Power Source

A toy robot needs a portable power supply, which is why this trade study is actually about batteries.

Identifying Alternatives

Based on previous experience of our team members, our team identified five battery types which are suitable for building robots:

1. Lithium Polymer Battery (Li-Po)

2. Lithium Ion Battery (Li-ion)

3. Lithium Iron Phosphate Battery (LiFePO4)

4. Nickel Cadmium Battery (NiCd)

5. Nickel Metal Hydrate Battery (Ni-MH) 

Since the properties of batteries vary within each type, which makes it harder for comparison, we chose one particular product within each type from www.all-battery.com and www.hobbyking.com by preliminary comparisons:

• Li-Po: Tenergy 11.1V 1300mAh 25C LIPO Battery Pack w/ Dean Connector

• Li-ion: Tenergy Li-Ion 18650 7.4V 8800mAh PCB Protected Rechargeable Battery Module w/ Bare Leads

• LiFePO4: 3.2V 10Ah LiFePO4 Rechargeable Battery (DGR-A)

• NiCd: 10pcs Tenergy 4/5 SubC 1300mAh NiCd Flat Top Rechargeable Battery w/ Tabs

• Ni-MH:Turnigy LSD 6.0V 2300mAh Ni-MH Flat Receiver Pack

For clarity the types of battery are used to denote the actual product in the following sections.

Ranking Criteria

Explanation for the ranking criteria is described below.Cost: Cost refers to the cost of batteries in terms of our project. A few batteries cost more than $50; the cost on batteries constitutes a small portion of the development cost, hence a low priority.

• Energy Properties: Includes voltage, electric charge and max discharge, with the first one being the most important because it directly affects the number of batteries needed to power the robot. Energy properties of the batteries enjoy high priority because they affect the functionality of every electronic component. High power is also likely needed by the robot to jump on a 30-inch table.

• Physical Properties: Includes dimensions and weight, with the latter being more important. Since both the weight and the size of the batteries strongly affect the weight and the size of the robot, respectively, the physical properties of the batteries enjoy high priority.

• Recharging Ability: Whether the batteries have recharging abilities affects the cost as well as the convenience level of using them. Consequently, recharging ability has medium priority.

• Consumer Safety: Consumer safety is crucial to a toy; however, the safety issue caused by alternatives is rather limited. As a result, consumer safety has medium priority.

Grading Criteria

This section explains the grading criteria. 

• Cost: Cost has a negative relationship with the grading. As the cost of the of product increases, its corresponding grade decreases.

• Energy Properties - Voltage: The relationship between battery voltage and the grading is shown in the graph. The curve goes up and has a gentler slope at smaller values, has a steeper slope at medium values, but decreases at large values. This curve illustrates that the correct battery voltage is very desirable and that being out of the best range creates inconvenience. Even a small voltage requires more batteries and a large voltage means the need to use voltage regulators.

• Energy Properties - Electric Charge: Electric charge has a positive relationship with the grading because greater electric charge means longer use time.

• Energy Properties - Max Discharge: The max discharge has a positive relationship with grading because higher the maximum discharge, higher the system performs.

• Physical Properties - Dimensions: Dimensions are measured in terms of the largest dimension among the length, width and height. They have a negative relationship with the grading but the curve has steeper value at large values because large batteries significantly influence the actual size of the robot.

• Physical Properties - Weight: Weight has a negative relationship with the grading but the curve has steeper value at large values because batteries with heavy weights significantly hinder the robot's jumping ability.

• Recharging Ability: Recharging ability is examined by checking whether the battery can be recharged and whether the recharging has memory effect.

• Consumer Safety: Consumer safety is examined at the existence of substances which are restricted by Restriction of Hazardous Substances (RoHS).

Results

Details of data collection and processing can be found in the Conceptual design review from Project Documents.The results of this trade study are illustrated in Table. Lithium Polymer Battery (Tenergy 11.1V 1300mAh 25C LIPO Battery Pack w/ Dean Connector) is selected from the trade study.