Vintage scientific calculator Texas Instruments T1-30

Texas Scientific Calculator from 1976 Excellent shape, works well! Comes with case, which has a couple of water marks on it Also has rechargeable battery, but I don’t know how to charge it. Works just as well with a 9 volt battery as seen in video. Technical details: Display is 8 digits, red LED. Four function, memory, scientific functions. Integrated circuit - Texas Instruments TMC0981. 78 x 146 x 35 mm (4.0" x 6.25" x 1.25"). Made in various countries including USA and Italy.