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/*/*
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