We can be reasonably satisfied with our off-grid power installation these days. Certainly, if we were starting from scratch, there are a few things I would do differently, but in general the system is remarkably self-running and trouble free. However, being off-grid can be a two-edged sword, as it also means that you have to take responsibility when things go wrong.
We get most of our power, for most of the year, from the solar panels, and have a relatively small wind turbine for the windy winter months, when there is not enough light at our high northern latitude to provide much, or any, power. At shoulder seasons, we may experience plenty of light along with strong winds, and that means excess power.
This power needs to be diverted away from the batteries, as they would suffer from over-charging, and the electronic equipment, most notably the inverters, will trip out from an excess of voltage. This diversion is done with a charge controller, which sends excess power to a large "dump load", a term that may well make a schoolboy giggle, but really means a large heating element; the excess power is turned to heat. You can usually hear this happening, as the controller switches power at a rapid rate, resulting in a loud hum from the dump load. Also, you can feel the heat coming off the unit, but it was stone cold.
This morning, after 24 hours of a good solid breeze, and with the batteries already full, I noticed that the voltage in gusts was well over 31v, far too high for comfort. Something was wrong. A quick look at the charge controller suggested it was unlikely to be the culprit, so I took a volt meter to the input of the dump load, and could see voltage there. The dump load had failed. I thought it might just be connections, so I shoved a pair of pliers against the connector bolts, and suddenly heard the dump load spring to life. After a little more fiddling, though, I noticed sparking along the top of the dump load. This wasn't easy to see, as there is a protective cover around the resistor element, for obvious reasons, as it can easily soak up a kilowatt or more. While all this was going on, I had disconnected the solar panels, to ensure they did not add to the voltage of the batteries, and I knew the voltage would soon be under control, as it is bread baking day, and the bread baker uses around 600w of power. I had time to carry out a fix.
It turns out that the dump load is really two resistance coils, joined at the top with a metal double connector. I started dismantling the dump load, and pieces of the fireproof backing material starting to come away. The unit is ten years old, and time, as well as heating and cooling cycles, have taken their toll. I got the device apart, though, and could see the erosion to the connector block, whose bolts had completely come undone.
Now the problem was how to join the two resistor elements, knowing that this is the hottest part of the unit. You can't solder them together, and I didn't have a suitable connector block. What I did have, though, was a crimping tool and a length of 8mm copper pipe. I have made plenty of joins to high amperage wire in this way, so am fairly confident in it, but the unknown factor here is the temperature to which the heater element can attain. I don't think it will become hot enough to melt the copper as the elements never get red-hot, but I will have to keep an eye on it. So a little while later, the join, now very tight, was made. The picture below was taken after the loads were put back, held loosely in place with a length of wire
Now could I repair the fireproof backboard? A glance said "not a chance." But I did have, lurking in the shed somewhere, some offcuts of fireproof board from installing our woodburners. It wasn't nearly thick enough, but I layered 3 together to get the required thickness, holding them in place with a bolt, centrally mounted.
Soon the whole unit was back together, and connected to the charge controller. The bread had finished baking and the voltage was again climbing. I switched on the solar panels to get back up to voltage quicker, and a little while later, the familiar, and now, soothing, hum, started coming from the dump load. Here, everything is once more in place, and you can easily see the resistance coils against the white of the fireboard.
Living as rurally as we do, when a journey to Inverness, the nearest place which might have suitable connectors etc, is a 200 mile round trip, we know we need to be resourceful and make do. So this was a reasonably satisfying experience. I have learnt a little more about the dump load, and hopefully sorted it for another ten years's service.