100% CPU load, battery drain with no app minimized/running in the background

I have a Xiaomi Redmi Note 4 S625 global variant (4/64GB). A day or so ago, I noticed a quick battery drain (from 100% to 24% after a whole night idle and 2-3 hours of heavy use the next day) and from there, it was dropping a percent every 2nd-3rd minute. It never happened before as it's a 4000mAh battery and usually lasts around 2 days on normal use. I'm new to the smartphone world and I was reading online about the usual causes of battery drain including background apps consuming it and overcharging issues damaging the battery. I got worried as last night it was indeed left plugged in for like 2-3 hours maybe after a full charge. So either the battery is damaged due to overcharging, or it's some software issue. Now what I noticed is the CPU usage is maxed with no app running in the background. I don't know if CPU-Z's usage info is accurate, but it's 100% when the battery is like 15% or lower and 40-50% when the battery is 20% or above. Maybe there's some background process or maybe it's normal, I don't know. I'm pretty clueless at this point. Here's a screenshot of CPU-Z: Really appreciate it if anyone helped me with this one. Thanks.