This is a high-performance replacement cordless battery for the Sony MDR-RF4000, guaranteed to meet or exceed the OEM specifications of your original. The cordless battery is 1.83 x 0.92 x 0.45 inches with an output of 2.4 volts. The chemistry is Ni-MH at 700mAH capacity. Manufactured by Empire Scientific, model number CPH-537. We offer a 1 Year Warranty with purchase. See this product elsewhere at a lower price? Call or email us; we have a Price Match Guarantee.