It's not hard to change the battery, either. ASUS designed it to be nonremovable, but you'll only need one screwdriver and a few minutes. You'll find many detailed disassembly and battery replacement guides/videos on google.
Original G751YJ battery is a Li-poly rated 15V @ 5800mAh (88Wh), ASUS part A42N1403.
It is compatible with parts A42LM93, A42LM9H, 0B110-00290000M, and BTC-AUG751NB.
Many vendors sell these batteries, new and used and (usually just as good) no-name knockoffs, you can buy them cheap on ebay or amazon or pay a little more to obtain from a more reputable source. I see prices ranging from $50 to $125 in Canada, $35 to $80 in USA - maybe not cheap, but certainly cheaper than a useful new laptop to replace your half-useless existing one.
Your laptop can run on external power (AC adapter) without a battery. Though I wouldn't personally risk reflashing firmware (upgrading BIOS) without backup power, risk of a problem is low but consequences of a problem could be catastrophic (bricked device).
You should check your BIOS version before worrying about upgrading BIOS version. You may already be running the latest version.
Run full scans for virus and malware. Only install NVIDIA drivers from the
official download site. Make sure you're logged into Windows as the "Admin", close all unnecessary programs, and "Run as Administrator" to install. You may need to reinstall DirectX. You may need to reinstall and repatch your game(s). I recommend isolating and eliminating software issues before assuming it's a hardware or firmware (BIOS) fault.
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