Most likely the clock doesn't run on electricity. It is a spring operated works with an electric winder mechanism. When it winds down, a contact is made causing an electromagnet to pull the winder away from the contact. Thus it only uses electricity for a zip of a second every so often. 32fire2's idea of a lantern battery might work. I've never seen a rechargeable one but I imagine a regular one would wind it a long time before discharging.

"It's just a phase. He'll grow out of it." Mama, 1964

1956 Chevy 1/2-ton 3100
1953 Chevy 6100 "The Yard dog"
1954 GMC Suburban Now with a new proud owner.