Most marine batteries onboard boats today are of the lead-acid type. This type of battery has been used for many decades. The major reason that the popularity of this type battery has continued is relatively low cost to purchase and maintain. The disadvantages of lead-acid batteries are the weight, the short useful life (2-5 years), the lead ingredient that is poisonous and the fact that it creates a potentially explosive mixture of hydrogen gas. 

Although most recreational vessels will generally use a combination lead-acid battery, there really are two types. You could have a starting battery that delivers a short burst of power to start your engine and then is almost immediately recharged by your alternator. You could also have a deep cycle battery that would operate your trolling motor, DC lights, instruments, VHF radio, etc. These are designed specifically to be drained to a low level of charge and then are recharged as necessary. The only difference in the two batteries is how much power is delivered and how long it needs to be delivered.

Battery Construction

                                                                                  Batteries are made of five basic components:

A resilient Plastic container.

Positive and negative internal plates made of lead.

Plate separators made of porous synthetic material.

Electrolyte, a dilute solution of sulfuric acid and water better known as battery acid.

Lead terminals, the connection point between the battery and whatever it powers.

The manufacturing process begins with the production of a plastic container and cover. Most battery containers and their covers are made of polypropylene. For a typical 12-volt   battery, the case is divided into six sections, or cells, shaped somewhat like one row in an ice-cube tray. The cover will be dropped on and sealed when the battery is finished.

The process continues with the making of grids or plates from lead or an alloy of lead and other metals. A battery must have positive and negative plates to conduct a charge.

Next, a paste mixture of lead oxide -- which is powdered lead and other materials -- sulfuric acid and water is applied to the grids. Expander material made of powdered sulfates is added to the paste to produce negative plates.

Inside the battery, the pasted positive and negative plates must be separated to prevent short circuits. Separators are thin sheets of porous, insulating material used as spacers between the positive and negative plates. Fine pores in the separators allow electrical current to flow between the plates while preventing short circuits.

In the next step, a positive plate is paired with a negative plate and a separator. This unit is called an element, and there is one element per battery cell, or compartment in the container. Elements are dropped into the cells in the battery case. The cells are connected with a metal that conducts electricity. The lead terminals, or posts, are welded on.

The battery is then filled with electrolyte - or battery acid -- a mixture of sulfuric acid and water, and the cover is attached. The battery is checked for leaks.

The final step is charging, or finishing. During this step, the battery terminals are connected to a source of electricity and the battery is charged for many hours. When the battery is fully charged, it moves to another line where the case is cleaned, if necessary, and the labels are attached.

A battery stores electricity for future use. It develops voltage from the chemical reaction produced when two unlike materials, such as the positive and negative plates, are immersed in the electrolyte, a solution of sulfuric acid and water. In a typical lead-acid battery, the voltage is approximately 2 volts per cell, for a total of 12 volts. Electricity flows from the battery as soon as there is a circuit between the positive and negative terminals. This happens when any load that needs electricity, such as the radio, is connected to the battery.

Most people don't realize that a lead-acid battery operates in a constant process of charge and discharge. When a battery is connected to a load that needs electricity, such as the engine starter, current flows from the battery. The battery begins to be discharged.

In the reverse process, a battery becomes charged when current flows back into it, restoring the chemical difference between the plates. This happens when you're driving without any accessories and the alternator puts current back into the battery.

As a battery discharges, the lead plates become more chemically alike, the acid becomes weaker, and the voltage drops. Eventually the battery is so discharged that it can no longer deliver electricity at a useful voltage

You can recharge a discharged battery by feeding electrical current back into the battery. A full charge restores the chemical difference between the plates and leaves the battery ready to deliver its full power.

This unique process of discharge and charge in the lead-acid battery means that energy can be discharged and restored over and over again. This is what's known as the cycling ability in a battery.

Each cell in a battery undergoes the same two chemical processes to create voltage within the cell. In its simplest form, two sheets of lead are housed in the acid resistant tank filled with diluted sulfuric acid (electrolyte). In this state nothing happens, the plates just sit there. However, if you add a current, such as from a battery charger, chemical reactions start to occur. The lead plate attached to the positive (+) side of the battery charger starts to turn brown as the lead surface is turned into lead peroxide. Simultaneously, the lead plate attached to the negative (-) side of the battery charger forms an oxide film that is plain metallic lead. When this chemical reaction has occurred the cell is now charged. If you use a multimeter to measure the voltage you will get a reading of approximately 2 volts.

As the battery charges, bubbles start to form on the plates. This is caused by the decomposition of water in the electrolyte. Oxygen bubbles collect at the positive (+) plate and hydrogen bubbles collect at the negative (-) plate. This reaction is producing hydrogen gas which is highly explosive when mixed with air.

Six of the 2 volt cells described above are connected in series (positive plate connected to negative plate) to make up a 12 volt battery as shown below.

Batteries Onboard

You now understand (hopefully) the general workings of a battery, which is probably more than you ever wanted to know. Let's get down to the important stuff. You may have heard some old salts at the marina talk about connecting batteries in series and connecting batteries in parallel. What's the difference. First we must define another variable in our battery; amp hours. Battery capacity is expressed in amp hours. This means that if you had a 60 amp hour battery and everything that you were running off that battery collectively drew 10 amps, your battery would only be good for 6 hours.

The difference in series and parallel lies in what is increased by connecting the batteries and what remains the same. We mentioned earlier how each individual 2 volt cell is connected in series in order to make up a single 12 volt battery. Using the same technique of connecting the positive terminal of one battery to the negative terminal of another you are, in effect, increasing the voltage while the amp hours remain the same. Some boaters use two 6 Volt deep cycle golf cart batteries to make up a 12 volt bank. This is done by connecting the two batteries in series.

You can arrange batteries in parallel to increase amp hours while leaving the voltage the same. You might ask; if I anchor out and use my lights, radio, anchor light, and 12 volt TV how long will my batteries last and still be able to start my engine in the morning? The answer is calculated by dividing the total number of amp hours in your battery(s) by the total of current (amps) drawn by the appliances.

In order to increase the number of amp hours available to run your onboard 12 volt appliances, you can arrange any number of 12 volt batteries in parallel. Arranging in parallel is exactly the opposite of arranging in series. Rather than connecting the batteries from one's negative to the other's positive, you connect the positives together and the negatives together as illustrated below.

When connecting batteries in parallel be sure to use the same type of battery, for example; two deep cycle batteries. The age of the batteries should be the same. A new battery connected in parallel to an older one will not get fully charged, and the older battery may get overcharged.