battery charge time and tests

Bas Wijnen wijnen at
Thu Feb 3 16:34:27 EST 2011

Op 03-02-11 20:53, Werner Almesberger schreef:
> Bas Wijnen wrote:
>> If anyone has info about how to do this (or pointers to Linux source
>> files which read the capacity), I'm interested. :-)
> I'd look at the schematics [1]:

The citation is missing, but your explanation is good enough. :-)

> the red LED is connected to the
> charger chip U5 and - with diode D4 preventing high VUSB flowing into
> the CPU - to the CPU.
> You could try to force it off by outputing a "high" level on CHARGE_N,
> "fighting" U5's FET pulling down, but you probably won't reach a
> sufficient voltage difference to accomplish much.

Not sure if this is useful anyway, but I looked in the Jz4720 datasheet
and can't find a CHARGE_N pin. The schematics I found so far weren't
readable enough (for me anyway) to conclude what was connected where, so
I have the question: is CHARGE_N just a gpio which is named because of
its use (by the kernel), or is it a missing pin on the Jz4720?

> Because of D4, you can't force it on.

Which makes it not very useful anyway...

> The only feedback on battery status you have is +VBAT on the ADC
> (ADIN0).

And now I see that this is located in the "touch screen" section of the
programmer's manual, which explains why I never saw it before. :-)

> A USB current < 400 mA would suggest that the PW_ON_N switch (Q3)
> isn't acticated. In the case, the nominal charge current (which
> includes overall system consumption as well) should be 1100V/R15 =
> 110 mA.  If PW_ON_N is set, it should raise to approximately
> 1100V/(R14 || R15) = 550 mA.

So you're saying that the battery always charges with (at most) 110 mA?
That's quite a lot less than the maximum USB current of 500 mA.

> Note that this is the current that leaves U5 and is shared by the
> battery and the rest of the system.
> Disclaimer: all this is according to schematics and data sheets.
> Reality may choose to differ :)

Of course. :-) The 180 mA I measured was for an empty battery with the
device turned off. It does still supply the rtc though (but I suppose
that's what you mean by the overall system consumption). When I remove
the battery, it takes 4.4 mA, so that should be subtracted.

I suppose 4.4 mA is also the amount by which it drains the battery when
it is turned off, leading to an empty battery in:
capacity: 850 mAh * 3.7 V = 11.3 MJ
Draining with 4.4 mA * 5 V = 22 W
leads to 11.3e6 / 22 = 515e3 s = 143 h = 6 days.

When switched on, it drains with values depending on its use, but the
maximum is around 500 mA. In that case, the result is multiplied by
4.4/500, leading to 75 minutes.


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