linux-kernel - Re: [PATCH v3 3/6] Documentation: DT: Document twl4030-madc-battery bindings

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Date:	Tue, 17 Mar 2015 14:59:13 +0100
From:	Pavel Machek <pavel@....cz>
To:	"Dr. H. Nikolaus Schaller" <hns@...delico.com>
Cc:	Belisko Marek <marek.belisko@...il.com>, mark.rutland@....com,
	Sebastian Reichel <sre@...nel.org>,
	Dmitry Eremin-Solenikov <dbaryshkov@...il.com>,
	David Woodhouse <dwmw2@...radead.org>,
	Tony Lindgren <tony@...mide.com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
	Linux PM mailing list <linux-pm@...r.kernel.org>,
	GTA04 owners <gta04-owner@...delico.com>
Subject: Re: [PATCH v3 3/6] Documentation: DT: Document twl4030-madc-battery
 bindings

Hi!

> >>>>> to introduce coefficients for temperature and discharge rate?
> >>>> What do you mean? Nothing like that is used in current driver why do
> >>>> we need to add it?
> >>> 
> >>> Well, conversion between Li-ion's voltage and state of charge at 0
> >>> current is well known:
> >> 
> >> We can’t measure at 0 current since the OMAP is driven from battery
> >> and charger and may also draw some mA…
> > 
> > Yes, but you know how many mA you are taking just now. So if you knew
> > the internal resistance, you could compute the voltage at 0
> > current. (And it should also work during charging, as long as you know
> > how much current is going in.)
> 
> As far as I understand the twl4030 charger and MADC it is not possible to
> separate these values. It is only reporting the inflow from charger to
> battery + system. So you don’t know how many mA are supplying the system
> and how many mA are left over for charging.
> 
> You can only assume how much the system is drawing while running (something
> between 50 and 600 mA but this depends on system activities, power state
> of peripherald and e.g. backlight being switched on).
> 
> I think your basic assumption that we know any time how many mA the system
> is taking is not given.

So.. you won't be able to get exact value while charging, but you
get one while discharging, which is what really matters...?

> > Yes, and that coefficient should be internal battery resistance ;-).
> 
> But where do you know this value from to write it into a DT file?
> Usually you can’t measure it easily and for some batteries you don’t have
> a data sheet.
> 
> Contrary, the calibration curves can easily be measured on the device
> (assuming that the charge level decreases/increases linearly over time
> between Full and Empty).

If you can copy it from the data sheet, that's the easiest option. If
not, you should be able to easily compute it from the charge/discharge
curves or from measured voltage at different loads.
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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