Capacitors vs Batteries

So the big question here is which is better, a capacitor (or supercapacitor) or a standard lead-acid battery? The capacitor weights significantly less and has an incredible service life and power output, but sucks as specific energy (amount of energy stored), and has a very quick discharge rate. The standard lead-acid based battery is heavy, has limited cycle life, and needs a good amount of time to re-charge but is capable of sustained energy discharge, high storage capacity, and voltage stability. Let’s take a brief look at how these units work and the pros and cons of each.

The Supercapacitor

The supercapacitor is the predecessor to the standard capacitor. The supercapacitor stores energy through static electricity. Inside of each supercapacitor is two metal plates coated with a thin layer of active charcoal to create a porous surface. Both of those plates are then soaked in an electrolyte and separated by a very thin separator called a dielectric. As you can see from the illustration below, the positive and negative electrical charges build up on the plates and the separation between them is what stores the energy - an electric field.

The amount of power that can be stored by any capacitor is directly related to the size of the metal plates within the battery. The larger the plate surface, the more energy the capacitor is able to store. This is why active-charcoal is commonly used in the supercapacitors - this creates a more porous plate that results in more surface area and more electrolyte absorption.

Advantages of the supercapacitor are:

• The weight savings
• Typically doesn’t contain harmful chemicals or toxic metals
• Extremely high cycle life, 20K +
• Instant power output

Disadvantages of the supercapacitor are:

• Low specific energy
• Linear discharge voltage
• High self-discharge
• High cost per watt

The Lead Acid Battery

The standard lead-acid based battery (this includes AGM, GEL, and Flooded) contains three parts - the Cathode (positive terminal), the Anode (negative terminal), and the electrolyte. Chemical reactions happen involving both the electrodes and electrolyte that converts the chemicals inside the battery into other substances thereby releasing energy in the process. The battery is considered fully discharged once the chemicals have all been depleted, and then the battery can be re-charged from an external power source to reverse this chemical process.

Lead-acid batteries dominate the automotive and commercial battery markets and show no sign of slowing down despite the introduction of new alternative battery storage solutions. Batteries will have a higher energy density meaning that they can store more energy than supercapacitors but have a latency transferring the chemical energy into electrical energy. So in other words, batteries are capable of sustaining power output longer than supercapacitors due to their higher energy density, but they are only able to discharge a limited amount of power at any one time due to the delay of the chemical energy creation process.

Advantages of the battery:

• Cost-effective
• Storage capacity
• Power density

Disadvantages of the batteries are:

• Limited cycle life
• Long charge times
• Limitations on current output

Can you use a capacitor in place of a battery: In short - no. The issue is that the applications om which we use batteries rely on the battery’s capacity to power the application. In vehicles the starter will continue to pull power until the car starts which could be some time depending on the engine. In stationary power applications, you have the same issue - the amount of power than can be stored is key.

In our line of work we typically use capacitors in parallel with one or more batteries to create a battery bank. The capacitor is placed at the front of the bank and takes the brunt of the impact of whatever system it’s connected to. We use this setup for semis, large audio systems, solar setups, and high compression starters. This setup will give you the best of both worlds, your battery bank will be able to produce instant power to flatten out potential voltage drops and give you the reserve capacity that your application needs to run. Having the capacitor take the brunt of the force will also help extend the life of your battery bank.

I hope that this article gave you a better idea of what and how a capacitor differs from a standard battery. If you have any questions please let us know!