Specifications

components, their configurations, or the specific interconnections to be used in the final

design. In the following sections, the different options and details about each option for

all major hardware components of the system will be discussed. Specific components

will not be selected here; this can be found in the design sections.

4.1.1 Batteries

Choosing the correct battery is one of the most critical parts of this design. Since there

is no guarantee that the sun will be out or that there will be wind at the time of the

blackouts, it is necessary for the power generated by the wind turbine and solar cells to

be stored in a battery as it is created, at the time these resources are available. In the

most extreme case, when the wind turbine and the solar cells aren‟t able to create any

power, the battery has to have enough energy stored to run a laptop and a projector for

at least two hours. Choosing a battery for this device may seem like an easy task at

first, but after closer consideration, it‟s a rather involved process. Since the goal is to

keep the entire unit as portable and modular as possible, the physical size and weight of

the battery must be limited. It‟s also important to consider the reliability, durability,

typical life span, ease of use, and the availability of the battery. There are many battery

types and sub-types that could be used for the intended application, a few of the most

viable of which will be discussed in detail below.

Lead acid batteries come in many forms. The most widely recognized form is known as

an SLI (starting, lighting, ignition) battery or a cranking battery. These are the typical 6V

and 12V automotive battery found in cars, motorcycles, and lawnmowers all over the

world. Cranking batteries can provide a high current for a short period of time and most

have a large enough storage capacity for our application. The problem is they are not

meant to be significantly discharged and re-charged, or deep cycled, very often. When

SLI batteries are subject to such deep cycle use, their life-span is quickly deteriorated.

The reason this occurs is cranking batteries are made with very thin and porous lead

plates, which maximizes the surface are in contact with the acid to in turn maximize the

instantaneous current they can provide. It is these thin lead plates that cause the battery

to fail quickly under repeated deep cycle operation, as the deep discharging and

recharging dissolves the lead. Once the lead plates are dissolved to a certain point, they

cannot be used. The lead plates are not replaceable, so the entire battery must be

replaced. Most of these batteries come in a form without caps to replace water that may

be lost, which causes a problem if the battery is overcharged multiple times. During

overcharging, the electrolyte inside the battery undergoes electrolysis, and the

hydrogen and oxygen gasses created can be released from the battery if the pressure

builds too high. If this occurs too many times, the water within the electrolyte can be

depleted, causing the lead plates to slowly lose contact with the electrolyte. These

batteries are usually designed with extra electrolyte already inside, which prolongs the

life of the battery in if overcharging does occur a few times, but it is limited. If the

electrolyte level on battery with removable caps gets low, the battery can be saved

simply by adding distilled water as often as necessary.

Another popular type of lead acid battery is known as a deep cycle battery, which are

also typically 6V or 12V. These batteries typically provide a more moderate current for a