[Gta04-owner] battery holder

[Christoph Mair]

On Mon, Dec 19, 2011 at 9:02 PM, NeilBrown <neilb at suse.de> wrote:
> On Mon, 19 Dec 2011 18:14:35 +0100 Andreas Kemnade <andreas at kemnade.info>
> wrote:
>
>> Hi,
>>
>> as there seems to be again problems with empty batteries and charging the
>> gta04.
>> I am thinking about printing a separate battery holder (I own a reprap) to be
>> able to charge the battery externally. How do I make stable contacts to the
>> battery? The electronics is not the problem.
>>
>> Or is the problem with empty batteries already solved? Unfortunately I have no
>> GTA04 yet to try it out. I hope that situation changes in the next days.
>
> It isn't solved but I have hopes of solving 99% of the problem.
>
> My experience is that if you leave a flat battery for a while it will gather
> up what charge it has and be able to deliver it - maybe only for a second.
> That much time should be enough for uboot to enable the battery-charge
> interface and run off USB power.
>
> So we would need to make sure that uboot enables charging very early, and
> make sure that Linux doesn't disable charging - even for a moment.
>
> One problem with this is that it is not correct to draw current from a USB
> cable unless we have determined that current is available, and determining
> current availability from a PC is (I think) a bit more complicated than uboot
> should be.  Detecting a charger is easy so you might need to use a charger to
> charge a nearly-empty battery.   I still have stuff to learn here though.
>
> Currently the device doesn't charge when off because we turn the battery
> charger off.   That should be easy enough to fix.
The GTA04 will automatically charge the battery if you connect it to
some USB Host. To meet the USB spec it charges only with 70mA, As soon
as the battery voltage reaches a specified level (IIRC, 3.2V) the
device will power on and try to boot. With neils current kernel it
takes to long to get to the point where the kernel enables fast
charging with 500mA and the GTA04 switches off again, but without all
those debugging features it is no problem reaching that point.

You can also prevent it from booting linux, and type "systest charge"
into your u-boot console. This enables fast charging.

> Also the gta04 board has the  possibility for being provided stable current
> and booting from that with no battery.  This current comes in through the
> same port that the serial cable is connected to.  I cannot think of a way to
> provide power through that port but if you could, you could charge a
> completely flat battery.

This is supposed to work, but due to a bug inside the TPS power
management chip it does not work. Texas Instruments has documented
this errata with "no workaround known". Charging the battery using
this method works fine. The input voltage should be about 5V. Absolute
max is 6.5V IIRC. Make sure you get the polarity right.

Best regards,
  Christoph