Working with Supercapacitors

I’ve always been interested in using supercapacitors for powering UKHASnet nodes, its something about the simplicity of how you can use them. As I’ve been rebuilding my network I’ve wanted to put a solar powered repeater on the roof to add some range and so thought this would be an ideal opportunity to use supercapacitors.

The aim is to use a solar panel to charge a supercapacitor (with a diode in-between) and then to run a low power LPC812 UKHASnet node (running on my EtnaNode PCB). Supercap technology has come a long way in the last few years – high farad caps are now much more affordable. I therefore got hold of two 2.7V 15F capacitors from PowerStor/Eaton (HV1325-2R7156-R) and put them in series. This allows them to act like a 5.4V 7.5F capacitor (theory here), while we have lost capacitance it means that our supercaps can safely cope with the wide variation in voltage from the solar panel.

The LPC812 is running direct off the supercaps and so can measure its own input voltage which allows us to both keep track but also adjust our power consumption depending on how much is available. The LPC812 has its brownout circuitry turned on which means that when the voltage is low it doesn’t get stuck in a reset loop, once the voltage gets above 2.2V it’ll turn on and initially just act as a beacon. Once the voltage gets to 2.5V or above it starts to repeat packets, regularly monitoring the supercap voltage and if necessary going back to beaconing to save power.

AI0 is the current experimental node, its sitting just below the roof window and with the recent cloud cover is turning on around after 0930 and lasting to about 2100, the amount of time spent repeating is quite dependent on how much sunlight there is.

Now that the node is at least working partially I have some plans to make it work a bit better. Firstly some code changes including getting it to measure how much time it spends in repeat mode vs beacon mode as this will provide some useful information. Ideally I’d like the setup to survive the night, this might require the low power beacon mode to be extended further (perhaps a transmission every 5 or 10 minutes at night). I’m also currently looking at using the LTC3105 IC as a dc/dc step up convertor to make better use of the supercapacitors – in the setup I’m using now the voltage needs to climb up to at least 2.2v before anything can occur, with something like the LTC3105 it will be able to start doing useful things at 0.3V.