Used this command and rebooted.

No change in space used. There is still 1.6% in use.

Reserved space is not included in the output of the df command. Add the numbers up and see.

reserved = size - used - available.

I have been using numbers that Gparted shows.

Reserved space is part of the filesystem and not overhead.

I know that some of the space is used for bad blocks.

Any disk drive made in the last decade remaps bad sectors to spare sectors internally and presents the OS with an error-free medium once this is done. Writing a bad sector with new content should be all that is necessary to correct the problem unless the drive is in such bad condition that no spare sectors remain. Yes, the filesystem does have a "bad block" inode, and a block can be assigned to that inode to prevent it from being used, but that list is typically zero-length and in any event takes very little space. A single disk block could hold a list of 512 bad block numbers, and the disk should be discarded long before that.

I decided to delete the unused partition and increase the size of another partition in use.

I am happy with Slacko and Ubuntu and no longer need to test other distros to see if there is "something better". :-)

P.S. I got a battery backup about a month ago.

Real happy with it especially since my power company has frequent brownouts.

I have my computer,router, and monitor plugged in.