DC Sources of Electricity
Cells and batteries
A cell converts chemical energy into electrical energy using two dissimilar electrodes in an electrolyte. Primary cells (alkaline, zinc-carbon, silver oxide) are not rechargeable; secondary cells (lead-acid, NiCd, NiMH, lithium-ion) are.
Lead-acid: lead dioxide positive plate, spongy lead negative plate, dilute sulphuric acid electrolyte; about 2.1 V per cell, so a 24 V battery has 12 cells and sits around 25.5-26 V open circuit when fully charged. It is cheap and delivers high current but is heavy and gives off hydrogen while charging, so ventilation and no naked flames. NiCd suffers memory effect and contains hazardous cadmium. Lithium-ion needs protection circuitry because of thermal runaway risk.
Ratings
Capacity in ampere-hours (Ah) = current x time. A 40 Ah battery can nominally deliver 4 A for 10 hours, though capacity falls at high discharge rates and in the cold. Series connection adds voltages and leaves capacity unchanged; parallel adds capacities and leaves voltage unchanged.
Internal resistance makes terminal voltage sag: V = E − Ir, and the lost power appears as heat inside the battery.
Traps
- Ah is charge, not energy; Wh = Ah x volts.
- Two 12 V 40 Ah batteries in series give 24 V 40 Ah; in parallel, 12 V 80 Ah.
- A thermocouple is a heat-to-electricity converter, not a battery.
- Do not confuse emf with terminal voltage under load.
Worked example
A battery of emf 24 V and internal resistance 0.2 Ω supplies 10 A. Terminal voltage = 24 − (10 x 0.2) = 22 V, and 20 W is dissipated internally.