help-echo from battery-mode-line-string fails

[Dave Love]

For some reason, the help-echo property on `battery-mode-line-string'
doesn't have an effect in the mode line.  It works in 21.3 and if you
insert the string in a buffer.  I can't see what's special about it.

By the way, battery.el should provide a minor mode that can be turned
on via Custom (using `display-battery').  I think it has to cancel the
timer and update global-mode-string when turning off the mode.