How to Wire an External BEC
By Brian Schneider
Most modern-day electric-powered radio control vehicles, whether airplanes, boats, or cars, have something built into their electronic speed controls (ESCs) that shunts power from the main battery to the receiver to power the radio system. This device is called a Battery Elimination Circuit — more commonly referred to simply as a BEC. While everyone that enjoys their electric R/C vehicles uses a BEC all the time, few realize it, and fewer still understand exactly what it is — and why you might want a better, more powerful one. So today, we're diving into the world of BECs: what they are, what they do, and why you might need something better than what comes built-in.
A Brief History of the BEC
To understand why they are called battery elimination circuits, you have to go back in time — way back to the early days of electric radio controlled vehicles. Back then, you had to have two batteries to power the vehicle — one to power the motor (like today) and another to power the radio system. This second battery pack, usually just a holster to hold four AA batteries, was plugged into a battery port on the receiver. This was inefficient, as the batteries were heavy and took up space. Eventually, someone came up with the concept of a battery eliminator circuit — a device that could step down the voltage from the main battery to make it safe for the radio system to use, and eliminate the need for the second battery (hence the name). The first BECs were built into the receiver, not speed controls, but eventually, as the need for more powerful BECs grew, the ESC manufacturers took it upon themselves to include them into their products. Thus the modern BEC was born.
A BEC is (usually) a switching voltage regulator that turns on and off very rapidly so as to allow only the necessary amount of energy through at a time. This allows for high efficiency BECs that can supply high amounts of power, and even have output voltages higher than the input voltages. The average current rating of today's built-in BECs hovers around 3 Amps, though some are much higher. Unfortunately, the demands of today's high-torque servos in large-scale airplanes, rock crawlers and much more means these 3A BECs are occasionally not sufficient, and now we have external BECs available to take up the slack.
Wiring a BEC
Using an external BEC isn't as easy as simply plugging it in. There is some nuance to using an external BEC correctly. Here's what you need to know.
Power is provided from the main battery, same as the built-in BEC on your speed control. However, that means the external BEC needs to be "tapped into" the power wires before they reach the speed control. There are multiple ways to do this: you could simply solder the positive and negative wires from the external BEC to the power connector on the ESC, or you could make a wiring harness that allows for a more modular, plug-and-play option.
The external BEC can be plugged into any open servo port on your receiver. If you don't have any open, you can also Y-Harness it into any servo or ESC-occupied port.
The external BEC replaces a speed control's built in BEC; they don't play nice together. Thus the red wire on the ESC's receiver lead must be either clipped or pulled out before it reaches the receiver. Failure to do this can result in the overloading of the built-in BEC on the speed control, damage to your servos, damage to the external BEC, or all three!
If your external BEC has adjustable voltage output, make sure to set it properly for your servos' specs - if the voltage is too high, you can damage your servos; too low, and you run the risk of brownouts or sluggish servos.
A Traditional BEC Setup
Note the soldered connection between the external BEC and the ESC's battery lead, right after the battery connector - this is the easiest place to tap into the power leads. Also note the clipped red wire on the speed control's receiver lead a must to prevent damage to your electronics.
A Modular BEC Setup
Here, we've added a couple more connectors between the ESC and battery, allowing the user to unplug the BEC from the assembly, perhaps for use in another vehicle. This also allows the user to protect the warranty of their speed control, because it does not require the connector be clipped off and re-soldered.
The red wire on the speed control's receiver lead still needs to be pulled, though this can be done without voiding a warranty as well, by sliding the wire out of the connector instead of clipping it.
Those are only a couple of the BEC wiring options you can do, but they should start as a launching point for your own R/C vehicle. So if you're experiencing brown-outs or slow actuating servos, maybe try an external BEC — it's a great way to ensure constant power to your electronics! Good luck!