linux-kernel - RE: [PATCH v7 1/4] gadget: Introduce the usb charger framework

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Date:	Mon, 18 Apr 2016 10:59:23 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Pavel Machek' <pavel@....cz>, Felipe Balbi <balbi@...nel.org>
CC:	Baolin Wang <baolin.wang@...aro.org>,
	Greg KH <gregkh@...uxfoundation.org>,
	Sebastian Reichel <sre@...nel.org>,
	"Dmitry Eremin-Solenikov" <dbaryshkov@...il.com>,
	David Woodhouse <dwmw2@...radead.org>,
	Peter Chen <peter.chen@...escale.com>,
	Alan Stern <stern@...land.harvard.edu>,
	"r.baldyga@...sung.com" <r.baldyga@...sung.com>,
	Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>,
	Lee Jones <lee.jones@...aro.org>,
	Mark Brown <broonie@...nel.org>,
	Charles Keepax <ckeepax@...nsource.wolfsonmicro.com>,
	"patches@...nsource.wolfsonmicro.com" 
	<patches@...nsource.wolfsonmicro.com>,
	Linux PM list <linux-pm@...r.kernel.org>,
	USB <linux-usb@...r.kernel.org>,
	"device-mainlining@...ts.linuxfoundation.org" 
	<device-mainlining@...ts.linuxfoundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v7 1/4] gadget: Introduce the usb charger framework

From: Pavel Machek
> Sent: 18 April 2016 11:40
...
> > >> > Actually, less is not stupid. Charging li-ion battery from li-ion battery might
> > >> > be stupid. Imagine I'm on train, with device like N900 (50% battery) and power bank
> > >> > (3Ah). I'm actively using the device. If I let it charge at full current, I'll waste
> > >> > energy. If I limit current to approximately the power consumption, it will run the
> > >> > powerbank empty, first, then empty the internal battery, maximizing total time I
> > >> > can use the device.
> > >>
> > >> why would you waste energy ? What the charger chip would do is charge
> > >> battery to maximum then just to maintenance charge from that point
> > >> on. Where is energy being wasted other than normal heat dissipation ?
> > >
> > > Physics 101, of course wasted energy goes to heat. Lets not waste
> > > energy by charging li-ion from li-ion when it is not required.
> >
> > your cellphone has no means to know that it's connected to a Li-Ion
> > battery. We don't have visibility on what we're connected to, just how
> > much it can source.
> 
> But cellphone user knows what he connected his charger to, and that's
> why it is useful to be able to lower the current. Even when you said
> "less is just stupid" I demonstrated it is not, at least in case when
> your power source is a battery.

It reality you may want the phone/tablet to be configurable to take power
from USB, but disable the li-ion charging circuit.
That will maximise the time you get when running from an external battery.
I connect my tablet to the 1A output (which discharges the internal battery
slowly) rather than the 2A one (which will charge it with some cables).

Getting 2A from a USB charger seems to be very device/cable/charger dependant.
We've two Samsung chargers that look similar, but have completely different
characteristics. Both claim 2A at 5v (one says 5.3v), one claims 1.5A at
something like 12v as well - not sure we should be using that one for
'random' devices.

	David