Random shutdowns occuring under high load with new ifixit battery

I recently replaced the original battery, which was at around 45% health with a new ifixit battery. The ifixit battery has excellent battery life, however, I've discovered an issue whereby when under a high load when not plugged in, the laptop will randomly shutdown. It does not shutdown gracefully, simply switching to a black screen immediately. I've tested this on a number of relatively resource intensive games, with the same results. This does not occur with the old battery. Lenovo Vantage reports the battery as being in good condition so I'm at a loss as to what is causing this.