PC shuts down from low battery even though battery is full

Hi! You have a serious problem. You should RMA to ACER or Authorized ACER Repair shop immediately. There is an issue with the motherboard or circuitry dealing with power usage.  Also possibility that it is unable to supply proper voltage / wattage to the GPU or CPU causing it to crash. Both CPU and GPU are hard soldered to motherboard and not replaceable. Again, get a RMA.(return merchandise authorization) - email Support or Call ACER as soon as possible.  Acer will either repair or send you a Refurbished at your expense. Unfortunately, since it is a 2018 Helios 300 and warranty has expired the cost is entirely on you.  You could try buying a replacement battery for $50 to $70 but must be for your specific model, just not any off from eBay. But I feel this is not the problem as you mention Battery is shown as being Full. When was the last time you had cleaned your laptop. Remove the bottom cover and use canned Air (Dust Buster or other) to blow out dust, avoid using anything abrasive and never use any liquids on electronic components. May use a small non-conductive brush to remove heavy dirt. The CPU or GPU could be immediately overheating causing the laptop to shuts down ASAP. Could redo/ reapply the Thermal Paste on the CPU and GPU using Arctic MX-4 or Kryonaut thermal paste. Never use thermal pads since they are not good. Watch YouTube videos if uncertain how to remove, clean and apply thermal paste. Never perform maintenance with AC Adapter plugged in.
 Other alternative, sell Helios 300 on eBay noting this problem exists in description. You may get $200 to $300 for it. Expect 50 to 70 percent less than you had paid for it. Also could list it for Parts Only.  Then take the money if desired and still want a ACER laptop as could use it toward a 2019 or new 2020 Helios 300.  2018 was okay, 2019 was improved, the new 2020 was not a step forward more like a step back to 2018. Avoid anything with nVidia RTX 2060.
Go with GTX 1660ti if have the money could opt for RTX 2070 Max-Q or RTX 2080 Super Max-Q.