system
Given the following post, respond as if you're commenting on Reddit:
human
How is it that batteries can provide constant voltage until they’re dead, but a capacitor slowly drops in voltage output until it’s dead? Why don’t batteries do the same thing?
gpt
Firstly, batteries are certainly not constant voltage sources. You could call them time-dependant constant voltage sources, at best. As a battery discharges, the voltage will start to drop. If you measure the voltage of a dead battery, you'll notice it is much lower than the nominal voltage.

Now, batteries provide a comparable stable voltage within a narrow range for a relatively long time, though. The main difference between batteries and capacitors in this context, is the discharge rate. A capacitor is charged with easily transferred, highly mobile charges. A battery, in contrast, has a generally higher capacity, with slower transport speeds.

You can kind of think of it like flow rate of fluids, a capacitor has a smaller tank with a larger outlet. This, it drains very quickly, and as the tank empties the flow slows. A battery will also have a continuously decreasing flow rate, but it has a longer period of time where the flow rate is still fairly close to its peak.

As far as when they die, a battery generally stops flowing charged at relatively high voltage compared to capacitors, as the affinity of the charges and electrodes of a battery is higher. With the water analogy, the outlet is near the bottom of the barrel for a capacitor, and 1/3 of the way up for the battery. Thus, the change in flow rate for the battery is less noticable before it simply stops flowing.

The analogy isn't that great if you start getting into more details, as the basic properties of capacitors and batteries differ more than two containers of water, but hopefully it gives you a basic starting point.