Just replaced the battery and its already dead/dying

Randal McDowell there is always a possibility that your charger is not fully charging the battery or that there are issues with the charging circuitry, but that is unlikely since “says it at 40% but instantly dies”. That leads me to believe that you have a bad battery. I would try to return it to wherever you got it from since it obviously is not working. Batteries are one of those commodities where there is a lot of mischief going on. Recycled batteries are being sold as new. Control boards are being “hacked” to display wrong information etc. So, I would definitely suggest you start with a new battery. Always make sure you buy from a reputable source (which most often excludes ebay and Amazon)