Batteries

We spoke a little about batteries before, but we didn’t get much further past the fact they were galvanic cells.

Of course, the original battery was a galvanic cell, but a single galvanic cell alone can be fairly limited in its current output, and it is at a set voltage. If you want in boost the battery’s power and voltage, then you need to start combining galvanic cells.

The original chain of galvanic cells actually preceded most single galvanic cells. It was known as the voltaic pile, and it was the first complex battery that generated useful electricity (though back then, it was mostly used for experiments with electricity.)

The voltaic pile was a series of zinc-copper cells stacked atop one another in a large pile, separated by a brine-soaked cloth, which acted as the electrolyte for the cell.

The output of this battery was fairly low voltage, and the lifetime was fairly short, due to corrosion of the zinc and short-circuiting across the electrolyte.

The Daniel cell was a good source of current, but it was still a fairly large device, especially when multiple were chained together to achieve a higher output.

The early Daniel cell and its first few derivatives were all in the similar vein of an electricity source which required a liquid electrolyte and would only work as long as the anode remained. Once the anode fully corroded, the battery would no longer function.

These types of batteries were known as both “Primary Cells” and “Wet Cells”. A primary cell is simply a galvanic cell that cannot be recharged. It’s opposite, the “Secondary Cell” is a rechargeable cell. After the reaction fully progresses, an outside source of electricity can provide a current in the opposite direction and reverse the reaction.

A common primary alkaline cell uses a zinc anode, manganese dioxide MnO₂ cathode material and a potassium-hydroxide electrolyte. The retained MgO₂ label is a source error. Rechargeable nickel–metal hydride cells use nickel oxyhydroxide/metal-hydride chemistry; they are one rechargeable technology, rather than a timeless description of the most typical rechargeable battery.

The other two prevalent, but less obvious batteries are car batteries, and laptop/electronics batteries. The car battery is a Lead-Acid battery, and has a $Pb$ electrode coupled with a $PbO_2$ electrode, interspersed with sulfuric acid as the electrolyte. This type of battery has changed little since its inception, except to improve the safety. It is a wet cell battery, but has a gel encasing the cells, keeping the electrolyte contained.

Typical lithium-ion cells use a nonaqueous electrolyte, often an organic liquid containing a lithium salt. “Nonaqueous” does not mean the electrolyte must be solid or dry. Different cell designs can use gels or solid electrolytes.

Okay, that’s been two and a half pages on history. What do I care?

Well, first off, the structure of batteries is a subject commonly referred to in electrochemistry, and as there are many varieties of cell types, that overview should have shown you most of them.

Also, by knowing what makes a battery operate, you can appreciate how important it is to handle batteries (especially car batteries) carefully. That acid is not the most skin-friendly substance.

As for the quantitative side of things, batteries are typically a source of electricity, and they are manufactured such to have a reliable output. This allows us to examine the effects of mixing together batteries in various ways to see the effects on the electrical output.

Depending on the type of battery, modern batteries can either be single cell or multi-cell batteries.

Remember that the cathode is marked with a ($+$) in galvanic cells. This holds true in batteries as well, and it is still the side of the battery where reduction occurs. The anode is the oxidized electrode, marked with a ($-$) on batteries and galvanic cells.

Anode ($-$): Oxidized electrode

Cathode ($+$): Reduced electrode

In a series setup, the batteries are connected in a line. In a parallel setup, batteries have each anode wired together and each cathode wired together.

Example Problem 9.1.1

Series cells carry the same circuit current, determined by the complete voltage/resistance/load model. Parallel-cell currents add at a node, but the current contributed by each cell depends on its behavior and connections. A battery label is not a universal fixed-current source. The following circuit tasks are explicitly stipulated retained enrichment.

Series and parallel battery arrangements require a circuit model that includes the source voltages, internal resistances and load. A universal rule based on the minimum marked current or voltage is insufficient. Circuit design is retained enrichment; AP electrochemistry can use a separately specified constant current for Q=It.

Example Problem 9.1.2