This is a FERRORESONANT BATTERY VOLTAGE SUPPLY
It uses incoming LINE FREQUENCY to saturate the transformer which stops voltage from climbing.
ANY LOAD accessory that you put on the output that has in its internals an electrolytic capacitor will serve to raise output voltage. The higher the capacitance the greater the LC voltage.
This charger uses a massive BLEED RESISTOR to Dracula some output wattage to maintain the house battery. Expect 4-7 maximum amperage battery recharge rate.
The output power quality is dirty. The hum you hear is from the transformer.
Do yourself and the world a favor and put this thing in "Send To China" recyclables. It is a battery killer. It is an accessory killer.
You will get lots of advice as to what to replace it with on this forum