Battery stereo system finally reduced by 50% (5)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

definition: NiCad meaning nickel-cadmium, a type of battery NICAM [wiki] is a system for broadcasting stereo sound the final M (equal to 1000 in Roman numerals) is reduced by 50% to D (500 in Roman numerals) edit thanks to DropBear in the comments – I had written L for 500 rather than D