We all know that the varying intensity of an athletic effort will have a varying "cost" or damage to our physiological system. If you go at an easy aerobic effort, the cost is very low and you can go seemingly all day. As your efforts increase and nears or exceeds your threshold, the cost will be much higher and the time to failure gets shorter and shorter. Your threshold is theoretically how long you can go for one hour, so if you go above your threshold, your ability to maintain that effort becomes much less because you're burning fuel and building up lactic acid faster than your body can flush it out. That's about as nerdy as well get here, but there's a great article below that digs in much deeper.
The battery is grounded in the W-prime methodology that quantifies how much energy you have available above your threshold. As the battery decreases, your ability to continue to perform above threshold becomes less. The higher above threshold you go, the faster your battery depletes. Conversely, as you maintain efforts below threshold, your body will recover (flushing lactic acid) and refill the battery.
Here's a great article from one of the thought-leaders on W-prime and one of Velocity's advisors Dr. Phil Skiba: