See The SuperCapacitor Difference

All batteries are portable energy sources featuring three basic components in each cell — an anode, a cathode and an electrolyte. Their properties relate directly to their individual chemistries. For instance, the characteristic voltage depends on the type of cell that makes up the battery: Nickel-Cadmium (NiCd) and Nickel-Metal Hydride (NiMH) cells are 1.2 volts, alkaline cells are 1.5 volts, and Lithium ion cells are 3.6 volts.

Batteries come in a wide range of sizes and shapes — from wafer-thin to button-size devices to very large industrial battery systems — and fall into two broad categories: Primary storage and secondary storage, or accumulator batteries.

Primary Batteries

A primary battery is designed to be used until it runs out, then disposed of or recycled. These include carbon-zinc, alkaline, silver oxide, zinc air and lithium batteries. Although sometimes consisting of the same active materials as secondary types, are constructed so that only one continuous or intermittent discharge can be obtained.

Primary batteries have the following properties:

• Designed as a single use battery, discarded or recycled after it runs out
• Very high impedance which translates to long life energy storage for low current loads
• Available in carbon-zinc, alkaline, silver oxide, zinc air and some lithium metal batteries (like lithium-thionyl-chloride)
• Lithium-thionyl-chloride (LTC) batteries come in cylindrical form factors of AAA to D
• Larger C and D size LTC batteries are a chemical hazard and cannot be transported by air
• Operating temperature range is - 40 ºC to + 85 ºC

Secondary Batteries

Secondary batteries such as NiCd, NiMH and Lithium Ion (Li ion), can be recharged, sometimes as often as 1,000 times, by the flow of direct current through them in a direction opposite to the current flow on discharge. By recharging after discharge, a higher state of oxidation is created at the positive plate (electrode) and a lower state at the negative plate (cathode), returning the plates to approximately their original charged condition.

Secondary batteries have the following properties:

• Designed to be recharged
• Can be recharged up to 1,000 times depending on the usage and battery type
• Very deep discharges result in a shorter cycle life, whereas shorter discharges result in long cycle life for most of these batteries
• Charge time varies from one to 12 hours, depending upon battery condition, depth of discharge (DoD) and other factors
• Include NiCd, lead-acid, NiMH, some lithium metal and Li-ion batteries
• Lead-Acid and NiCd batteries are toxic and are subject to stringent disposal regulations

Some of the limitations posed by secondary batteries are limited life, limited power capability, and low energy-efficiency and disposal concerns.

Types of Secondary Batteries

Lead-Acid. The life of lead-acid batteries is directly related to its depth of discharge and duty cycle. It has been only in the last three to five years that lead-acid batteries become available to survive constant cycling to below 50% charge. This means that a typical car battery with a rated capacity of 500 watt-hours has only 250 watt-hours of capacity. Their low energy efficiency requires an extra 40% to be supplied to store to capacity.

Nickel-Cadmium (NiCd) and Nickel-Metal Hydride (NiMH). These batteries offer much better energy density than lead-acid batteries. NiCd batteries perform best when they are regularly discharged completely and then recharged completely; otherwise, they display the memory effect, which limits their depth of discharge and usefulness. NiCd batteries can last for about 1,000 charge-discharge cycles and function well in extreme temperatures.

Many countries now impose strict disposal regulations on lead-acid and NiCd batteries. The heavy metals used in their manufacture can cause serious environmental pollution if not recycled or stored. Compliance with these regulations may add significantly to the cost of these batteries.

NiMH batteries lasts approximately 40% longer per charge than comparable Nickel-Cadmium batteries. They are lighter in weight and last up to 700 charge/discharge cycles.

Both NiCd and NiMH batteries cost substantially more than lead-acid batteries.

Lithium ion. Lithium ion batteries (Li ion) offer twice the energy per charge of NiMH and approximately 500 charge-discharge cycles. They are being used increasingly in mobile phones and notebook computers. However, they cannot sustain high currents at temperatures below 0°C and are relatively expensive.

Lithium polymer. Lithium polymer batteries can be made in thin, flat or shape fitting forms and their biggest plus is that they won't leak corrosive electrolyte. They provide 500 charge-discharge cycles, but require smart chargers to monitor them closely. Lithium polymer batteries are not suitable for high-power applications, are limited to the operating temperature range 0 - 65° and are relatively expensive.

The Supercapacitor Difference

In contrast to batteries, supercapacitors can last virtually indefinitely if kept within their design limits, and their energy efficiency rarely falls below 90%. Their energy density is lower than batteries, but almost all of this energy is available reversibly. For example, a typical lead-acid battery has an energy density of 30 Wh/kg, of which only 15 Wh can be used without reducing battery life. To reversibly store this 15 Wh, about 21 Wh must be supplied. For supercapacitors with 9 Wh/kg of energy density (i.e., already developed supercapacitors), 1.7 kg will provide the same storage capacity (cf 1kg batteries) and only 16.5 Wh will be needed from the source.

Conventional capacitors, like supercapacitors don't involve chemistry to store charge. They store energy on the surfaces of metallized plastic film. Their energy density is at least two orders of magnitude less than a supercapacitor, but their power density can be much higher than a supercapacitor. Conventional capacitors are useful on time scales of less than one thousandth of a second.

Compared to batteries, supercapacitors can be used to deliver frequent pulses of energy without any detrimental effects or reduced life; charged very quickly and safely where batteries are damaged by fast charging; and cycled hundreds of thousands of times without significant degradation in performance.

A supercapacitor can replace a battery where the battery is being used primarily to provide power rather than energy. (SEE "Power vs. Energy" report.) The benefits will be improvements in cost, weight, size and operating temperature range. A supercapacitor can replace multiple capacitors where the capacitors are being used primarily to provide energy for load leveling rather than power. The benefits will be improvements in cost, weight and size.

 

For more information on batteries, visit Battery University.