Which uses more energy (battery), Bluetooth or self-contained speakers?

Bluetooth. The Bluetooth protocol (software) itself takes a certain amount of energy just to do the necessary running of the program.

Then, producing the sound (energizing the speakers) takes additional power.

Self contained speakers will take less overall power. (not by an enormous amount, but enough to matter)

If by “self-contained” you mean the speakers in your Android, and by “energy (battery)” you mean the battery in your Android, not the battery in your Bluetooth speakers, then I would expect the Android’s battery to use more of its own battery running its own physical speakers, since the speakers use their own battery to run themselves.

But it really comes down to everything the device does in each mode and how efficient that is.

I don’t think there’s enough information to know the answer. Streaming audio signals over bluetooth requires cpu cycles as well as powering the bluetooth antenna that all burn up energy, also powering internal speakers requires energy. There are many factors that would affect the outcome including how much energy the internal speakers use, how efficient the bluetooth antenna is, how efficient the cpu is, how optimized the code is (which itself could change based on updates). Perhaps one tech uses significantly more energy which would make one answer better than the other in call cases. I would think running an experiment would be the best way to find out. If the total amount of time between the two options is close, then I’d imagine the answer could be different for different android phones and versions of the operating system.

So this will vary a bit when you add the consideration of playing on speaker mode versus headphones as well. It will also vary depending on how loud you drive your self contained speakers.

Studies done on both Android and IOS devices show a negligible increase playing music to a bluetooth speaker vs contained speakers. Most devices shut off the audio amplifier circuits when playing over bluetooth which balances out the power draw from the bluetooth circuit.