HP ProBook 4520s Battery will be compatible with:

587706-751, 587706-761, 593572-001, BQ350AA, HSTNN-CB1A, HSTNN-CBOX, HSTNN-DB1A, HSTNN-Q78C, HSTNN-Q78C-3, HSTNN-Q78C-4, HSTNN-Q81C, HSTNN-UB1A, HSTNN-W79C-5

HP HP 420, HP 421,HP 425, HP 4320t, HP 620, HP 625

HP ProBook 4000 SeriesProBook 4320s, ProBook 4321s, ProBook 4325s,ProBook 4326s, ProBook 4420s, ProBook 4421s,ProBook 4425s, ProBook 4520s, ProBook 4525s,ProBook 4720s

COMPAQCompaq 320, Compaq 321, Compaq 325, Compaq 326, Compaq 420, Compaq 421,Compaq 620, Compaq 621