezombiekat

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    Oct 6, 2014
    Safe USB battery extender


    I am building a custom case for my ASUS tablet PC. I would like to include a supplemental battery so it will last a full day of use.


    The tablet charges from USB but came with a wall charger (that takes a USB cable) its output is 5V 2A.


    I have found many methods on the net for making this on the net all are very simple, almost identical and most carry a warning that you cant just leave the unit charging untended because the batteries could overcharge, over heat and leak.


    Commercial units don’t have this problem so I assume they have a circuit built in that prevents overcharging.


    Is it possible for me to build that circuit into my tablet case, where would I find instructions.


    Thankyou


    The Zombie Kat
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Welcome to our forum.
    Could you post a link to one of your findings on the net?

    I wonder why leaving the tablet plugged into the charger should damage it. Typically the external charger is nothing more than a voltage source 5V, xxA. All the charging is done internally in the tablet including voltage regulation, supervision etc.
    These warnings were probably posted as a disclaimer only in case something happens to the tablet when using a charger other than the original one from ASUS.
     
  3. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
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    Nov 28, 2011
    Harald, I think the overcharging danger may relate to the battery in the USB charger, not the battery in the tablet.
     
  4. thezombiekat

    thezombiekat

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    0
    Oct 6, 2014
    that is quite correct. allegedly the external battery pack is at risk while plugged into the wall (via USB charger). the tablet is safe (well other than proximity to leaking batteries).

    here is a link to one of the ones i found and didn't quite trust
    http://www.wikihow.com/Make-a-Portable-and-Rechargeable-USB-Charger
     
  5. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Sorry, this wasn't all too clear for me from the original post. The "circuit" (I barely dare call it a such) linked in post #4 definitely is at risk of overloading the batteries. No charge controller is being used.

    You should use a charging circuit that is suitable for the supplementary battery you are going to use. For a NiMh battery you could use e.g. this controller chip. It will provide the necessary protection for the battery pack. Controllers similar to this are available from many manufacturers and for other types of battery, too.
     
    KrisBlueNZ likes this.
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