Chosen Solution
Hi It could be a faulty battery However It could be that the battery needs recalibrated as new battery’s need to “learn” how to read their percentage properly I have had loads of them that act like this and it’s usually that or the battery connector just needs reseated I would try charging it up fully then draining it till flat a couple times to help recalibrate it and if the issue persists contact iFixit to see about getting a replacement or a refund iFixit support is great so they will make it right:-) Hopefully this helps Any questions please ask Thanks:-)
