I'm using just a generic oem LG battery from eBay on stock Android 5.1.1 straight from Google's Nexus 5 factory image archive. With this I'm able to spend maybe 4 or more hours just watching YouTube videos on an old battery that's permanently lost like 20% of its original capacity

When you start to think about the nine year old nexus 5, think about computer CPUs and CPU generations. An older generation is gonna be less power-efficient. I figured that modern android is going to be filled to the brim with tons of background processes and other powerhungry security redundancies that are going to be a big no-no on an older, less efficient CPU like the one in the Nexus 5. So I just stuck with 5.1.1 and magically I have battery life to spare