I've been experimenting with this for about a year. I have retrofitted 3 old MTH Premier PS1 locomotives to DCC Battery Power with the Air Wire system from CVP products. I installed sound boards from Soundtraxx. So far I am pleased with the results.I put the battery pack in a MTH Express reefer so I don't have to buy a battery for every loco.I am using Lithium Ion 14.8 4400MA-Hr. The AirWire system is proven technology and they're based out of and made in Texas.
http://www.cvpusa.com/airwire_system.php
The technology will improve over time. My plan is to convert my locomotives one by one when their TMCC or DCC boards fail.