Supercaps team with RF-transmitted power for “perpetual power supply”

A couple of years ago Powercast made the news through its installation in the Pittsburg zoo of RF-transmitted power for a wireless sensor network in the penguin exhibit.

Here’s an excerpt from an EDN article that mentioned Powercast’s technology: “Powercast has developed the Powercaster transmitter and Powerharvester receiver chips operating at 900 MHz, through which you can broadcast and receive energy. Their range is several meters, and the power level is as much as 100 mW. This low power level may not be the restriction it seems: If your device, such as a node on a wireless-sensor network, has periods of higher power needs followed by long sleep states, consider adding batteries to the device and use the RFpower receiver to trickle-charge the unit. The devices sell for less than $5 each (production volumes).”

The installation in the penguin exhibit allowed for occasional replacement of batteries since the wireless aspect was more about eliminating wires into the exhibit than making the sensors maintenance-free.

However, Powercast wanted to get into the general market for wireless sensors, where maintenance costs for replacing batteries, even on an occasional basis, can make the cost of deploying wireless sensors prohibitive. As Harry Ostaffe of Powercast puts it, “Our goal was to come up with a “perpetual power supply” – one that’s battery –free.”

So the company teamed up the supercapacitor manufacturer, CAP-XX, to replace the alkaline batteries with supercaps, which have a virtually unlimited lifetime due to their unlimited charge-recharge cycles and tolerance of  temperature extremes. While supercaps are more expensive than batteries, labor costs to replace a battery even once dwarf the price premium of a supercap.

As Powercast and CAP-XX detailed in their paper last week at the nanoPower Forum, their RF power platform can send power continuously, scheduled, or on an on-demand basis. The supercap acts to store up power and then burst it out to the sensor network. The sensor network spends most of its time asleep, waking up to briefly gather data from the sensor and then transmit the data to the remote network  processor/host.

One of the challenges when designing with supercaps is allowing for the ageing that’s related to the operating temperature and voltage. Ageing results in some increase in ESR and some decrease in capacitance over time. The paper gave a design rule-of-thumb of doubling the supercap’s ESR and reducing the capacitance by a third when initially sizing the supercap. In addition, supercaps require a minimum charging current of at least 20uA to charge, with the minimum increasing for larger capacitance values.

You can download slides for the paper here – it’s a 0.6MB pdf file.

Front and back pictures of the supercap powered sensing unit:

Full picture of the unit with antenna: