A voltaic cell consists of two different conductors as electrodes immersed in an electrolyte. The voltage output depends only on the chemicals in the cell. The current capacity increases with larger sizes. A primary cell cannot be recharged. A secondary or storage cell can be recharged.
A battery is a group of cells in series or in parallel. With cells in series, the voltages add, but the current capacity is the same as that of one cell. With cells in parallel, the voltage output is the same as that of one cell, but the total current capacity is the sum of the individual values.
The carbon-zinc dry cell is a common type of primary cell. Zinc is the negative electrode; carbon is the positive electrode. Its output voltage is approximately 1.5 V.
The lead-acid cell is the most common form of storage battery. The positive electrode is lead peroxide; spongy lead is the negative electrode. Both are in a dilute solution of sulfuric acid as the electrolyte. The voltage output is approximately 2.1 V per cell.
To charge a lead-acid battery, connect it to a dc voltage equal to approximately 2.5 V per cell. Connecting the positive terminal of the battery to the positive side of the charging source and the negative terminal to the negative side results in charging current through the battery.
The nickel-cadmium cell is rechargeable and has an output of 1.2 V.
A constant-voltage generator has very low internal resistance.
A constant-current generator has very high internal resistance.
Any generator has an internal resistance ri. With load current IL, the internal ILri drop reduces the voltage across the output terminals. When IL makes the terminal voltage drop to one-half the no-load voltage, the external RL equals the internal ri.
Matching a load to a generator means making the RL equal to the generator's ri. The result is maximum power delivered to the load from the generator.
