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Tuesday, October 16, 2007

Leeming SHS: Day 2: October 16

Early down to the pits.

Well, relatively early, anyway: we wind up leaving about 45 minutes later than planned. This is pretty typical for day 2 -- it's our first morning in Darwin, so it takes a bit longer to get mobilised.

We put the lithium batteries into the car. Our testing in Perth was done with our nickel metal hydride batteries, from the previous challenge, because NiMH are easier to manage, but now it's time for the real deal.

We set up the lithium battery safety unit, which is a piece of kit which watches all the batteries, and if their voltage is too low or too high or they're too hot or too cold, cuts them off. The cut-off works via a relay, the relay is powered by the 12V power supply, and the 12V power supply is supplied by the battery, which is cut off. So the relay won't activate until the 12V power supply is on, and the 12V power supply won't be on until the relay is active.

We need to buy a pushbutton from Jaycar to work around this. When we get there, Nuna are there; before we leave, Aurora arrive. Nuna (winner of the last three challenges) let us know that they'll be on the track this afternoon.

The pushbutton doesn't work first try: it eventuates that we've neglected to attach an important wire. Once we do, it all goes. We put it in the sun and start charging.

Nuna arrive, get out on the track, and zoom around at about 10,000 km/h.

We've got a damaged cell in the front array: the cells are mechanically very weak and break under the slightest force. This is my first opportunity to test it under load. I crawl around with multimeters for 15 minutes or so, getting ambiguous numbers as the sun comes and goes under patchy cloud, before figuring out that it's definitely munted: whenever the sun goes into cloud, the cell gives a positive voltage, but whenever the sun comes out, it can't keep up, and it and its five friends get bypassed. We hatch plans to electrically remove the cell from the array tomorrow.

Steve Morgan goes for a lap, we come back and talk about it, then do two more laps. All the voltages and temperatures seem right; currents are a bit greyer.

The car was a bit more damaged than we thought: the composite panel around the catch at the top of the array is broken, and the solar cell nearest it is cracked from end to end. But when I test the cell, it's fine: the cell has cracked in parallel with all its little tracks that collect the current, so it's essentially divided itself into two parallel cells that added together still do the job.

We muck about with Ayres panel to replace the broken section, and leave the Sika adhesive to cure overnight.

Sample dialog from the evening back at the apartment:

John: Laura, this is the fourth set of clothing I've seen you in today.

Laura So?

-- Doug, Leeming Hammerhead