the state file is kinda static, as in it shows the current state, from which one cannot deduce the percentage.

The info file is much more informative:

$ cat /proc/acpi/battery/BAT0/info
present:                 yes
design capacity:         5144 mAh
last full capacity:      5144 mAh
battery technology:      rechargeable
design voltage:          10800 mV
design capacity warning: 200 mAh
design capacity low:     100 mAh
cycle count:              0
capacity granularity 1:  100 mAh
capacity granularity 2:  100 mAh
model number:            Primary
serial number:           49233 2011/02/25
battery type:            LIon
OEM info:                Hewlett-Packard
[download]

Combining the two gets you to:

$ cat /proc/acpi/battery/*/* | grep capacity:
design capacity:         5144 mAh
last full capacity:      5144 mAh
remaining capacity:      5144 mAh
$
[download]

Or in perl:

$ perl -MDP -e'while(<>){/^(.*)\s+capacity:\s*((\d+).*)/ and$i{$1}=[$2
+,$3]}print"$_: $i{$_}[0], "for keys%i;printf"%5.1f%%\n",100*$i{remain
+ing}[1]/$i{design}[1]' /proc/acpi/battery/*/*
remaining: 5144 mAh, design: 5144 mAh, last full: 5144 mAh, 100.0%
[download]

I use this code in gnu screen's statusbar:

backtick 1 1 1 sh -c 'perl -ne "/^(la|re|st)[^:]+:\s+(\S+)\s/and \$h{\
+$1}=\$2}{\$h{st}=~s/of.*/-/;\$h{st}=~s/on.*/+/;printfq(%.0f%%%s),\$h{
+re}*100/\$h{la},\$h{st}" /proc/acpi/battery/BAT0/* /proc/acpi/ac_adap
+ter/ACAD/state'
[download]

But what if you have two batteries? Some laptops allow that. You will have multiple lines.
Information doesn't want to be free. It wants to be feline.

Accumulate? Untested concept:

$ perl -MDP -e'while(<>){/^(.*)\s+capacity:\s*((\d+).*)/||next;$i{$1}[
+0]+=$2;$i{$1}[1]//=$3}print"$_: $i{$_}[0], "for keys%i;printf"%5.1f%%
+\n",100*$i{remaining}[1]/$i{design}[1]' /proc/acpi/battery/*/*
[download]


Enjoy, Have FUN! H.Merijn