A battery works by splitting a chemical reaction in two, so that the electrons one material gives up can only reach the other material by travelling through an outside wire, and that flow is the current. The voltage is fixed by the pair of materials, about 2.05 V for lead and lead dioxide in sulfuric acid, and building the cell bigger does not change it. Capacity, the charge a cell can deliver, depends on how much active material it holds.
Part of Battery Building and Maintenance
The principles that turn a chemical reaction into electric current, and why lead-acid is the cell a settlement can build and keep running.
Two Materials And An Electrolyte Make Current Flow Through Your Wire
Some chemical reactions release energy when two materials are put together. If the materials touch directly, the energy comes out as heat. If instead one material is kept at one electrode and the other at a second electrode, with an acid or salt solution between them that carries ions but not electrons, the reaction can only proceed if electrons run from one electrode to the other through an outside circuit. That outside flow is the electric current, and it does the work of lighting a lamp or turning a motor (Oxidation-Reduction sets out the two half-reactions).
At the negative electrode a material gives up electrons (oxidation). At the positive electrode another material takes them (reduction). The electrolyte completes the circuit inside the cell by moving ions. Take away any one of the three and nothing flows.
Voltage Belongs To The Chemistry, Capacity To The Mass
Cell Voltage gives the rule: cell voltage is the positive electrode’s potential minus the negative electrode’s potential, and it cannot be changed by how the cell is built. For zinc and copper that is 0.34 − (−0.76) = 1.10 V. For lead-acid it is the lead dioxide potential, +1.69 V, minus the lead potential, −0.36 V, which is 1.69 − (−0.36) = 2.05 V. The carrier article quotes 2.1 V for a fully charged cell; the small gap is the effect of acid strength and conditions, which the standard-potential figure ignores.
Current and capacity are separate matters (Battery Fundamentals). Current is limited by electrode area and how well the electrolyte conducts. Capacity, in amp-hours, is limited by how much of the active material the cell holds. Two cells of the same chemistry therefore have the same voltage however large they are, and the larger one simply runs longer or delivers more current.
Lead-Acid Is Chosen Because It Can Be Built And Rebuilt
A cell that can be recharged is a store of energy rather than a one-time source. Primary cells such as the voltaic pile or the Daniell cell are used up; the lead-acid cell reverses cleanly when current is pushed backward through it, and the corpus rates it as highly reversible over hundreds of cycles. Lead can be cast, the acid can be made or salvaged, and glass or hard rubber holds both. Nickel-iron cells last longer but are harder to build.
What You Need
- Two active materials — lead dioxide at the positive plate, sponge lead at the negative, both made from cast lead
- An electrolyte — dilute sulfuric acid (Electrolyte Preparation)
- A separator and a case — to keep plates apart and hold the acid (Cell Assembly)
Worked Example: How Many Cells Make 12 V
A fully charged lead-acid cell reads 2.1 V (the carrier’s figure). For a nominal 12 V battery: 12.6 ÷ 2.1 = 6 cells in series, which the carrier also states. Under load a cell gives less than its open-circuit voltage by the internal-resistance drop. With the cell-voltage article’s example of 0.05 Ω internal resistance and a 10 A draw, V = 2.05 − (10 × 0.05) = 1.55 V per cell, so six cells give 9.3 V at that load. This is why a battery that reads full on a voltmeter can still fail under a heavy load.
Common Mistakes
- Expecting a larger cell to give a higher voltage — it gives more current or run time, never more volts per cell
- Reading voltage alone as the state of charge — the voltage changes little until the cell is nearly flat, and specific gravity is the better gauge (Specific Gravity Control)
Safety
A lead-acid cell holds acid, lead and a source of hydrogen
Every cell contains sulfuric acid that burns skin and eyes, lead that poisons by dust and fume, and, once it is charging, hydrogen that rises and explodes in air between 4% and 75%. Learn the three before you build anything: Safety is the starting point.
What’s Next
- Learn next: Galvanic Cell Principles — what happens at each electrode
- Combine with: Lead-Acid Chemistry — the specific reactions of the cell this tree builds