I have installed 30 of Gunrunner John's YLB 19500 and 19510 Batteries in the tenders of my Lionel steam engines and they work perfectly to end the sound drop-off that was occurring on my Steam engines when going over Atlas #5 switches and Double Cross-overs. They are available from Henning's Trains website, under subheading: Electrical.
Connecting the red power wire to the center roller power wires is easy to do if you can strip a wire and solder a connection. The 10500 has a battery clip for connecting to older engines with a battery cable. The 10510 has a plug for the sound board that connects directly to the board. The red wire then goes to the center roller power wire.
The nice thing that occurs is that my steam engine tenders continue to make sound to shut down after I have shut down the transformer, like in conventional use. I am running Legacy, so I usually shut down each engine individually, but the battery provides benefits in conventional use.
I just installed a 10510 YLB with plug into my new 1931810 Hudson J3a #5415 from Mr. Muffins Trains. It helps the PT Tender sound system continue uninterrupted over my Atlas O #5 switches or other center rail gaps. The PT Tender has both pick-up rollers close together to accommodate the Water Spray Fill Effect three smoke output units on each side, making the YLB battery a necessity on my train layout.
Note: PT Tender Center Pick-up roller wires are Red or Gray.
Sincerely, John Rowlen