Electrochemical Cells (Batteries)

1. Electrochemical Cells (Batteries) Why is this important? Brief history of batteries Basic chemistry Battery types and characteristics Roll of batteries in digital technology:

2. Two cells A real battery Another battery More precisely Duracell batteries 9v battery 6v dry cell

3. Why is this important The battery pack forms the essential life giving power source to all modern portable digital or analogue electronics, commercial or military Lighter smaller packages leads to greater miniaturisation of products It helps to save the environment if the battery can be recharged Allows the use of technology in remote and harsh environments where mains or generated electricity is not available

4. Electrochemical Cells Cell is part of a battery, a battery is a collection of cells. Cells are classified as either primary or secondary. In a primary cell, chemical reactions use up some of the materials in the cell as electrons flow from it. They can’t be recharged. When these materials have been used up, the cell is said to be discharged and cannot be recharged.*

5. Primary Cells Primary cells can be further classified as either wet or dry. The primary wet cell was first developed in 1800 by Italian scientist, Alessandro Volta. This cell is therefore called the voltaic cell.

6. Primary Wet Cells A wet cell is made up of two pieces of metal that are placed in a liquid. The metal plates, usually zinc and copper, are called electrodes. The liquid in the cell is called the electrolyte. An electrolyte is any liquid that conducts an electric current.

7. Primary Wet Cells Electrons collect at the positive and negative terminals of the cell. http://video.google.com/videoplay?docid=-6226504780579469841 .

8. DANIELL CELL

9. Primary Dry Cells The dry cell is similar to the wet cell, but the electrolyte is a moist paste instead of a liquid. When most of the negative electrode has been used up by the chemical reaction, the electrons stop flowing and the cell is discharged. A group of cells make up a battery.

10. Secondary Cells • Unlike primary cells, a secondary cell can be discharged and recharged many hundreds of times. • Secondary cells are often referred to rechargeable batteries. • Secondary cells are so named since there are two chemical processes involved: • one to discharge the cell • one to charge the cell • A car battery consists of a group of secondary cells.

11. The charger strips electrons from the cathode leaving it with a net positive charge and forces them onto the anode giving it a negative charge. The energy pumped into the cell transforms the active chemicals back to their original state. http://www.mpoweruk.com/chemistries.htm#intercalation

12. The Electrochemical Series Iron Zinc Aluminum Magnesium Sodium Potassium Lithium Most wants to oxidize (lose electrons) Most wants to reduce (gain electrons) • Gold • Mercury • Silver • Copper • Lead • Nickel • Cadmium

13. Battery Characteristics • Size • Physical: button, AAA, AA, C, D, ... • Energy density (watts per kg or cm3) • Longevity • Capacity (Ah, for drain of C/10 at 20°C) • Number of recharge cycles • Discharge characteristics (voltage drop)

14. Further Characteristics • Cost • Behavioral factors • Temperature range (storage, operation) • Self discharge • Memory effect • Environmental factors • Leakage, gassing, toxicity • Shock resistance

15. Secondary (Rechargeable) Batteries • Nickel cadmium • Nickel metal hydride • Alkaline • Lithium ion • Lithium ion polymer • Lead acid

16. Nickel Cadmium Batteries • Chemistry • Cadmium (-), nickel hydroxide (+) • Potassium hydroxide aqueous electrolyte • Features • Rugged, long life, economical • Good high discharge rate (for power tools) • Relatively low energy density • Toxic

17. Nickel Metal Hydride Batteries • Chemistry • LaNi5, TiMn2, ZrMn2 (-), nickel hydroxide (+) • Potassium hydroxide aqueous electrolyte • Features • Higher energy density (40%) than NiCd • Nontoxic • Reduced life, discharge rate (0.2-0.5C) • More expensive (20%) than NiCd

18. NiMH Battery Discharge