Does anyone know what was used to coat terminal ends? Like ground wires/battery cables?
Solder. The process is called "tinning", and it's best done with a soldering iron, not a soldering gun. Other "tools" needed are a small bowl of water, a small piece of sponge (I prefer a 1 X 1 X 3 inch chunk), 60-40 solder, patients. The sponge is dipped in the water to clean the iron's tip, the hot piece being tinned, and to remove excessive solder from the hot piece. I normally use rosin core solder vs. acid core (availability mostly, it's what i got). Professionally, a solder pot is used (What? You don't have one?).
Here's one link from many that describes the process.
https://chicagoselectrician.com/mcc/how-to-tin-a-wire/Be careful, it's hot. Ask me how I know.
Jim
Certified assembly technician/engineer, USN (three times), NASA, USA, USAF (twice)
PS: Add a pair of small pliers or a stainless medical hemostat to the tools section.