Solar Voltage

[thinwater]

The disagreement is still alive. My point remains that the controller can output power into a dead short. OCP at the battery is irrelevant, as the controller is the power source.

Consider the picture below. View attachment 226812

Fusing the panels was not the point, although the discussion was interesting. We are in agreement that they do not need to be fused in most circumstances. A disconnect is a good idea though.

Good illustration.

If the controller is rated for 30 A, the panels are 30 A, and the wire is 30 A, and there is a 30 A fuse near the battery (DC bus in the above case--not all controllers wire to the DC bus) what is going to heat until failure? Obviously, everything from the controller to the bus (battery is rated at 30 A). In fact, by fusing at the controller you expose the wire length from the DC bus to the controller fuse to over current. I think what he was suggesting is that it would be best to place the 30 A fuse close to the DC bus (or battery).

Yes, the controller is a power source, but even with the fuse at the battery, the DC bus remains the much, much more powerful energy source.

What we have, I believe, is a failure to communicate.

 

[mermike]

I agree that the DC bus is the more powerful energy source. My thinking is that if properly fused, the OCP on other power sources connected to the bus will open, leaving only the controller putting power into the short.

My assumption, and perhaps this is the error in my thinking, is that the controller or wire will fail with a constant 30A current into a dead short. I supposed if they are all rated for the current, and the controller output doesn't exceed that current, then all will be well.

 

[thinwater]

I agree that the DC bus is the more powerful energy source. My thinking is that if properly fused, the OCP on other power sources connected to the bus will open, leaving only the controller putting power into the short.

My assumption, and perhaps this is the error in my thinking, is that the controller or wire will fail with a constant 30A current into a dead short. I supposed if they are all rated for the current, and the controller output doesn't exceed that current, then all will be well.

THanks. Just making sure I was not missing anything.

In fact, mine is wired as you suggested, but more because the fuse was easier to install at the controller than because of a reasoned choice. There is a battery fuse. In my case, the main bus is 10 feet away, so I wired to the battery (after the battery fuse). The battery is on starboard and the bus is farther forward on port.

 

[American_Mainsail]

I agree that the DC bus is the more powerful energy source. My thinking is that if properly fused, the OCP on other power sources connected to the bus will open, leaving only the controller putting power into the short.

My assumption, and perhaps this is the error in my thinking, is that the controller or wire will fail with a constant 30A current into a dead short. I supposed if they are all rated for the current, and the controller output doesn't exceed that current, then all will be well.

Hi Mike,

I think there’s a fundamental misunderstanding in your thinking at work here.

The wire won’t fail with a constant 30 amp current. The wire is sized for that current, and the controller’s output is fixed at its maximum output (30 amps in this case). It can be idealized as a fixed current source.

The battery, on the other hand, is more or less a fixed voltage source. It will essentially supply current in proportion to the resistance in the circuit. This is Ohm’s law at work: I = V/R.

In a short, resistance in the circuit drops to essentially nothing, as the loads in the circuit are bypassed. We have (idealized) V = 12V, and R = a vanishingly small number. Current is near infinite (in reality thousands of amps, probably somewhere in the ballpark of 10x to 100x what the wire can physically handle before it overheats and potentially catches fire.

The fundamental problem isn’t “dumping power into the short”, it’s creating a fault circuit that sends uncontrolled, high amperage into wiring that was never meant to handle that kind of amperage.

The other day, I was having a conversation with one of the tech support guys at Battle Born. He mentioned a story about someone dropping a wrench that landed perfectly to span the battery terminals, creating a perfect short circuit located right at the terminals. As it was relayed to him, the wrench melted almost immediately. That’s the kind of amperage we’re talking about here. That’s why we need a fuse that’s engineered to blow safely.

On a side note, this is why a fuse is required at the batteries, as well as any additional circuits that step down the wire size, usually at the DC+ bus where the positive controller wire feeds into the bus. I think we’ve created a little confusion by saying “at the battery”. For most, a fuse is already located at the battery, and the solar controller would be wired into the house bank at the positive bus. Unless you’re using battery-sized cable at this connection, a fuse is needed here as well to protect the smaller gauge wire leading to the controller.

Fusing the controller at the controller is offering 0 over current protection to the wires.