Sojourner S’19
Solving the Power Issue with an External Battery
Author/s: Cole Bailey
Introduction
This blog post will describe the things we learned in terms of supplying power to our system, and what we did to fix the issue of the power cutting out.
The Power Issue
The power budget of the Sojourner was extremely important because we had to ensure we could drive all 6 motors and 4 servos at the same time to drive the Sojourner. The issue with the original battery fitted for the 3DoT was that it did not have enough output current to drive all 6 motors. We chose a Nitecore RCR123A Li-Ion battery with 4A of instantaneous output current and 650mAh capacity. At first we thought this would be enough to drive all of the motors and servos but we did not take into account the large spike of current to get the motors to start turning from the stopped position. We found that the voltage of the battery would drop too low if we tried to start all the motors at the same time, and the 3DoT would cut out and lose connection.
The Solution
To solve the power issue, we decided to use an external LiPo battery to power the motors and servos. We used the Tattu 2 cell 7.4 V battery rated at 25c so we had plenty of current to handle the initial current draw of the motors.
To use the external battery with the 3DoT was easy. All you need to do is:
- De-solder the battery short pin on the back of the board to disconnect the 3DoT battery from the motors and servos.
- Connect the power wire from the battery to the VM pin on the 3DoT.
- Connect the ground wire from the battery to the GND pin on the 3DoT.
After doing this, the battery will now be supplying the motor and servo drivers from the 3DoT.
Conclusion
In general, you should make sure your power source (battery) should be able to supply enough power to drive the robot in the worst conditions. LiPo’s have a large discharge rate so they do well with the initial current surge, but because of this, they will die faster. With the combination between the powerful Tattu external battery and the Nitecore powering the 3DoT, you should have plenty of power for the Sojourner.