A dangerous, potentially catastrophic, situation developed with my solar system and I was almost unaware of it. Thankfully I caught it in time and learned a couple lessons along the way. And that is what I will share with you today.

I run a 150/70 Victron MPPT SmartCharger…a charge controller that converts the solar panel power into usable power that charges my batteries and provides power to the inverter during daylight hours. The 150 represents the incoming voltage from the solar panels, the 70 represents the amperage (current) going into the inverter and batteries.

Normally we push about 82vDC from the panels into the charge controller. And the charge controller runs about 45 – 50amps into the batteries/inverter under normal conditions. It can run a high as 70amps in early fall or on some summer days. In the winter it can go as low as 35 – 40amps due to the solar elevation. Obviously it can go down noticeably as cloud cover increases.

When I installed the charge controller I used a standard wire size calculator for 70a, less than 10’ (round trip) circuit. That showed that 6AWG wire was plenty good enough, well within the requirements. And I wanted the circuit fused to protect the charge controller and the rest of the system, so I went with an 80a ANL fuse. And since I didn’t want the wire to act as a fuse I upgraded the wire to 4AWG, which is capable of 80a when used in a less than 15’ (round trip) circuit. Yup, all set!

About 2 months ago I was looking over the system on a particularly sunny early fall afternoon with the sun at the perfect solar elevation for my panels. I was hitting the 70a max for the charge controller. But not to worry, the charge controller will throttle the output amperage at 70a and not let it go above that limit. It does that by limiting the input power from the panels. Well, something prompted me to touch the 4AWG wire between the charge controller and the ANL fuse. Ah…hot!

For some reason, and I mean unknown, I had used 2AWG between the main busbar and the ANL fuse, but used 4AWG between the fuse and the charge controller. Please don’t ask me why, I don’t remember. I probably ran out of 2AWG and didn’t want to go buy a 1’ piece of 2AWG…but I wasn’t worried about it because 4AWG was a full size above what was required anyways.

But the 2AWG wire on the battery side of the fuse was not hot at all. Hummm….. I wrote it off as being the peak 70a current due to the sun intensity, panel angle, and solar elevation. A month later I was looking over the system again and noticed a slight discoloration to the 4AWG wire, I touched it, it was real warm, not hot, but real warm. The 2AWG below the fuse was room temperature. Problem! But I couldn’t figure it out, not to worry, the fuse would protect the system no matter what…and it was high quality 105c degree rated insulation on the wire so I wasn’t particularly.

A couple of weeks later I was once again looking over the system and noticed my red electrical tape on the aforementioned 4AWG wire, used as makeshift heat shrink, had flagged on me and an end was sticking out. 40A current showing on the charge controller and the wire was real warm. OK, I am not the brightest bulb on the tree but I realized I had to cure this problem. I wasn’t sure what the problem was…but it had to be fixed whatever it was. And since I didn’t know for sure what the problem actually was, it was my intention to replace the 4AWG wire with 2AWG, new terminal lug, proper heat shrink, and replace the 80a ANL fuse, maybe even the fuse holder if needed.

Now…left turn for a minute. While researching my upcoming system upgrade I learned that using wire ferrules on the battery wire going into the charge controller is considered “best practice” for a number of reasons, all of which made perfect sense. So I was sure that my wire overheating came from that lack of wire ferrule when I first put the system together 2.5 years ago. I now have the tool and wire ferrules…this “fix” would be a great trial run for me.

Well, the day came for the “great fix”…full batteries, informed the wife of the need for low power consumption, and gathered all of my tools and parts. Shut down the panel power input, disconnected the batteries from the charge controller, tested all the wires to make sure the charge controller was isolated, and double checked everything once again.

I went to disconnect the charge controller to battery ring terminal from the fuse and immediately knew what the problem was.

About a year ago I was reading on how to maintain a solar system and one of the points made was to check all electrical connections, then tighten or replace as needed. Ah, well, the nut holding down the ring terminal on the charge controller side of the fuse was actually loose. Yup, loose…and that was the problem.

You see if a terminal connection is loose it becomes a high resistance connection and that generates heat. The hotter the terminal gets, the weaker it becomes, and this continues until it shows up as a critical failure. But the question remains…why did it become loose? I know for a fact that when I did the initial install I tightened it correctly…period. But, as I thought about it…all the times the ring terminal connection heated up, then cooled down, then heated up, then cooled down…over and over again nearly 2,600 times. The nut simply loosened itself due to the continuous heating and cooling. And I never once checked my connections to see if everything was still tight.

Well, I replaced everything as needed, making sure everything was properly tightened. Everything is fine now, the system is running as designed, wire is room temperature, and I am sleeping a little better. Then I took the old fuse apart this morning. Oooooppppppsssssss…

See the picture for more information, but, the fuse was burnt as it should be but not 100% separated at the element. Thus, current/power was still flowing to the batteries/inverter but was several impeded.

Needless to say…I will be setting up a schedule to perform maintenance just as advised.

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