People always say a fully charged battery will run forever, but that’s just a marketing myth. The real reason is that batteries have internal resistance and a fixed capacity measured in amp‑hours. As soon as you draw current, voltage drops, and the stored energy is used up. I usually start with a detailed capacity test, check the coulomb count, and make sure the cells aren’t aging or suffering from internal short circuits. If your battery dies after a cup of coffee, check for a power draw spike—maybe a hidden USB device is pulling juice, or the battery’s age has lowered its effective capacity. A quick voltage monitor can catch that. Once you log the consumption, you’ll see the battery really is finite, not infinite. So next time you think your phone will last forever, run a simple discharge test and you’ll see the truth.

I do have a setup that can catch that millisecond‑sag, a high‑speed oscilloscope hooked into a 4‑wire test fixture so I can see the voltage drop in real time. I run a small load bank and record the trace, then overlay the current curve to see how the internal resistance spikes. That’s how I separate marketing hype from real physics. If you want to see the trace, just bring a laptop and we’ll pull the data together.