Fall 2016 Biped Trade Off Study- Battery

Battery Selection

By: Alan Valles (Electronics and Control Engineer)
Approved by: Ijya Karki (Project Manager)

Table of Contents

Introduction

A Battery must be chosen in order to provide power to the entire system of the Biped. The motors and servos will be the main power sink onboard our system. The rest of the unit will be nominal in comparison to the continuous rotation that will be provided to the motors during normal walking operation.

Study

The default design of the 3Dot board was chosen by the Arxterra team to utilize a 3.7V RCR 123A. However, the team designed the 3Dot board to be flexible and allows for an external battery utilizing a JST connector. In order to meet scheduling requirements, we will assume that the proper discharge rate is used across entire system.

Battery	Mfg	Voltage	Capacity	1hr power	Chemistry	Mass	Cost
RCR123A	Varies	3.7V	650mAh	390=>1.4W	Li-ion	~50g	Free
B0072AEHIC	Turnigy	7.4V	1000mAh	600->4.4W	LiPo	~23g	9.89
LP-503562	Adafruit	3.7V	1200mAh	720->2.6W	LiPo	~23g	9.95
34117	Tenergy	7.4V	2200mAh	1320->9.7W	LiPo	~136	14.99

Conclusion

As an estimation, the motors should draw anywhere between 150mA – 300mA at 5V (1.5W)during normal operation. Another estimation is that the cutoff voltage will be reached at .4V capacity. Thus the entire battery capacity cannot be utilized. However, the servos utilized will be have a operating voltage range of 4.8-6V. Although they can be utilized directly at 3.7V if they are connected directly to the battery, the voltage will decrease as the battery voltage lowers over time. Thus, a LDO will be used to regulate the voltage to the servos. Thus, a 7.4V battery will be used in order to raise the voltage delivered to the servos, since the mission profile calls for a worst case of 60 minutes. The battery should be able to supply at a minimum 1.5W continuously. Therefore, the Turnigy B00 will be selected due to its compromise between cost, mass, and capacity. Furthermore, it operates at the necessary voltage. Since it is used for hobby RC cars it also has necessary discharge rates.