"Panasonic P-P507 / P-P507A / PQP50AA61 / TYPE 18 / CPB-478B (Single Pack), The Panasonic P-P507/P-P507A is a replacement battery for 2.4GHz phone models
For base stations only. Features: Voltage: 2.4vComposition: NiMH Replacement Battery For The Following Panasonic 2.4GHz Phone Models KX-TG2000B (BACKUP POWER) KX-TG4000B (BACKUP POWER)"
Great replacement for the manufacturers battery, sometimes with replacements you can have fitting issues. No problems installing seems to keep a good charge on the phone.