Module 3.5 Sources of DC electricity: cells, batteries, thermocouples, photocells

Knowledge levels

Sub-module	A	B1 / B2 / B2L	B3
3.5 Sources of DC electricity	1	2	2

Content: primary and secondary cells (lead-acid, nickel-cadmium, lithium, other alkaline), series and parallel connection, internal resistance, thermocouples, photocells.

Primary and secondary cells

• A cell converts chemical energy into electrical energy; a battery is several cells connected together.
• Primary cells cannot be recharged (the chemical reaction is not reversible): zinc-carbon (1.5 V), alkaline manganese (1.5 V), lithium primary cells (about 3 V, used in ELTs and some memory back-up).
• Secondary cells can be recharged by reversing the current: lead-acid, nickel-cadmium, nickel-metal-hydride, lithium-ion.

Every cell has two electrodes (anode and cathode) and an electrolyte.

Lead-acid cells

• Positive plate: lead dioxide (PbO₂); negative plate: spongy lead (Pb); electrolyte: dilute sulphuric acid.
• Nominal 2 V per cell: a 12 V battery has 6 cells, a 24 V battery 12 cells.
• On discharge both plates turn to lead sulphate and the electrolyte becomes more water-like, so specific gravity falls. State of charge is checked with a hydrometer (about 1.275-1.300 fully charged, lower when discharged).
• Charging produces hydrogen and oxygen - ventilate battery boxes; no sparks or flames.
• Never use lead-acid tools, water or electrolyte near NiCd batteries (and vice versa): the electrolytes neutralise each other and destroy the cells.