Years before, the typical (at the time very common at least here) Motorola MicroTac and later models:

https://en.wikipedia.org/wiki/Motorola_MicroTAC

had a battery which was the actual "back cover" (it was connected to the phone via sliding), and everyone had at least two batteries, there were all kind of slim to bulkier sizes.

Some of them (medium sized) simply contained 5 (Ni-Cd or Ni-Mh) AAA batteries.