Power bank would be the term or battery brick/bank if you wanted to meld both terms.

I think "brick" started because laptop chargers used to have the adapter like an actual brick (most still do).

Since phones can get away with smaller power cables, the brick got stuck onto the regular power socket.

And since they use(d) less juice, the brick isn't a brick anymore.

AFAIK batteries are power banks, but brick and bank sound alike so I guess it got lost in translation a bit.