Mobile Battery Charge

Capacitors and (rechargeable) batteries can both be used to store and retrieve electrical energy, and both are used for this purpose. But the way they store electrical energy (charge) is different, which leads to different characteristics and hence different use cases.

A capacitor directly stores charge on what is essentially two plates of conductors. The fact that the charge is stored in conductors makes it readily accessible (low impedance, quick to react to changes), but the fact that its storage is essentially a 2-dimensional pair of plates severely limits the amount of energy that can be stored. (Higher-capacity capacitors use a 2.5-D storage at the expense of much less conducting plates). A capacitor stores charge, which means that when the capacitors discharges (delivers current), its voltage drops (linearly when the current is constant).

A battery stores energy in chemical reactions. This means that energy is stored in a 3D volume, so much more energy can be stored, but as ions don't change their speed as quickly as electrons, a battery can't respond as fast to changes of current as a capacitor. Chemical reactions are never perfectly reversible, so a battery wears out, much quicker than a capacitor. But a chemical reaction has a fixed 'activation voltage', so the voltage of a battery stays (more or less) the same while it is discharged.

Hence batteries and capacitors have different use cases, that seldom overlap. If you need

• high capacity => batteries
• fixed voltage => batteries
• quick response => capacitors
• 'infinite' (component) lifetime => capacitors

In fact batteries are often too slow for electronics, but capacitors would not be able to store enough energy, so in practice you often want

• high capacity + quick response => use batteries + capacitors

An external defibrillator will deliver a biphasic waveform usually. This would look like something like this:

The waveform can also be monophasic or triphasic. So we are not talking about continuous charge. Current is equal to charge per second.

A defibrillator circuit is fairly simple. Generally speaking, you would only need a battery, capacitor, inductor, and a set of paddles. The most reduced version of the circuit would look like this:

When the switch is connected to 'A', the capacitor is charging. When it is connected to 'B', the capacitor is discharging through the inductor and the paddles. I am assuming the inductor is there to control/limit the amount of current surging through the paddles. Current is what will kill you, not voltage.