I bought a homemade BCR unit for the 3 volt replacement of the double or triple A batteries which works excellently and the cost was much cheaper than buying one through the other company. I'll let the person who made these post his thoughts if he wants to. I have one in my MTH JETS engine and it works great.
As for putting in a battery for a TMCC engine, you only need it if you run the engine in conventional operation. If you operate in TMCC mode, it is a waste of time putting in a 9 volt battery. It could leak and cause you more problems.