linux-kernel - Re: [PATCH v2 4/4] power: supply: Add Spreadtrum SC27XX fuel gauge unit driver

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Date:   Wed, 26 Sep 2018 10:09:20 +0200
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Baolin Wang <baolin.wang@...aro.org>
Cc:     Sebastian Reichel <sre@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Linux PM list <linux-pm@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        yuanjiang.yu@...soc.com, Mark Brown <broonie@...nel.org>,
        Craig Tatlor <ctatlor97@...il.com>
Subject: Re: [PATCH v2 4/4] power: supply: Add Spreadtrum SC27XX fuel gauge
 unit driver

On Wed, Sep 26, 2018 at 5:00 AM Baolin Wang <baolin.wang@...aro.org> wrote:

> This patch adds the Spreadtrum SC27XX serial PMICs fuel gauge support,
> which is used to calculate the battery capacity.
>
> Original-by: Yuanjiang Yu <yuanjiang.yu@...soc.com>
> Signed-off-by: Baolin Wang <baolin.wang@...aro.org>
> ---
> Changes from v1:
>  - Use battery standard properties to get internal resistance and ocv table.
>  - Change devm_gpiod_get_optional() to devm_gpiod_get().
>  - Add power_supply_changed() when detecting battery present change.
>  - Return micro volts for sc27xx_fgu_get_vbat_ocv().

Acked-by: Linus Walleij <linus.walleij@...aro.org>

I see that you need to have the internal resistance as parameter to
this equation so it needs to come in. (I guess as factor/typical
internal resistance as suggested.)

IIUC this reliance on an uncertain internal resistance is the reason
why fuel gauges / coloumb counters are seen as crude ways to
estimate battery capacity, and their accuracy and reliability worsen
over the lifetime of the battery, and this is why safety-critical devices
include ways to measure the internal resistance as well.

But the hardware is as it is, and probably for some good reasons,
like cost.

Yours,
Linus Walleij