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

Pavel Machek
2015-03-17T13:59:13Z

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
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