Department of Marine Science
These ROVs were designed to be used by inexperienced children near salt water, so electrical safety was the primary consideration. We therefore limited our power source to a single low voltage battery. To achieve adequate power at modest cost under this design constraint, we opted to use a 12 volt, 12 amp-hour, absorbent glass mat (AGM), sealed, valve-regulated, lead-acid battery. This battery powers every aspect of the ROV, both on the surface and below the water line.
The diagram below provides an overview of all the circuitry in the Catalina ROV System. You can see that all of the power is derived from the single 12 volt battery located at far left in the diagram.
Electrical power is conducted from the battery to the electronics box (pictured below mounted on the Pilot’s Console), by a red and black pair of 14 AWG speaker wire.
The red and black insulations hues serve as a color code to reduce the chance of reversing the battery polarity. Red goes to the positive (+) terminal of the battery, and black goes to the negative (-) terminal. We stripped the ends of the wires and crimped on 1/4" female spade disconnects to fit the battery’s F2 terminals.
Once inside the electronics box, the red (positive) lead of the power cord goes through a 20 amp automotive blade fuse in an in-line fuse holder. This fuse is an important safety feature to reduce the risk of fire in the event of an electrical short somewhere in the system. (These batteries are low voltage, but they are still capable of delivering enough current to make things very hot and potentially start a fire. )
Immediately after the fuse, the power line goes to the main power switch, a rocker switch located on the left outside surface of the electronics box.
From the other side of the switch, it goes to a barrier strip with pairs of of electrically-connected screw terminals that provide a convenient way to distribute the +12 volt DC power to the various electrical subsystems. One of these subsystems is just a big blue LED that lights up whenever the main power switch is turned on to let the pilot know that power is flowing to the circuitry.
Most of these subsystems can (and do) run directly off the 12 volts from the battery; however, the TV monitor and the GoPro camera used on this ROV each require a different voltage and therefore require an adapter of sorts.
The TV monitor is a small screen designed to be used in combination with a small TV camera mounted on the back of a recreational vehicle or other large vehicle to supplement the rear-view mirror, so the driver can see more clearly what is directly behind the vehicle while backing up. It therefore comes with a convenient power adapter box (with its own fuse and filtering) designed to connect to a standard 12 volt car battery. We simply hooked that adapter box to the 12 volts coming from our Pilot’s Console battery.
The GoPro camera has an on-board battery that can be recharged though the camera’s USB port, which supplies 5 volts; however, that USB port (or the corresponding wires accessible through the 30-pin GoPro bus socket located on the back of the camera) can also be used to power the camera directly from a +5 volt DC source. We therefore use a 5 volt DC-to-DC converter to convert some of our 12 volt supply to 5 volt, which we then send down the tether to power the GoPro camera from the main 12 volt battery. WARNING: Do not connect 12 volts directly to the GoPro, or you will probably kill the camera!
All of the return current from the various subsystems comes back to the battery through other screws in the barrier strip, which is used as a ground bus. The black wire from the battery connects to this bus.
Some of the other details of the electrical box and the control system, including how the TReX motor controllers use joystick signals to route battery power to the motors, are described on the page that deals with the control system.
A final note about the power system and the tether: One limitation of using a low voltage power source rather than a high-voltage source is that a significant fraction of the available power is lost to resistive heating in the tether wires. This loss can be reduced by using thicker wires, but that results in a thicker, heavier, stiffer tether. The losses can also be reduced by using a shorter tether, but that limits maximum depth and maneuverability. Our tether represents a compromise; it uses 16 AWG speaker wire in 15 meter (50 foot) lengths to transmit power to the thruster motors.