SONY BP-3 battery adapter

thingiverse

The battery adapter lets you replace your old 6V lead acid battery with a normal battery, either AA or 14450. To do this, you'll also need to order some additional materials: copper foil and conductive gasket. You can use four 1.5v AA batteries for this purpose, although it's recommended that you use 1.5v Li-ion batteries for even better operation time. Alternatively, you can use two 3.2v Li-Fe 14450 batteries along with two dummy batteries to achieve the same result. However, be careful not to use 14450 Li-ion batteries (with a voltage of 3.7v x 2) as they may cause damage to your discman due to their high voltage.