Problem Statement
A typical mobile phone consumes a modest 0.9 W electrical requiring a 18 g Li-ion battery for 3 hours of talk time.
The trade-off is particularly severe in the design of powered prosthetic joints that need to be lightweight while performing their sophisticated task over a full day of typical use.
Improvement to the performance requires an alternative to current battery technology.
Biomechanical energy harvesting–generating electricity from people during daily activities–is a promising alternative.
Wearable knee-mounted energy harvesting device that generates electricity during human walking.
A small lightweight device that efficiently converts intermittent, bi-directional, low speed and high torque mechanical power to electricity, a technology having great potential for multiple applications like charging cellular device.
Requirement
Design and build a knee henergy harvester that can generate sufficient energy to charge a cell phone as a person walks.
Generate 5V of constant DC power
Low cost
Lightweight
Comfortable
Non-Intrusive
How long does it take for a full cycle charge on an iPhone?
Based on the wattage, it takes anywhere between 1 hour and 40 mins to 3 hours and 19 mins to fully charge the latest iPhone model. There is significant decrease in charge time from 5 watts to 18 watts but since iPhone optimizes its charging to prevent overheating and prolong battery life, there is no advantage in increasing the max wattage from 18-20 Watts.
The graph on the left, the blue line represents the 5-watt power adapter. As one can see, it takes about 3 hours and 19 mins to fully charge the phone. The graph follows a linear pattern for the first 2 hours and 45 mins then when the battery reaches about 90%, the charge starts to trickle and slows down until it reaches 100%.
Now, if we look at the orange line which represents the 12-watt power adapter, it is much faster and only takes about 2 hours and 35 mins to fully charge the phone. It also follows a linear pattern for the first hour and 47 mins when it reaches 90%, then again starts to trickle charge until 100%.
There is a significant decrease in the overall charge time between 5 and 12 watts, however as you increase the wattage output, there is no advantage after a certain point. While looking at the 18, 20 and 65 watts, there is virtually no difference. This is because the device software optimizes charging to make sure the battery does not overheat and lose its maximum capacity faster over time.
Voltage vs Current vs Power Distribution
