How can my battery die if it's consistently plugged in to a USB?

Hi Susan.

Lots of devices only use the battery to power themselves, while plugging in, it only charges the battery it doesn’t actually power the device, so once the battery is dead, boom the device also dies, even plugged in.

Assuming it’s a USB plug, you can try a larger charger, one that outputs more amps. The Speakerphone will only draw as much as it can and circuitry allows, so no guaranty that would solve your issue. Another possibly is that it’s just a weak battery and needs replacement.