To give either battery or non-battery speakers the ability to run disconnected or play longer, you will probably want to pick up a Portable Power Station or power bank.
Power Stations for speakers that need AC or USB
If you need to power speakers over AC, make sure the Power Station you are choosing can handle the wattage of your speaker plus 25%. For very big speakers that will be around 1500w. Not doing so will cause your power station to shut down entirely or sag causing audible clipping.
Computing runtime for a power station will be a little difficult for speakers as you are looking for average power and speaker power is exponential. Assume that low volume is 1/40th of rated power, medium volume is 1/10th of rated power and full volume is 1/4 of rated power. So for something like a Ecoflow Delta mini with a 882Wh battery running a 240w Partybox 310 that would be 147 hours (882 / 240 * 40) at low volume, 36 hours (882 / 240 * 10) at medium volume and 14 hours (882 / 240 * 4) at full volume. These are just estimates and also depend on how much bass boost you are using.
Note most Power Stations cannot be taken on airplanes as they have a 100Wh limit for batteries
Power Banks for speakers that need USB
For “large size” and smaller speakers that charge over USB you can use a power bank depending on the run time you want out of your speaker.
Boombox size speakers generally have the equivalent of a 20,000mAh 3.7v battery inside. Large size speakers generally have the equivalent of a 10,000mAh 3.7v battery inside. Pocket/Small/Medium speakers generally have the equivalent of a 2500-5,000mAh 3.7v battery inside. This should give you an ideal of how much battery you would need for the runtime you desire. Double the battery would be double the play time.
I’d recommend one with USB-C PD (Power Delivery) which is higher speed charging for larger speakers, laptops and phones.
Power banks up to 26,800mAh 3.7v (26.8 * 3.7 = 99.2Wh) can be taken on an airplane.
I haven’t had any issues with any brands I own, just check the reviews to make sure though.