Measuring Amps from a power brick

Discussion in 'General Hardware' started by Extraordinary, Jun 23, 2016.

  1. Extraordinary

    Extraordinary Guest

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    Bought a Raspberry Pi 3, and the charger it came with states 5.1v 2.5A (Recommended specs for the Pi 3), connected the gf's Nexus 7 2013 to it while I was using the tablet, and the battery level is dropping while I'm using it

    Connected my S5 adapter, 5.2v 2A to it, and the battery is charging while I'm using it

    So from that, I'm guessing I've been stung with a lower spec power adapter for the Pi, lower than 2A if the S5 charger is charging the Nexus, and the Pi charger is not

    Any way to measure it?

    EDIT - Looks like I need something like this

     
    Last edited: Jun 23, 2016
  2. alanm

    alanm Ancient Guru

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  3. Extraordinary

    Extraordinary Guest

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    I bought it from eBay as a package, Pi, adapter, sdcard, case

    Would need some sort of proof it wasn't 2.5A if I wanted to get anywhere with a return

    Telling them my S5 charger charges an N7 faster than the one he sold me, isn't really going to cut it :)
     
  4. AsiJu

    AsiJu Ancient Guru

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    AFAIK the 0.1 V difference between the output voltages of the adapters is enough to account for that I think.
    It's the voltage that determines whether a battery gets charged or not (the potential difference between the battery and the charger must be high enough for charging to happen).

    Amperage affects charging speed, more amps equals faster charging, if the battery can take it that is.

    That's actually how your phone/tablet shows the charge level, by measuring the voltage level of the battery.
    The difference in voltage between a fully charged and an almost empty battery is only 0.X volts so a 0.1 V difference in charger output can be the difference between charging and not charging.

    What's the nominal voltage for the battery btw?
     

  5. XL_ence

    XL_ence Maha Guru

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    Many phones need to have the USB data pins shorted in order to have fast charging through USB connector.

    Your S5 adapter probably has the data pins configured this way, so fast charging is enabled on your tablet.

    Raspberry Pi charger may not have the data pins shorted, and thus your phone only has slow charging mode with it. It does not necessarily mean your Raspberry Pi charger cannot support 2.5A.
     
  6. Extraordinary

    Extraordinary Guest

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    The tablet is not charging with the Pi adapter though, it's discharging while connected to the Pi adapter

    I have a few other adapters, all rated 5v or slightly higher 5.2v, tablet requirement is 5v, as is the Pi, and it works with the Pi, so the voltage is not the problem

    1.35A is the required amperage for charging the N7, all adapters are way higher than 1.35A, the lowest being 2A, all can charge the N7 while it is being used

    The Pi adapter is rated the highest at 5.1v 2.5A and that can not charge it while it is in use

    Anyway, I contacted the seller, he sent me another adapter, this one works, has the correct output, or at least enough to charge and use, but it plays games with another windows tablet I have, that tablet uses the same MicroUSB port to charge as it does for peripherals, all adapters I own work flawlessly with it, apart from the new Pi adapter, that somehow causes a single touch on the touchscreen to become multiple random fast touches in a horizontal line across the screen, meaning I can not use the tablet because it's trying to press 10 things at once

    Disconnect the Pi charger, and it works fine again

    I'm guessing the guy has a batch of bad Pi adapters, I've sent him a message saying I'm not looking for any replacement or refund, but suggest he gets those adapters checked in case they all have a potentially dangerous fault/short
     

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