how to cite a website without getting an "underfull hbox" warning?

Question

I am trying to cite a website but I am getting this message

Underfull \hbox (badness 1939) in paragraph at lines 9--11 

my citation is

@MISC{K&J,
author        = {k\&J magnetics},
year          = {2011},
title         = {The original K\&J magnet calculator},
howpublished  ={\url{https://www.kjmagnetics.com/calculator.asp}},
note = {Accessed 2020-2-30},
}

the packages I am using

\documentclass[5p,preprint]{elsarticle}

\journal{Energy}
\setcitestyle{square}
\usepackage{times}
\usepackage{hyperref}
\usepackage{graphicx}
\usepackage{subfig}
\usepackage{amsmath}
\usepackage[RPvoltages]{circuitikz}
\usetikzlibrary{patterns}
\usepackage{array,booktabs}
%\usepackage[round, sort & compress]{natbib}
\usepackage{tikz}
\usepackage{float}
\hypersetup{pdfauthor=whatever}

Answer 1

Some suggestions and observations:

• To avoid getting warning messages about either underfull or overfull lines because of URL strings, load the xurl package before loading hyperref.

• Since the elsarticle-num bibliography style recognizes a field called url, use it, i.e, replace howpublished = {\url{https://www.kjmagnetics.com/calculator.asp}}, with url = {https://www.kjmagnetics.com/calculator.asp},.

• Encase the string K\&J in the title field in curly braces to keep it from getting lowercased by BibTeX.

• Change the author field from {k\&J magnetics} to {{K\&J Magnetics}} so that the author is recognized as a "corporate" author and is sorted under K, not M.

• With the elsarticle document class, it's better to specify times as a document class option than it is to load it as a package.

\documentclass[5p,preprint,times]{elsarticle}
\journal{Energy}
\begin{filecontents}[overwrite]{testbib.bib}
@MISC{K&J,
author        = {{K\&J Magnetics}},
year          = {2011},
title         = {The Original {K\&J} Magnet Calculator},
xxhowpublished= {\url{https://www.kjmagnetics.com/calculator.asp}},
url           = {https://www.kjmagnetics.com/calculator.asp},
note          = {Accessed 2020-2-30},
}
\end{filecontents}

\bibliographystyle{elsarticle-num} % information gleaned from an earlier posting
\setcitestyle{square}

\usepackage{graphicx,subfig,amsmath,array,booktabs}
\usepackage{tikz}
\usepackage[RPvoltages]{circuitikz}
\usetikzlibrary{patterns}

\usepackage{xurl} % <-- new
\usepackage[colorlinks,allcolors=blue]{hyperref}
\hypersetup{pdfauthor=whoever}

\begin{document}
\cite{K&J}
\bibliography{testbib}
\end{document}