Owned this MZ-R55 for 20+ years and only noticed today that you only need one AA battery (not two) in the sidecar. Makes sense now I think about it as the gum-stick
Amazon.com: HQRP Battery Compatible with Sony D-777, D-E01, D-E888, D-E900, D-E905, D-E990, D-E999, WM-EX900 MZ-NF810 MZ-NF810CK Portable CD/MD / MP3 Player : Electronics
Battery for Sony MD (MZ-R50/MZ-R5ST) - China Sony battery and Battery manufacturer price | Made-in-China.com
Estry Battery Replacement for LIP-12 MZ-R30 MZ-B3 MZ-E3 MZ-R2 MZ-R3 MZ-R35 MZ-R4 MZ-R4ST LIP-12H : Health & Household - Amazon.com
5400 Maverick / 5400 MZ Magnum / 5400 ZT Elite Bad Boy replacement battery
Walkman Battery 3000mah Lip-12, Lip-12h For Sony Mz-b3, Mz-e3, Mz-r2, Mz-r3, Mz-r30, Mz-r35, Mz-r4, Mz-r4st - Mobile Phone Batteries - AliExpress
Estry Battery Replacement for LIP-12 MZ-R30 MZ-B3 MZ-E3 MZ-R2 MZ-R3 MZ-R35 MZ-R4 MZ-R4ST LIP-12H : Health & Household - Amazon.com
NiMH battery retrofit for Metz 76 MZ-5, 70MZ-5 type 76-56 | for Lamps | Accumulators | Products | Akkuplanet