You charge the XG300 boombox speaker with power bank via the USB-C DC power input port. Plus to get the quickest charge time and fullest recharge, use the charger that comes with Sony SRS XG300, or an equivalent. Details follow.
Sony SRS XG300 Charge Time: How Long to Fully Recharge
Firstly, according to the specs for this speaker the time it takes to charge the SRS XG300 Bluetooth speaker is five (5.0) hours, using the 5 volt, 3 amp, 15 watt charger that Sony supplies with this speaker, or equivalent.

How Long the XG 300 Plays on a Full Charge
On a complete charge, the speaker can play for up to twenty-five (25) hours. But when you play it loudly, especially with booming bass, you will indeed see much shorter playing times per charge cycle. In fact, we see this speaker die in around ten hours when we play at louder volume. So to get the longest playing time per recharge, power saving is useful. Just play it only as loudly as you need to, to hear it well throughout your listening space.

The Play Time Gets Shorter as the Sony SRS XG300 Speaker Ages
Now if you own this speaker over a few years, you may notice that the time it plays from a single charge shrinks as the speaker gets older. This is normal for devices like this one, that rely on lithium ion batteries.
But by deep cycling the battery now and then, you can slow this charge capacity decrease. So to exercise the XG300, run the battery all the way dead, and then recharge it again every three to six months.
Also, we advise that you avoid leaving the speaker connected to a charger once it reaches full charge. Right after the charging lamp goes out, unplug the speaker as soon as you can, to avoid overcharging it and aging the battery early.
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References
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- 2022-09-27: First published.