How do batteries work?

Electricity, as you probably already know, is the flow of electrons through a conductive path like a wire. This path is called a circuit.

Batteries have three parts, an anode (-), a cathode (+), and the electrolyte. The cathode and anode (the positive and negative sides at either end of a traditional battery) are hooked up to an electrical circuit.

The chemical reactions in the battery causes a build up of electrons at the anode. This results in an electrical difference between the anode and the cathode. You can think of this difference as an unstable build-up of the electrons. The electrons wants to rearrange themselves to get rid of this difference. But they do this in a certain way. Electrons repel each other and try to go to a place with fewer electrons.

In a battery, the only place to go is to the cathode. But, the electrolyte keeps the electrons from going straight from the anode to the cathode within the battery. When the circuit is closed (a wire connects the cathode and the anode) the electrons will be able to get to the cathode. In the picture above, the electrons go through the wire, lighting the light bulb along the way. This is one way of describing how electrical potential causes electrons to flow through the circuit.

However, these electrochemical processes change the chemicals in anode and cathode to make them stop supplying electrons. So there is a limited amount of power available in a battery.

When you recharge a battery, you change the direction of the flow of electrons using another power source, such as solar panels. The electrochemical processes happen in reverse, and the anode and cathode are restored to their original state and can again provide full power.

Table 1 summarizes the characteristics of the common batteries. The figures are based on average ratings at time of publication. Note that nickel-cadmium has the shortest charge time, delivers the highest load current and offers the lowest overall cost-per-cycle but needs regular maintenance.


Table 1: Characteristics of commonly used rechargeable batteries.

1) Internal resistance of a battery pack varies with cell rating, type of protection circuit and number of cells. Protection circuit of lithium?ion and lithium-ion-polymer adds about 100mW.
2) Cycle life is based on battery receiving regular maintenance. Failing to apply periodic full discharge cycles may reduce the cycle life by a factor of three.
3) Cycle life is based on the depth of discharge. Shallow discharges provide more cycles than deep discharges.
4) The discharge is highest immediately after charge, and then tapers off. The capacity of nickel-cadmium decreases 10% in the first 24h, then declines to about 10% every 30 days thereafter. Self-discharge increases with higher temperature.
5) Internal protection circuits typically consume 3% of the stored energy per month.
6) 1.25V is the open cell voltage. 1.2V is the commonly used as a method of rating.
7) Capable of high current pulses.
8) Applies to discharge only; charge temperature range is more confined.
9) Maintenance may be in the form of 'equalizing' or 'topping' charge.
10) Cost of battery for commercially available portable devices.
11) Derived from the battery price divided by cycle life. Does not include the cost of electricity and chargers.

In subsequent columns I will describe the strength and limitation of each chemistry in more detail. We will examine charging techniques and explore methods to get the most of these batteries.

The lithium-ion battery

Pioneer work with the lithium battery began in 1912 under G.N. Lewis but it was not until the early 1970s when the first non-rechargeable lithium batteries became commercially available. lithium is the lightest of all metals, has the greatest electrochemical potential and provides the largest energy density for weight.

Attempts to develop rechargeable lithium batteries failed due to safety problems. Because of the inherent instability of lithium metal, especially during charging, research shifted to a non-metallic lithium battery using lithium ions. Although slightly lower in energy density than lithium metal, lithium-ion is safe, provided certain precautions are met when charging and discharging. In 1991, the Sony Corporation commercialized the first lithium-ion battery. Other manufacturers followed suit.

The energy density of lithium-ion is typically twice that of the standard nickel-cadmium. There is potential for higher energy densities. The load characteristics are reasonably good and behave similarly to nickel-cadmium in terms of discharge. The high cell voltage of 3.6 volts allows battery pack designs with only one cell. Most of today's mobile phones run on a single cell. A nickel-based pack would require three 1.2-volt cells connected in series.

Lithium-ion is a low maintenance battery, an advantage that most other chemistries cannot claim. There is no memory and no scheduled cycling is required to prolong the battery's life. In addition, the self-discharge is less than half compared to nickel-cadmium, making lithium-ion well suited for modern fuel gauge applications. lithium-ion cells cause little harm when disposed.

Despite its overall advantages, lithium-ion has its drawbacks. It is fragile and requires a protection circuit to maintain safe operation. Built into each pack, the protection circuit limits the peak voltage of each cell during charge and prevents the cell voltage from dropping too low on discharge. In addition, the cell temperature is monitored to prevent temperature extremes. The maximum charge and discharge current is limited to between 1C and 2C. With these precautions in place, the possibility of metallic lithium plating occurring due to overcharge is virtually eliminated.

