DIAGNOSIS A parasitic drain is occurring, most commonly caused by a connected USB cable drawing power or a stuck Bluetooth/Wi-Fi module from a failed update.
FIX First, physically unplug any cable from the speaker's USB-C port. Fully charge it, then perform a factory reset (power on, hold Volume Up + Volume Down for 10 seconds until it reboots).
IF THAT DIDN'T WORK Check if you have the JBL Portable app installed with any background features enabled. Disable 'Auto Power Off' in the app, then re-enable it to reset the logic.
HEADS UP The battery meter (5 white LEDs) is an estimate. For an accurate test, fully charge it, play at 50% volume, and time it. You should get near 12 hours. If not, the battery may be degraded.