This is a case for two AA batteries with a DC plug that can be connected via USB. It's designed for use with AIWA cassette players, and the polarity is reversed. The outside is positive, and the inside is negative. Since it uses two AA batteries, it can be used with 2V-3V reverse-polarity cassette players (e.g., AIWA). Specifically, it worked with the AIWA PX370 (2V) and the AIWA RS250 (3V). If you have a compatible gadget, you'll find this useful. Thank you.