Hello,

battery.el doesn't allow the customizing of the regexp used to filter
folders under /sys/class/power_supply/.  On more exotic systems, this
doesn't necessarily have the name BAT[0-9].  For example, on my machine
I have:

matthew@armson /sys/class/power_supply $ ls
bq27500-0  gpio-charger

Attached is a patch that allows this value to be customized.

Thanks,
Matt

In GNU Emacs 25.1.50.1 (armv7l-unknown-linux-gnueabihf, GTK+ Version 3.18.9)
 of 2016-04-03 built on armson
Repository revision: f7c5f79ca565d3ef3feeb1e0af5ca261f1bcf58a
Windowing system distributor 'The X.Org Foundation', version 11.0.11802000
Configured using:
 'configure --prefix=/usr/local'

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LIBXML2 FREETYPE XFT ZLIB TOOLKIT_SCROLL_BARS GTK3 X11

Important settings:
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Mark saved where search started
battery-linux-sysfs-regexp
C-x C-g is undefined
Saving file /home/matthew/Development/emacs/lisp/battery.el...
Wrote /home/matthew/Development/emacs/lisp/battery.el
Mark set
Mark saved where search started [2 times]
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Saving file /home/matthew/Development/emacs/lisp/battery.el...
Wrote /home/matthew/Development/emacs/lisp/battery.el

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Memory information:
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-- 
Matt