An old powered speaker picked up at the scrapyard (under ¥1); fuse good, a hum on power-up, transformer output normal. The plan was to feed the Bluetooth module’s audio output into the original board’s audio input, but it didn’t amplify; switched to driving the speaker straight from the Bluetooth board, which works. So everything original except the driver comes out and it becomes rechargeable — 6 × 18650 spot-welded into a pack with a 3S60A balancing BMS, fish paper insulation and fibre tape; a diode and a CC/CV buck module on the charge side; a fast-charge port, charge-level display module and switch in the case; wired up, powers on, back in the case, phone pairs and plays, ‘sound quality is decent for its size’. The top comment points out that without audio amplification the result is only so-so.