They are all marked if you look carefully.

The original Motorola phones with the big batteries are 6 volts. Many other
phones of that vintage are also 6 volts.

The next generation is 3.3 volts. Examples are the Nokia 5110, 6110, etc.

Look at the battery type. NiMH or NiCad batteries can be easily used. LiON
batteries usually can not, but save them. Almost all NiCad and NiMH batteries
will be suffering from "memory" they need to be compeletly discharged and
recharged a few times to be useable.

I make a point of after asking permission, to go through battery recycle
bins and taking out the rechargables I can use.

Lithium batteries must be problery charged or they will break open and
possibly catch fire, or corrode everything they touch. If you open many
of them you will see a 3.3 volt cell with a little charger board. If you
remove the board, you can use the cell in another device after replacing
the board from the (dead) battery that is in it.

If the phones are GSM and unlocked or Nokia, you could stuff them in an
envelope and send them to me. I'll find homes for them. Since landline
phones are very expensive and cell phone usage is cheap here (you only
pay to make calls), there is a constant demand for cheap phones or
if they are U.S. only, temporary phones for travelers.

Geoff.

If you have to ask, it's probably not an undertaking you should
attempt.

What's the voltage?

Varies from battery to battery.

What are the 3 terminals?

Varies from battery to battery.

If you don't have the equipment to determine the voltage and what each
contact does - you could easily fry your mp3 player.