  The $q$-Onsager algebra $O_q$ is presented by two generators $W_0$, $W_1$ and
two relations, called the $q$-Dolan/Grady relations. Recently Baseilhac and
Koizumi introduced a current algebra $\mathcal A_q$ for $O_q$. Soon afterwards,
Baseilhac and Shigechi gave a presentation of $\mathcal A_q$ by generators and
relations. We show that these generators give a PBW basis for $\mathcal A_q$.
Using this PBW basis, we show that the algebra $\mathcal A_q$ is isomorphic to
$O_q \otimes \mathbb F \lbrack z_1, z_2, \ldots \rbrack$, where $\mathbb F$ is
the ground field and $\lbrace z_n \rbrace_{n=1}^\infty $ are mutually commuting
indeterminates. Recall the positive part $U^+_q$ of the quantized enveloping
algebra $U_q(\widehat{\mathfrak{sl}}_2)$. Our results show that $O_q$ is
related to $\mathcal A_q$ in the same way that $U^+_q$ is related to the
alternating central extension of $U^+_q$. For this reason, we propose to call
$\mathcal A_q$ the alternating central extension of $O_q$.
