LG
LG April 17, 2012 01:38

Charging specs for the P920 and other phones with standard micro USB

I have a P920. Most of the time I need it connected to one of my laptop's USB 2.0 ports. The battery never fully charges. In fact even if the phone is 100% charged from LG's official charger, it will actually discharge very gradually while connected to the laptop. I don't know whether this is good for the long-term health of the battery, and is very annoying that I can't use the data at the same time as charging.

So the phone is using more than 500mA@5V (2.5W). Please can someone provide me with an official LG source of how much power the phone can draw from the LG charger, and what USB charging standards LG complies with.

USB 3.0 can provide upto 900mA current (4.5W), if the device requests it, but is the P920 able to request this power from USB 3.0? I'm not aware that the P920 supports USB 3.0, so I'm not sure whether buying a powered USB 3.0 hub will help. And is 900mA enough for optimal charging?

Another issue is that when travelling and using the USB cable for data, the phone insists on draining my laptop's battery to charge it's own battery. The phone's battery lasts much longer than my laptop's, and charging one battery from another is inefficient energy wise. I would like to disable charging while travelling.

Ideally, I'm looking for a micro USB adapter/cable that has 2 USB type-A plugs - one for USB 2.0 high-speed data that draws zero or minimum power from the host, and one for power that draws maximum power from LG's official charger. If the adapter must still draw power from the laptop, it should be no more than 100mA (the minimum current for USB 2.0 and lower).

I don't think this is as simple as splicing the power (pin1/VBUS and pin4/GND) and data(pin2/D- and pin3/D+) from 2 cables into one. I don't think this would meet USB specs or even work without a small hub-like adapter.

I would expect the adapter/cable to look similar to this:

Adapter hub cable with power and data host plugs

It should cost no more than å£20. Maybe a bit more if it does everything I'd expect: draws zero or minimum power from laptop, draws maximum power from LG's official charger, ability to function without the charger while still drawing zero or minimum power from laptop, abilty to still charge from the charger when disconnected from laptop.

Thank you,

James.

[from James Haigh]
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  • James Haigh
    James Haigh May 07, 2012 21:07

    The battery never fully charges. In fact even if the phone is 100% charged from LG's official charger, it will actually discharge very gradually while connected to the laptop. I don't know whether this is good for the long-term health of the battery, [...]


    I have recently found out that gradual charge/discharge of Lithium-ion batteries is better for their long-term health than rapid charge/discharge. Also, maintaining the charge level of Lithium-ion between 40 and 60% is better for the health than full charge-cycles from 0 to 100%. Full charge-cycles should only be used when needed, for maximum time without mains power.

    So it seems that slowly charging from my laptop is actually /better/ for long-term battery health than LG's charger (which makes me wonder why LG discourage the use of other chargers in the user guide). So an adapter to combine LG's power with the laptop's data is pointless with regard to battery health.

    I'm pleased with this finding.


    Another issue is that when travelling and using the USB cable for data, the phone insists on draining my laptop's battery to charge it's own battery. The phone's battery lasts much longer than my laptop's, and charging one battery from another is inefficient energy wise. I would like to disable charging while travelling.


    This is still an issue.

    I've discovered that most Android devices have software control over charging exposed in:
    /sys/class/power_supply/battery/

    This sysfs directory is device-specific. Unfortunately, the LG p920 does not seem to have support for disabling charging.

    There is a useful discussion on XDA. Solutions (for other devices) start to appear on page 2:
    http://forum.xda-developers.com/showt...

    I was very impressed to see that this device has exact control over the charging current in mA (see "om battery charger-limit [0-500]"):
    http://manpages.ubuntu.com/manpages/m...

    Another, idea is to cut VBUS from a USB cable. Apparently this does *not* work because VBUS and thus power is /required/ for device detection. And no detection means no data.

    Well, I think I've explored all avenues on this second issue, unfortunately with no solutions for this phone.

    James.

    Ps. Yay! Get Satisfaction!
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  • Katrinna LG
    Katrinna LG June 04, 2012 19:41
    I'm sorry, we do not have the resources to support phones outside the USA within this forum. Here is the support page for International phones:

    http://www.lg.com/hk/mobile-phone/all...
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