Battery Blog Post
Conducted by: Luis J. Martinez (Mission, Systems, and Test Engineer)
Battery (Duration, Powertrain)
As selected from our previous trade-off studies, the battery of choice for powering our Prosthetic Arm, in conjunction with the Prosthetic Hand, recognized as the Upper-Limb Prosthetic System was the 14.8V (4S), 1600–mAh Zippy Compact Lithium-Ion Polymer (LiPo) battery.
Incorporated into our system to fulfill the requirements for duration (L1-3) and powertrain (L2-7), associated tests were conducted in the verification and validation phase of the Prosthetic Arm system and are summarized as follow:
Duration (L1-3)
Following preliminary calculations calling for a 1500-mAh capacity, the battery was first charged to its full capacity through use of the Imax B6AC Professional Balance Charger/ Discharger in accordance with LiPo battery safety guidelines as referenced from a previous semester. Setting the charger to top off the LiPo cells at a maximum of 4.2V each, the battery was charged to a maximum voltage of 16.8V in 50:04 minutes.
Upon reaching its full charge, the battery was then set up in a configuration to discharge at a constant rate until the voltage dropped to 12V, a minimum voltage recommended by LiPo battery safety guidelines.
The duration test concluded in approximately 3 hours and 45 minutes, drawing an average of 0.317 Ah (amp-hours) during hour 1, 0.4 Ah during hours 2 and 3 each, and 0.3 Ah during the final 45 minutes, totaling to 1.417 Ah in capacity over the entire duration.
Based on a 3.2A total draw on 12V for the Upper-Limb Prosthetic System, where the Prosthetic Arm and Prosthetic Hand systems are estimated to draw 2A and 0.7 A on 12V, and 1A and 0.1A on 5V respectively, the test yield of 1.41A is considered passing due to the fact the Upper-Limb Prosthetic System will not be drawing a constant 3.2A during the entire 20 minute duration time.
Factors such as a two-digit significant figure on the current draw for the battery discharger, and given the battery was programmed to stop discharging upon reaching an overall 12V per precautionary guidelines to avoid permanent damage to the cells, the 1.417 Ah yield is conservative and considered to have enough head room to permit a 20 minute meal duration.
Powertrain (L2-7)
For the powertrain test, calculations were made indicative of a 3.43 ohm, 42 W load to test a 3.5A draw on 12V, and a 5 ohm, 5W load to test for a 1A draw on 5V. After powering our Printed Circuit Board (PCB) and connecting the 3.43 ohm load on 12V, with an in-series ammeter to measure current, a measured current draw of 3.056A were effectively verified drawn from the 12V source – this due only to the fact that a 42W resistor was not available, and our test was conducted with a 25W resistor instead. Afterwards, the load was swapped out to the 5 ohm power resistor corresponding to the 1A test. Respectively, this experiment yielded a measured current draw of 0.949A, effectively verifying the current draw from the 5V source. In conclusion, the powertrain test proved the capacity of the Prosthetic Arm system to deliver 3.5A on 12V and 1A on 5V respectively, should the Upper-Limb Prosthetic System need it.