[Gta04-owner] Battery charge profile

NeilBrown neilb at suse.de
Sat Feb 25 22:14:56 CET 2012

Does anyone know much about battery charging?

I've been looking at what the battery charger is actually doing.
Attached are two graphs created by taking 5 minute samples of various numbers
while the GTA04 was on with the screen lit, so it was drawing over 200mA

The first, charge-current.png compares the current seen by the battery with
the current seen by the USB port.
It just gives a simple picture of what is happening.  The battery is charged
for a while.  As it gets full the current flowing in to it slows and stops.
Then the charger switches off for about 100 minutes, then turns on again
and charges the battery back up.

The second, charge-volts.png, compares the voltage at the battery with the
charge in the battery (i.e. how much charge it holds).

Here we see that when charging stops, the voltage and the charge steadily
drop until the voltage reaches 3.95V, then charging starts again.  This
agrees with the docs.

When the voltage is 3.95V, the charge (on this battery at least) is
approximately 75% of full.

My inclination is that I would rather it didn't get quite that low.  90% is
about the minimum I feel comfortable with, 95% is probably better.

I can program that voltage cut-off level to anything from 3.75V to 4.1V
(approximately) in 23.4mV steps.
4.1V corresponds to a charge of about 94%.

So to my question:  What is a good profile for a battery to encourage long
life, or to balance that sensibly with usefulness?

How much would it hurt the batteries to make 4.1V the default so that when
plugged in to charge, it never lets it get below 94%??  Or should I aim for
90% (4.05 V).

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