Monday, December 5, 2011

LiPo Batteries and more!

Today, we finished our experimentation and verification of our new power supply. Previously we were using 16AA batteries to power the system. 8AA (8*1.5V = 12V) were for the Kinect, and 8AA going through a LM7805 voltage regulator which stepped the voltage down to 5V for the BeagleBoard xM and mbed.  We had to use this set up since the LM7805s were readily available and we could not get 5V from AA batteries. Unfortunately, this set up had many setbacks:

  • added a lot of unnecessary weight
  • required the use of a very large and dangerous heatsink -- the LM7805 could handle 1.5A and our system used up to 2A
  • required us to fiddle with two power supplies to when powering and shutting down the device
  • AA batteries are expensive for the amount of time they lasted (approximately 30 mins)

So motivated by these issues, we decided to switch to a slim 2000mAh, 3 cell LiPo battery that fits within the profile of the stripped down Kinect.

A 2000mAh, 3 cell LiPo battery delivers 11.V (3 * 3.7V) and can run at 2A continuously for an hour. Since our system averages less than 2A, it will last for over an hour on a single charge. Also, from our previous tests, we have verified that the Kinect -- which is rated to work at 12V -- will function as low as 8.5V. Not everything is perfect, however. We still need a voltage regulator to get 5V for the BeagleBoard and LiPo batteries require much more care to ensure they do not get overcharged or over-depleted.

For our new voltage regulator, we are using a more robust component that can take from 6-23V and output 5V at a maximum of 3A. This will allow us to eliminate the heatsink and use power directly from the LiPo instead of having a second power supply.

The final circuit we need for this system to work is a voltage cutoff circuit. Since LiPo cells lose their ability to accept a full charge if they are discharged beyond 3.0V, we need a circuit that cuts off power to the system when the 3 cell LiPo discharges to 9.0V (3 * 3.0V). Since this is the absolute limit, we built our circuit, with the help of Dan B., to cutoff at 9.5V. Dan's circuit is operated by a single momentary switch that turns the device on, but no switch to turn it off. With our modifications, we will be able to operate the belt with a single (ON)-OFF switch -- 'ON' in parenthesis indicates that the switch is momentary ON, static OFF. The circuit is shown below in schematic and PCB form:

Below is a video demonstrating proper function of these components, using a power supply instead of a LiPo to show the voltage cutoff.

Voltage Regulator
Voltage Cutoff