linux-kernel - Re: generic-adc-battery volatge-to-percent formula

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Date:   Tue, 11 Apr 2017 19:31:18 +0200
From:   Pavel Machek <pavel@....cz>
To:     "H. Nikolaus Schaller" <hns@...delico.com>
Cc:     Sebastian Reichel <sre@...nel.org>, pali.rohar@...il.com,
        kernel list <linux-kernel@...r.kernel.org>,
        ivo.g.dimitrov.75@...il.com, patrikbachan@...il.com,
        serge@...lyn.com, abcloriens@...il.com,
        Belisko Marek <marek.belisko@...il.com>,
        Linux PM mailing list <linux-pm@...r.kernel.org>
Subject: Re: generic-adc-battery volatge-to-percent formula

On Mon 2016-10-24 15:09:26, H. Nikolaus Schaller wrote:
> Hi Pavel,
> 
> > Am 24.10.2016 um 14:34 schrieb Pavel Machek <pavel@....cz>:
> > 
> > Hi!
> > 
> >> On Mon, Feb 22, 2016 at 06:58:13AM +0100, H. Nikolaus Schaller wrote:
> >>>>> static inline int fuel_level_LiIon(int mV, int mA, int mOhm) {
> >>>>> ...
> >>>>> }
> >>> 
> >>> To which header file should this go?
> >> 
> >> I think it should get its own header file in include/linux/power/.
> >> Maybe something like "generic-fuel-gauge.h". I'm open to other
> >> solutions, though.
> > 
> > I'd like to use use this formula in my own code. Is it somewhere in
> > the kernel already?
> 
> Not that I am aware of. Marek is still working on the generic-adc-battery
> driver augmented by DT + iio-ADC + formula before publication.
> 
> Here is the latest draft so you can cherry-pick it into your work:
> 
> 	http://git.goldelico.com/?p=gta04-kernel.git;a=patch;h=22ab047ae296e998379c1aa29fe1210043cfa040
> 
> But beware: I think it is quite wrong using the sqrt() function above 19.66%
> and doing linear interpolation below.
> 
> By using the sqrt() function it has a steepness that goes to infinity when reaching
> 19.66% from above. This makes a quite non-realistic curve with a sharp bend at 19.66%
> (which is equivalent to 3.756 V).
> 
> No real battery I have seen and measured with a coulomb counter has such a strange
> bend at 3.756 V...
> 
> IMHO it would be a better approximation to adjust the factors so that e.g. a realistic
> voltage for "empty" (e.g. 3.3 V) is taken as the 0% point where steepness goes through
> the roof.
> 
> It should then be something like:
> 
> 	SOC = sqrt((Volt - 3.3 V) / (4.2 V - 3.3 V))
> 
> This goes more smoothly between 100% and 0%.
> 
> Or we could even use an exponential function with an exponent different from 1/2
> (1 being linear interpolation):
> 
> 	SOC = pow((Volt - 3.3 V) / (4.2 V - 3.3 V), factor)
> 
> But this would probably need floating point arithmetic in the kernel or some numeric
> approximation algorithm.

Thanks!
									Pavel



-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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