by Dermot Tynan in Microtransat

Posted on Monday, August 19, 2013 at 08:00

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Power supply in brine

There are four power supplies on board the boat. The main voltage rail is 12 volts, give or take. The solar panels feed into that via two separate DC to DC converters. Ideally, they would provide a 13.7v "float" voltage to the main rail. That voltage is the ideal float voltage for a sealed lead-acid battery at 20 degrees C.

There are two problems with this. Firstly, it is unlikely the battery will be pegged at 20C. In the Northern latitudes, we can expect the night-time temperature to drop well below that level. Likewise, once the boat catches the trade winds, the battery temperature will increase dramatically. I have concerns that the temperature could get well up into the 40's, due to the fact that the battery is enclosed inside the hull.

As the battery temperature increases, the float voltage decreases. In an ideal world, I would use a temperature-compensated power unit for charging the batteries, but this isn't an ideal world. Choosing the best float voltage or charge voltage under the circumstances, is little more than a guessing game as I cannot accurately predict the average charging current or the hull temperature. Choosing 13.7 is a safe bet, but slightly too low.

As you can see from the image, the float voltage is actually closer to 14.1 volts. I wish I could say there was some solid science behind that. I decided to adopt the conservative voltage, and then 3D-printed a case for the PSU. I then installed the DC/DC converter in the case and filled it with West epoxy. If I'd had proper potting compound available to me, I'd have used that. However, I am absolutely sure that the West epoxy is immune to salt water, which is more than I can say about your average potting compound. Maybe they are, but I don't want to be the first use-case.

Interestingly, the epoxy seems to have changed the value of the DC output by almost half a volt. It has risen to 14.1, which is why that voltage appears on the DVM.

To test it, I added a known load and dunked the whole power unit into a glass of water (see picture). I left it submerged for a week or two, with no noticeable effects. It's safe to say, that DC/DC converter is ready for the hull.

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