So I pulled the replacement battery. I used a Rayovac Recharge Plus NiMH battery. Replaced it with a fresh one. Now the engine responses to reversing unit commands from the remote, the coupler fires, but no sound no lights with the replaced battery. This behaviour was maintained after doing a factory reset from the DCS remote.
The battery the I installed yesterday and reads at 7.84vdc. Out of the box they are fully charged and are rated at 8.4vdc.