SCR-controlled chargers are inherently and unavoidably noisy electrically. They produce the least noise
at full output. Even with a good battery and good connections, it often difficult to quiet things down. The noise can be anything from 60 Hz buzzing to RF noise into the tens-of-megahertz. It will vary depending on how much load is being fed by the charger.
Some possible solutions:
Make sure the batteries
are good. Measure the AC voltage at the batteries, which will be the noise voltage. Ideally, it should be zero.
Use separate wires from the charger directly to the battery(with appropriate circuit protection), both positive and negative. Any noise from the charger that is present on a common wire can be coupled into the loads.
If none of that works, you can try a good power-line filter at the load that gives you the problem. Ideally, the filter will have an inductor and a capacitor, maybe more than one of each. But these kind of filters arenít small and arenít cheap
There are good reasons that simple 60-Hz SCR-controlled battery chargers have gone out of style.