Aging is a concern with most lithium-ion batteries and many manufacturers remain silent about this issue. Some capacity deterioration is noticeable after one year, whether the battery is in use or not. The battery frequently fails after two or three years. It should be noted that other chemistries also have age-related degenerative effects. This is especially true for nickel-metal-hydride if exposed to high ambient temperatures.

Manufacturers are constantly improving lithium-ion. New and enhanced chemical combinations are introduced every six months or so. With such rapid progress, it is difficult to assess how well the revised battery will age.

Storage in a cool place slows the aging process of lithium-ion (and other chemistries). Manufacturers recommend storage temperatures of 15°C (59°F). In addition, the battery should be partially charged during storage. The manufacturer recommends a 40% charge.

The most economical lithium-ion battery in terms of cost-to-energy ratio is the cylindrical 18650 (18 is the diameter and 650 the length in mm). This cell is used for mobile computing and other applications that do not demand ultra-thin geometry. If a slim pack is required, the prismatic lithium-ion cell is the best choice. These cells come at a higher cost in terms of stored energy.

Advantages

• High energy density - potential for yet higher capacities.
  
• Does not need prolonged priming when new. One regular charge is all that's needed
  
• Relatively low self-discharge - self-discharge is less than half that of nickel-based batteries.
  
• Low Maintenance - no periodic discharge is needed; there is no memory

Limitations

• Requires protection circuit to maintain voltage and current within safe limits.
  
• Subject to aging, even if not in use - storing the battery in a cool place and at 40% charge reduces the aging effect.
  
• Moderate discharge current - not suitable for heavy loads.
  
• Transportation restrictions - shipment of larger quantities may be subject to regulatory control. This restriction does not apply to personal carry-on batteries.
  
• Expensive to manufacture - about 40 percent higher in cost than nickel-cadmium.
  
• Not fully mature - metals and chemicals are changing on a continuing basis.

The lithium Polymer battery

The lithium-polymer differentiates itself from conventional battery systems in the type of electrolyte used. The original design, dating back to the 1970s, uses a dry solid polymer electrolyte. This electrolyte resembles a plastic-like film that does not conduct electricity but allows ions exchange (electrically charged atoms or groups of atoms). The polymer electrolyte replaces the traditional porous separator, which is soaked with electrolyte.

The dry polymer design offers simplifications with respect to fabrication, ruggedness, safety and thin-profile geometry. With a cell thickness measuring as little as one millimeter (0.039 inches), equipment designers are left to their own imagination in terms of form, shape and size.

Unfortunately, the dry lithium-polymer suffers from poor conductivity. The internal resistance is too high and cannot deliver the current bursts needed to power modern communication devices and spin up the hard drives of mobile computing equipment. Heating the cell to 60°C (140°F) and higher increases the conductivity, a requirement that is unsuitable for portable applications.

To compromise, some gelled electrolyte has been added. Most of the commercial lithium-polymer batteries used today for mobile phones are a hybrid cells and contain gelled electrolyte. The correct term for this system is lithium-ion-polymer. This is the only functioning polymer battery for portable use today.

With gelled electrolyte added, what then is the difference between classic lithium-ion and lithium-ion-polymer? Although the characteristics and performance of the two systems are similar, the lithium-ion-polymer is unique in that solid electrolyte replaces the porous separator. The gelled electrolyte is simply added to enhance ion conductivity.

Lithium-ion-polymer has not caught on as quickly as some analysts had expected. Its superiority to other systems and low manufacturing costs has not been realized. No improvements in capacity gains are achieved - in fact, the capacity is slightly less than that of the standard lithium-ion battery. lithium-ion-polymer finds its market niche in wafer-thin geometries, such as batteries for credit cards and other such applications.

Advantages

• Very low profile - batteries resembling the profile of a credit card are feasible.
  
• Flexible form factor - manufacturers are not bound by standard cell formats. With high volume, any reasonable size can be produced economically.
  
• Lightweight - gelled electrolytes enable simplified packaging by eliminating the metal shell.
  
• Improved safety - more resistant to overcharge; less chance for electrolyte leakage.

Limitations

• Lower energy density and decreased cycle count compared to lithium-ion.
  
• Expensive to manufacture.
  
• No standard sizes. Most cells are produced for high volume consumer markets.
  
• Higher cost-to-energy ratio than lithium-ion

Ask us about our low cost guarantee for camcorder, laptop, digital camera ,cordless phone, scooter, power tool, dvd player and other portable devices and more. If a product is not listed, Please call 1-877-462-2883.