This looks sort of ok if the styling and functions suit you:

https://sidedeal.com/deals/2-pack-infinity-x1-stereo-wireless-rechargeable-700-lumens-worklights-with-blue

Two speaker/flashlights for $25, bluetooth, claimed lumens and runtime suggests there are two 18650’s inside each one. Quick search for reviews indicates they have USB-C in and USB-A out (you can use them as power banks), they look fairly solid, but that the speaker volume is not all that high. I’m gonna resist though.

Meh had some quite nice ones a few years back that I still sort of regret not getting but it would have just been more crap around the house. Edit: found them:

https://sidedeal.com/deals/libratone-zipp-zipp-2-360-wifi-mutl-room-speakers-with-airplay-2-alexa-7

The large ones (original Zipp) without Alexa seemed very decent though the micro USB charge port is ugh these days. I saw a teardown indicating four 18650 inside. People said the audio was good. I see them on fleabay now for $100+. IDR what Meh was selling them for but it was definitely less. They are indoor speakers though, not waterproof or anything like that.

Other idea: I have a fairly decent non-Bluetooth (wired only, non-portable) speaker and I’ve thought of hacking up some cheap Bluetooth headphones to connect the headphone audio to the speaker’s analog input. Never bothered though. Something like this, but DIY from scrouged headphone so: https://www.homedepot.com/p/Bluetooth-Car-Audio-Receiver-ABA9-1003-BLK/310710754

Added: Here’s a page full of overpriced BT speakers with aux input. Some of them do look nice. https://www.crutchfield.com/g_385550/Portable-Bluetooth-Speakers.html#nvpair=AG_Connections|YCAux_Inputs

I’ve got a Tribit Maxboom and it’s great. Water resistant, line in plus Bluetooth, has a handsfree button (triggers Siri on iPhones) and volume up/down, plus a bass boost mode that uses more power.

It doesn’t drive as much sound as my portable guitar amp, but for a battery based speaker, does pretty well. If you buy two of them, they’ll figure out how to work together to provide surround sound, otherwise a single one will play stereo audio to fill the area.

Had it for around 6 years and it still holds a charge like the day I bought it (I usually need to charge it every couple of months).

They exist, you just have to look at specs of ones that you like. You can also get an analog to bluetooth adapter and vice versa. You don’t say whether you want a big speaker or a small one, or what your budget is. That could help. I guess the adapters need power, which isn’t so great for portability.

I know that power tool companies make some job site speakers with aux in, that run on tool batteries. They are on the large side though.

I just have a dinky little one, idk what brand, but it was cheap and suffices for personal listening, and has both 3.5mm and bluetooth inputs. It has an internal battery so I guess I’ll have to dismantle it to replace the battery someday. Be careful of internal batteries regarding BIFL. I know some of the older bigger ones used 18650’s inside and that was attractive. I didn’t buy one though.

My phone has analog output (I made sure to choose a phone that has that). In principle it should be possible to forward bluetooth audio in to analog audio out, or route the bluetooth input to the phone’s internal speaker, but I haven’t looked for an F-droid app that does this.

Keyword to look for in portable speakers is “aux”, referring to the analog input. A quick search just found this, which looks sort of cool and BIFL, if the 2600mah battery capacity refers to an 18650 inside:

https://www.bhphotovideo.com/c/product/1750016-REG/sangean_wr_7wl_wood_cabinet_mini_bluetooth.html/specs

I’ve got this one:

https://www.amazon.com/Soundfreaq-SFQ-04-Sound-Bluetooth-speaker/dp/B007XOQI4S

We’ve had it for 8+ years and it’s still going strong. Nice sound and bass, has aux and Bluetooth capability.

i have a Anker soundbar that has a aux plug that I use as my home server speaker

it’s got Bluetooth but I just use it with a audio cable and a usb for power

it’s years old, still kinda kicking it but the battery isn’t very long lasting anymore

I would not try to buy a product with a built-in battery for life. If you want a battery-powered speaker and want to keep it for a long time, I’d go for something that has standardized, removable batteries.

EDIT: It looks like a lot of the products out there (a) are Bluetooth, which I don’t know if you want or not (but I wouldn’t consider that something that will be a buy-it-for-life feature either, since Bluetooth has steadily seen protocol change over time and I doubt that twenty years down the road, the state-of-the-art in Bluetooth will be what it is today) and (b) have built-in lithium batteries. What you might consider doing is getting a radio with a built-in speaker with aux-in, as it’s easy to find those with removable batteries and without Bluetooth.

Do the JBL ones not come with an aux jack anymore? My JBL Charge 3 from early 2017 has one, as does the JBL PartyBox On-The-Go I picked up dirt cheap last year.