About batteries

The production of electrical energy involves the transfer of electrons. That transfer of electrons can be harnessed to generate an electric current. A device constructed to do just this is called a voltaic cell, or cell for short, usually consisting of two metal electrodes immersed in a chemical mixture (called an electrolyte) designed to facilitate such an electrochemical (oxidation/reduction) reaction. A series of these cells is a battery. Most batteries are made up of individual cells. The voltage produced by any particular kind of cell is determined strictly by the chemistry of that cell type. The size of the cell is irrelevant to its voltage. To obtain greater voltage than the output of a single cell, multiple cells must be connected in series. The total voltage of a battery is the sum of all cell voltages. A typical automotive lead-acid battery has six cells, for a nominal voltage output of 6 x 2.0 or 12.0 volts: 




Batteries also have internal resistance. Batteries connected in a series string will tend to have higher internal resistance than batteries connected in a parallel bank of batteries connected in a string. In a series connection you would be adding the voltage of each battery to the total (2 volts plus 2 volts = 4 volts) however the capacity (amp hours) remains the same.


The current is equal at all points in a series circuit, so whatever amount of current there is in any one of the series-connected batteries must be the same for all the others as well. For this reason, each battery must have the same amp-hour rating, or else some of the batteries will become depleted sooner than others, compromising the capacity of the whole bank.


Conversely, the current in a parallel bank of batteries is additive in nature, in other words the power (amps) of the batteries is added together. If you have two one amp hour batteries connected in parallel you will have 2 amp hours of power available. Logic would dictate that if a series string of batteries is sensitive to the amp hour rating of each battery that a parallel bank would not be. That is incorrect. As an example if you connect a 2 amp hour battery and a 1 amp hour battery in a parallel connection, the 1 amp hour battery will cause an additional strain on the 2 amp hour battery as it's capacity is drained over time, reducing the resulting 3 amp hour parallel bank to just 1.75 amp hours of capacity. Therefore, it is recommended that a parallel bank of batteries also be matched to be of equal capacities.

Drawing energy from the battery bank:

The process of the cell providing electrical energy to supply a load is called discharging, since it is depleting its internal chemical reserves. Theoretically, after all of the sulfuric acid has been exhausted, the result will be two electrodes of lead sulfate and an electrolyte solution of pure water, leaving no more capacity for additional ionic bonding. In this state, the cell is said to be fully discharged. In a lead-acid cell, the state of charge can be determined by an analysis of acid strength. This is easily accomplished with a device called a hydrometer, which measures the specific gravity (density) of the electrolyte. Sulfuric acid is denser than water, so the greater the charge of a cell, the greater the acid concentration, and thus a denser electrolyte solution. This is exactly why you do not want to discharge any deep cycle battery any farther than 50%. The work required to remove the lead sulfate from the lead plates can be enormous and time consuming as well as require great amounts of energy better used for powering equipment.

Capacity of a battery:

A battery with a capacity of 1 amp-hour should be able to continuously supply a current of 1 amp to a load for exactly 1 hour, or 2 amps for 1/2 hour, or 1/3 amp for 3 hours, etc., before becoming completely discharged. In an ideal battery, this relationship between continuous current and discharge time is stable and absolute, but real batteries don't behave exactly as this simple linear formula would indicate. Therefore, when amp-hour capacity is given for a battery, it is specified at either a given current, given time, or assumed to be rated for a time period of 8 hours (if no limiting factor is given). 

This is why we use Peukerts when we consider the capacity of a battery bank.


The physical size of a cell has no impact on its voltage. What does dictate it's voltage is resistance, which in turn affects the maximum amount of current that a cell can provide. Every voltaic cell contains some amount of internal resistance due to the electrodes and the electrolyte. The larger a cell is constructed, the greater the electrode contact area with the electrolyte, and thus the less internal resistance it will have.

Sources:

Jason Starck (June 2000): HTML document formatting, which led to a much better-looking second edition.

John Anhalt (December 2008): Updated Lead-acid cell chemistry.

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