EET Electronics Survey Chapter 17 - Batteries

The Voltaic Cell Basic device for converting chemical energy to electric energy. Electrodes - metal plates - are placed in a solution. –The chemical reaction of the solution with electrodes, results in the collection of electrons at one electrode(-) while a lack of electrons gains at the other electrode (+)

The Voltaic Cell Sulfuric acid as the electrolyte with copper and zinc as electrodes. –Zinc electrode has a surplus of electrons The potential for electron flow electrons move from copper to zinc Metal in one of the electrodes will be eroded as a result of the chemical reaction

The Voltaic Cell Single cells are connected in series to get a larger voltage. –A BATTERY, is this arrangement of cells. A Primary cell is one in which an electrode will be consumed during discharge A Secondary cell can be recharged, extending the life of the battery

Battery ratings Voltage - the electric potential –usually 1-3 volts –dependent on material used Energy Capacity –rated in ampere hours (Ah) –a rating of the current the battery is able to supply over time eg. A 16 Ah battery will supply 16 A for 1 hour

Battery Ratings –Generally, energy capacity is directly proportional to size. Internal Resistance –The resistance of the internal parts of the battery - generally of the electrolyte note the electrons must travel through the electrolyte –Terminal voltage drops when connecting a load - this is caused by internal resistance.

Battery Ratings Energy Density –ratio of stored energy to size/weight Shelf Life –life expectancy of the battery if not connected to a circuit Temperature –chemical reactions slow at lower temperature

Primary Dry Cells In dry cells, the electrolyte is actually a paste –in wet cells the electrolyte is a liquid Carbon-Zinc Cells –The Leclanché cell –typical flashlight batteries –zinc container and carbon rod are electrodes –ammonium chloride is the electrolyte

Primary Dry Cells Zinc-Chloride cells 1.5v –an improved version of the carbon-zinc cell –Chemical mix results in a 50% increase in capacity Alkaline Cells (alkaline-manganese) 1.4v –physical design and chemistry results in a higher capacity and greater efficiency. –The voltage-discharge curve is better suited to modern high current demand devices

Primary Dry Cells Mercury Cells and 1.4 v –zinc and mercury oxide –expensive, but has a very high energy density and longer self life Lithium Cells –high output voltage (3V per cell) –high energy density and very long (10yr) self life.

Serial and Parallel batteries In series, the voltage is increased - the sum of the cells in series –Energy capacity is the same as that of a single cell (current = electrons/second –if unmatched cells are connected in series, the capacity will be that of the weakest cell.

Series and Parallel Batteries Parallel connection –increases the energy capacity of the battery set. –Cells must be matched for voltage –unmatched cells result in voltage with only internal resistance. Series-Parallel connections –combination of series and parallel calculations

Testing Visual inspection of dry cells is not effective (other than for leakage) Measuring the internal resistance is a good check of quality of the battery –measure open circuit voltage –measure short circuit current R(internal)= V/I –high resistance indicates a bad cell

Lead-Acid battery Two different types of lead are used –sponge lead - (Pb) and lead peroxide + (PbO 2 ) –electrolyte is 36% solution of sulfuric acid (H 2 SO 4 ) –2.13 volts per cell As current flows, the acid content of the cell diminishes and lead is replaced with lead sulfate

Lead-Acid battery By charging the battery - forcing current to flow in the opposite direction - this chemical reaction is also reversed The electrolyte gives off gaseous oxygen (+) and hydrogen (-) during charge –This poses a severe fire hazard. Batteries require maintenance of electrolyte level and routine charge

Maintenance-Free lead-acid batteries –made with calcium in the grids –calcium reduces gassing of battery - and therefore the loss of water Rating of battery given in CCA - cold cranking amps - current delivered in 30 seconds at 0 degrees F while maintaining a terminal voltage of 7.2 volts

Testing a lead-acid battery Measuring the specific gravity of the electrolyte will indicate battery condition. 80deg F fully charged. –A difference between cells of.05 or more indicates a bad cell. A complete test must include a capacity or heavy-load test. At full load (starter motor on) 9.6 v is good

Nickel-Cadmium Rechargable Nickel and Cadmium are used as electrodes rechargeable up to 1000 times cell voltage is 1.2 V may develop a ‘battery memory’ –completely discharging Ni-Cad batteries eliminates this memory.

Battery Charger Charger is connected + to + and - to - leads. Slow charging –low current over long period (preferred) Fast charging –higher current over shorter period –heat dissipation can be a problem Trickle charge or float charge –constantly under charge