Specifications

Draining the battery to a voltage below 10.5V may also cause permanent damage to the

battery.

5.7 Power Inverter

When it came to the power inversion part of the design, we had two options. We could

either build an inverter from scratch, or we could purchase a commercially available

alternative. In building one from scratch, the inverter would be custom and specific for

this application, and it would also cost significantly less than a comparable commercially

available option. On the other hand, building one from scratch would require a lot of

time and effort, and in the end we would not have really accomplished anything new

since there are so many different inverters already available for purchase. The length of

time required to design and build an inverter from scratch may actually be long enough

to take up an entire senior design project, especially if building a modified sine wave or

pure sine wave variant. Thus, after careful consideration, it was decided that an inverter

would just be purchased.

Next the decision had to be made as to a specific power inverter to use in the system.

While the improved efficiency of a pure sine wave inverter would be useful in this

application, the ability to operate sensitive equipment is of little importance. As noted in

section 4.1.4, price sharply increases on pure sine wave inverters over comparable

modified sine wave inverters. Since one of the main goals of this project is to maximize

efficiency at the lowest cost, this made our choice especially difficult. But in the end, the

decision was eventually made to use a modified sine wave inverter. In order to come to

this decision, it was realized that the small gain in efficiency of the power inverter

wouldn‟t really translate to a lower cost in the battery, wind turbine, or solar cells since

the power output from these were already quite small when compared to all of those

available. There are a few situations where a pure sine wave inverter may produce

actual monetary savings. One such situation is when a when an older less efficient

inverter is already in place on a large system that the required runtime has been

increased. Rather than spend lots of money increasing the size of the battery bank, it

may be possible to simply swap out the older inverter for a new pure sine wave version.

Also, in some instances, extra generated power can be sold back to the power

company. Pure sine wave inverters are required for these situations, as the power that

is put into the electrical grid must match that which already occupies the grid.

As previously calculated in section 5.1, the wattage necessary to power a laptop and

projector should not exceed 600W continuous power. Thus, any inverter with a power

rating larger than this should work fine for this application. It was also decided that an

inverter with a USB output should be used, as this would further simplify the design of

the power usage monitoring to just between the inverter and the cigarette lighter plug.

The 5VDC from the USB would be able to provide the power for the microcontroller and

the USB output power port, so a separate power line would not have to be run from the

battery to these which would need a current sensor on each to calculate power usage. It

is also desirable to have an input and output overload alarms/shut down, low battery

alarm/shut down, temperature alarms/shut downs, and short circuit shut down. A

comparison chart with a few possible power inverters can be viewed in the figure below.