Installing the 24 volt to 12 volt Converter

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The new off grid system which we have in place puts out 24 volts DC, which then goes to the inverter and is converted to 240 volts AC, so it can power all of the 240 volt stuff in the house. However, there are parts of the old 12 volt system still in use, including all the lights, a small pump and a few other bits and pieces. Rather than go and replace everything, I wanted to keep it operating as it was, also one bank of batteries is still working and I want to keep it running as long as I can.

The 24v to 12v Converter Wired Up

The answer to my prayers was a small DC to DC converter (ie 24 volt to 12 volt converter), which would enable me to run the old stuff on the new system. I still wanted to be able to run the old stuff on the old lead acid battery bank if I needed to. Having the lights running on the old system proved to be particularly handy when I was bedding the new system down. Every so often I would get things wrong, over stretch the new system so that the inverter shut down and we lost power. If this happened at night, we would have lost the lights as well but because they were still operating on the old system, I could at least see to work out what I had done wrong with the new system.

If I wanted to keep operating both systems (12 volt DC for lights and 24 volt DC – 240 volts AC for everything else) I needed some way to switch between the two, and the easiest way was to install a battery isolator switch. These are used in 4WDs and RVs which have a battery to run appliances as well as a battery to start the vehicle. The battery isolator switch allows you to draw current from either or both batteries so that if need be, you can isolate the staring battery from the system and avoid flattening it, thereby stranding yourself out in the bush somewhere.

Battery Isolator (front)

Battery Isolator (rear)

The picture in my mind of a battery isolator switch was a rotary switch that would take the positive and negative wires from the 24 volt system (through the converter to 12 volt), the positive and negative from the original 12 volt system and the positive and negative from the house circuit and I could just change between them. When I got hold of the switch it only had room for 3 wires – battery 1, battery 2 and the common wire. It turns out that it only switches the positive wires and I had to do something else with the negative wires of each circuit. It dawned on me that the way to run it was to gather all 3 negative wires and run them to a neutral link so they were connected.

You learn something new every day and when I went to my local electrician supply place I found that they made neutral links with varying hole size and I was able to get one with the size which would take the (quite thick) 12 volt wires easily.

Neutral Link

Neutral Link (wired up)

Neutral Link (cover in place)

So to set all this up, I ran wires from the positive and negative terminals of my 24 volt battery bank to the input side of the 24v to 12v converter. I then ran a negative wire from the output of the converter to the neutral link. I took the negative wire which ran from the 12 volt batteries to the negative supply point for the house circuits, cut it in half and ran both ends into the neutral link, thus completing the negative side of the circuit.

To complete things I took off the back of the battery isolator switch, revealing 3 threaded terminals fitted with a nut and lock washer which accepted spade or ring terminals. I ran a wire from the positive terminal of the converter to the “1” terminal of the switch and then broke the positive wire from the 12 volt batteries to the positive supply point for the house circuits, soldered on a couple of ring terminals and then connected the battery side to the “2” terminal and the house supply side of the wire to the “C” (or “common”) terminal.

Battery Isolator (wired up)

This means that when the switch was turned to “1” the lighting was powered by the 12 volt system and when it was turned to “2” it was powered by the new 24 volt system through the 24 volt to 12 volt converter. (obviously when the switch was turned to “1 + 2” it would draw power from both systems).

Now the acid test! Would everything work when switched to the new system? Thankfully it did, working well when switched to either the old 12v or the new 24v to 12v side. Also, so far, running the lights directly off the system seems to work well and they do not consume that much power over and above what we were already using. The system is now quite flexible, allowing me to run the lights on either the old or new system, as required.

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