Doc Brown's walkie talkie was one of a pair of Archer Space Patrol walkie talkies[1] that Doc bought new in 1985A while Marty was at Biff Tannen's Casino & Hotel interviewing Biff. Doc had one of the walkie talkies with him when the DeLorean was accidentally sent to 1885, then sealed it in Delgado Mine along with the DeLorean for 70 years.
Since the battery in Doc's walkie talkie could not have lasted for 70 years, and was probably removed to avoid leakage, Doc Brown's 1955 counterpart provided fresh batteries for both walkie talkies, duct taping each one to a large Burgess battery.
Doc and Marty used the two walkie talkies with oversized batteries to communicate between the train and the DeLorean it was pushing during its final trip.
References
- ↑ The brand name of the walkie talkies is visible when Marty removed his from the packaging.