In our early field experiments, we determined that transceivers powered by DC operated better than those powered by AC for for instream applications (Downing et al. 2004). In AC systems, inline EMI from grid power interfered with transceiver operation, which contributed to lower performance. Most importantly, DC–systems could be deployed where grid power was unavailable.
To best serve the needs of the research, the choice of power system should be informed by the following questions:
For example, for a project of only one season or 1–2 months, it may be least costly to operate the system on batteries.
For long–term projects where the PIT–tag systems will be operated at the same location for several years, grid power is by far the most convenient and least costly alternative. Researchers using grid power may still need to provide a backup power source to prevent missing detections of fish during a power outage. Transceivers will generally restart when power is restored, but peripheral devices such as laptop computers may not.
If no backup power is provided, then the power system design is composed of a linear power supply (e.g., a 24–V DC converter) connected directly to a 110–V AC outlet box. The power supply converts 110–V AC grid power to the 24–V DC power needed by the transceiver. This configuration isolates the transceiver from EMI generated by AC grid power, which will interfere with tag reading.
If a backup power system is chosen, there are several design options to consider. It is essential to choose a design that does not include electronic equipment that produces EMI (specific brands and models are available from the authors).
One design that produces minimal EMI employs a high quality, 24–V power inverter/charger. The inverter is connected to the AC power grid, and both converts power to 24–V DC and charges a pair of 12–V batteries.
A backup power system with one pair of 12–V batteries will operate from several hours to several days, depending on factors such as battery size and condition, temperature, and the power requirements of the transceiver and ancillary equipment.
A second and less expensive option is to switch transceiver power between two pairs of 12–V batteries (Downing et al. 2008). One battery set powers the system while the other is being charged by a 24–V AC charger. Power to the batteries can be switched as often as every 2 hours using a timer and two relays (details and parts list available on request).
It is important to purchase a battery charger that has sufficient amperage (minimum of 8 amps) to bring the battery bank to full capacity within a few hours; otherwise batteries will not be at full backup capacity when switched after a power outage. It is also important to use a charger that is electromagnetically quiet.
A switched power–system design is the quietest electronically because transceiver operation is continuously isolated from the AC power source. This design is also more economical, since a charger is less costly than a high–quality 24–V power inverter/charger.
During a power outage, a switched backup system can provide 5–7 d of operation time; power is drawn from both sets of batteries, and the switching system continues to operate during the outage. When grid power is restored, the charger will commence charging the battery banks one at a time as they are switched. Since backup power systems rarely discharge their batteries fully, the batteries typically last a number of years.
One option that should be approached with caution is using off–the–shelf, uninterruptible power systems. We have found that most of them produce high EMI levels at frequencies that directly interfere with tag reading. Furthermore, these systems provide only a relatively short backup period (hours rather than days) unless a very large and expensive unit is installed.