SONY Discman battery BP-2 adapter

thingiverse

Here is a replacement battery solution that lets you utilize Li-ion batteries instead of old lead acid ones. Simply swap out your 4V lead acid battery for a 10440 Li-ion battery. To get started, you'll also need to order some additional materials: copper foil and conductive gasket. For added power, consider connecting two 10440 batteries in parallel, allowing them to work together to provide a combined voltage range of 3.7-4.2 volts.