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While these batteries can be charged safely without balancing, the use of a balancer helps improve longevity. Fitting every battery with a built in balancer board would be expensive and difficult to accomplish given the minimal space available in our battery packaging solution. Instead we charge the batteries using FMA Powerlab 6 chargers from ProgressiveRC. These chargers can give a high 40 amp charge rate and a decent 1 amp balance current. Unlike many other chargers, the FMA Powerlab can also be configured to charge a 100Ah battery without running into any safety timer or capacity limit issues. We mount two of these chargers and a 27volt, 40 amp power supply in a pelican case as a mobile charging station.
A RoboteQ LDC1430 motor controller is used to drive the the thruster. Having tried several motor drivers, the LDC1430 is the most reliable and provides several extra features such as voltage monitoring, current sensing, and PWM frequency adjustment. Noise tests with the motor driver and acoustic modem showed a spike at the motor's PWM frequency and another at twice the frequency. By setting a 17kHz PWM frequency (34kHz second spike), we were able to minimize noise near 25kHz as long as the motor is operating at less than 75% thrust. The motor driver is housed inside its own box that connects directly to the batteries and the motor. This same box also provides power and serial communication with the motor driver to the cpu box and connects to the e-stop switch. This design keeps the power electronics isolated from the various electronics inside the CPU box.
Communication
RF communication with the vehicle is provided by two different radios. A 2.4 GHz Bullet M from Ubiquiti provides typical WiFi functionality and bandwidth while increasing range - we usually see good performance out to 600 meters using an omni-directional antenna (~7 dBi) on the vehicle and a sector antenna on shore (~12 dBi). The Bullets are also easy to configure and weatherproof so they can be installed in the kayak without any sort of housing. Power and data are provided together via ethernet.
More recently, a longer range communication option has been added in the form of a Freewave FGR2-PE 900 MHz ethernet radio. These radios are lower bandwidth (~115Kbps shared among all active radios) but are much longer range (many miles theoretically, still testing). They enable us to more easily perform experiments in open water that test the range of the acoustic communications, which could be several kilometers. When in use, the freewave radios are installed in their own box much like the motor driver along with an ethernet switch and power switching board. Due to the small size of the kayak, the two antennas cannot be separated enough to avoid interference. Instead, only one radio can be powered on at a time on the kayak while the ethernet switch will automatically select the functioning radio to pass packets to and from the shore.
In addition to the data communication channels, an RC controller can also be used to drive the kayak over short ranges. The details of this link will be discussed further in the next section.