Why can't we big capacitors instead of batteries to store energy?

While capacitors can store energy, they are generally not suitable as a direct replacement for batteries due to their lower energy density, shorter discharge times, and inability to maintain a stable voltage during discharge. 

Here's a more detailed explanation:

1. Lower Energy Density:

Batteries store energy through electrochemical reactions, allowing them to pack a large amount of energy into a relatively small volume and weight. 

Capacitors store energy electrostatically by accumulating charge on their plates. This method results in significantly lower energy density compared to batteries. 

For the same amount of stored energy, a capacitor would be much larger and heavier than a battery. 

2. Shorter Discharge Times:

Capacitors discharge energy very quickly, while batteries can provide a sustained and stable power output over a longer period. 

A capacitor's voltage drops linearly as it discharges, making it unsuitable for applications requiring a constant voltage supply. 

Batteries, on the other hand, maintain a more stable voltage output until they are nearly depleted. 

3. Inability to Maintain Constant Voltage:

As a capacitor discharges, its voltage decreases, which can be problematic for many electronic devices that require a stable voltage input. 

Batteries offer a relatively constant voltage throughout their discharge cycle, making them a better choice for powering devices that need a consistent voltage supply. 

4. Applications:

While not ideal for long-term energy storage, capacitors are well-suited for applications requiring bursts of high power or quick charging and discharging, such as regenerative braking systems in vehicles. 

Batteries are more appropriate for applications requiring sustained power delivery over extended periods, like powering portable electronics, electric vehicles, and energy storage systems. 

In summary: While capacitors and batteries both store energy, their fundamental differences in energy density, discharge characteristics, and voltage stability make batteries the preferred choice for most long-term energy storage applications.