Every RoboClaw motor controller includes a built-in 5V Battery Eliminator Circuit (BEC). The BEC converts main battery voltage down to a regulated 5V that runs the controller’s logic and can also power microcontrollers, RC receivers, and encoders, eliminating the need for a separate 5V supply in most builds.
A Battery Eliminator Circuit allows a system to have more than one voltage available for components while using a single power source. In robotics, a common scenario is the need to run motors at 12 volts DC or higher while at the same time needing to supply 5 volts DC for microcontrollers or an RC receiver. A BEC eliminates the need for multiple power sources at different voltages.
What Does the BEC Do?
The BEC on the RoboClaw is a high efficiency switching voltage regulator. It takes the higher voltage from the main battery and steps it down to a steady 5V output. As a result, two voltages are available on the board: the main battery voltage, which drives the motors, and the regulated 5V from the BEC.
The 5V output does double duty. It powers the RoboClaw’s own logic circuits, and it can supply external 5V components such as microcontrollers, RC receivers, and encoders. Almost anything on your robot that needs 5V can run from the BEC as long as the combined draw stays within the BEC’s current rating.
Where Is the 5V Output Available?
The 5V power is available at several locations on the RoboClaw. On RoboClaw boards with pin headers, 5V power is available at the positive (+) pins of the S1-S5 headers as well as the encoder power headers. On RoboClaw boards with screw terminals there is a single 5V terminal powered from the BEC. Be aware that some components will also need to share a common ground with the RoboClaw. Ground can be accessed at the negative (-) pins of the headers used to get power from; screw terminal boards have a single ground pin that can be used for the same purpose.
Do not short the BEC’s 5V output to ground or to the battery voltage. A short on the 5V rail cuts power to the RoboClaw’s logic while the motors may still be energized and can permanently damage the controller.
What Are the BEC’s Electrical Limits?
The regulator used in the RoboClaw is a switching type regulator and has a dropout voltage of 0.5 volts. This means that the input voltage to the BEC must be a minimum of 5.5V to ensure that the BEC can provide a stable 5V output. Also note that the RoboClaw itself may have a separate and different minimum voltage to operate. The table below lists the battery voltage limits for each RoboClaw model.
| RoboClaw Model | Minimum Main Battery | Logic Battery Range |
|---|---|---|
| Solo 30A, Solo 60A | 6V | 6-34V |
| 2x7A | 6V | 6-34V |
| 2x15A, 2x30A, 2x45A, 2x60A | 6V | 6-34V |
| 2x60AHV 60VDC | 6V | 6-24V |
| 2x200A 60VDC, 2x300A 60VDC | 10V | 6-24V |
| Solo 300A | 10V | 6-24V |
BEC output current ratings vary by model and hardware revision. Check the datasheet on your model’s product page for the BEC output rating before sizing external loads.
Can a Logic Battery Feed the BEC?
While a BEC allows for the use of a single battery in a robot, the RoboClaw can also utilize a second battery, known as a logic battery. A logic battery provides an independent source of power for the onboard BEC. It ensures that the RoboClaw logic and accessories will always have a stable source of power and not suffer from brownouts and other power interruptions. Brownouts can occur when motors driven by the RoboClaw are under heavy load or when motors make sudden direction changes at high speed. A logic battery is recommended when the primary power source for the board will be heavily loaded down by the motors. On the RoboClaw board there is a pin header for connecting the logic battery. The allowable logic battery voltage range varies by model; see the table above.
What Precautions Apply?
There are several things to look out for when using the onboard BEC, with or without a logic battery. As discussed earlier, a separate logic battery is suggested when the RoboClaw will be driving heavy loads. Also note that electrically noisy components connected to the BEC, such as some encoders, can cause problems with the proper operation of the RoboClaw.
The BEC also powers the RoboClaw’s own logic circuits, so its full rated output is not available for external components. Leave headroom for the controller itself when budgeting BEC current. Exceeding the rating can cause the RoboClaw to behave erratically or reset.
Next Steps
These guides cover choosing a battery and configuring the RoboClaw’s voltage limits:
Thumbnail photo by Mataresephotos / Wikimedia Commons
