scrimma avatar image
scrimma asked

100/50 SmartSolar MPPT Misbehaving?!

Hi all,

I've recently installed a solar setup on my sailboat. 2x327w Sunpower panels in parallel, BMV712 and the 100/50 SmartSolar MPPT.

I've attached screenshots of my app showing the output over the past couple of weeks. In the first photo, you can see that things are consistent from 5th to the 15th of March. My wife and I were living aboard on our mooring and did not start the engine but had full 100% batteries by midday (520Ah house bank).

After the 15th things seem to take a strange turn. I have a circuit breaker between the solar and the MPPT that's 20Amps as the panels have a rated output of just over 12amps. After the controller i have the largest gauge wire the MPPT will accept running a short distance of 400mm to the battery connection to minimize voltage drop etc. On the positive wire between the MPPT and the battery I have a 60Amp circuit breaker. I used to have a 50Amp, but found that was too small and was advised by a marine electrician to up it to 60Amp circuit I did. From the 5th to the 15th, no issues. After that, the MPPT seems to get quite hot and the positive wire running from the MPPT to the battery also gets very warm and eventually the circuit breaker pops. This was never an issue in that first week. I live in the tropics in Australia, and here it's summer, so things are warmer for sure, but every day is basically the same of around 30c + humidity.

Does anyone have an idea what is going on? you can see from the other detailed screenshots that it just trips when in bulk mode and there is no absorption or float. The batteries are definitely not getting their full charge. It seems to happen the most in mid morning when the sun is getting really strong and the ambient temperature isn't too hot outside yet.

Is the heat possibly from the circuit breaker running at close to it's maximum? Again, it's a 60A on the battery side of a 50A MPPT. The wire seems to be hottest closest to that breaker so that to me makes the most sense, and then I assume when it gets too hot it trips itself? Should I upgrade to a 80A circuit breaker? Also, what happens to the 100/50 SmartSolar when it gets really warm/some would call hot? Will it just derate itself, or is this normal?

The Controller is located in an aft cabin with small hatches above it for ventilation and it's always open to the saloon and rest of the boat which is constantly ventilated and shaded with boom tents etc.

Any help would be really appreciated. Nothing worse than running a noisy diesel engine in a tranquil secluded island bay!

Thank you in advance!

MPPT Controllers
mppt-output.jpeg (348.6 KiB)
mppt-detailed-1.jpeg (481.4 KiB)
mppt-detailed-2.jpeg (441.2 KiB)
mppt-detailed-3.jpeg (436.6 KiB)
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4 Answers
Mark avatar image
Mark answered ·

The 'cheap' DC circuit breakers that you find online are useless & can be a fire risk. Unless you have a good quality DC breaker you are better off using a simple suitable fuse & fuse holder (like a mega or midi fuse).

Once a cheap DC breaker trips, the contact resistance can increase & things only get worse from there. The breaker trips based on heat effecting the internal bi-metallic strip.

There is HIGH RESISTANCE somewhere close by that is causing the wiring & circuit breaker to excessively heat up.

I would recommend to size the breaker/fuse at ~1.5x the expected load if the load can be continuous. As a minimum I would recommend for it to be at least ~1.25x the max load. So in my opinion even 60A is cutting it a bit fine, but it also depends on the 'trip curve' characteristics for the particular breaker or fuse being used.

Also, please check/ensure that;

A) All your connections are good - check both crimped lugs & clamped terminal connections as this could also be a contributor. Ensure that the insulation is neatly stripped & that the correct length of wire is exposed. Ensure that the wire strands are straight & that none of the insulation is under the clamped/crimped area. If crimping make sure that the correct size crimp lugs & crimping die is being used.

B) The wire you are using is not only suitable in terms of total cross section area, but also that the individual strand diameter is fine enough to provide sufficient contact surface area at the connection/terminal (the individual strand diameter should be less than 0.4mm in diameter, but a strand diameter of 0.25mm or 0.32mm is preferred/better).

Regarding the MPPT - yes the unit will automatically limit the current output if it senses that it is overheating (via its internal temperature sensor).

1554170449243.png (113.7 KiB)
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JohnC avatar image
JohnC answered ·

G'day Scrimma. Good advice from Mark, and your symptoms suggest you have something like this:

They get hot internally and will dissipate that heat down the wire. I've used them, and 25% oversizing isn't enough - I ended up at 150A for an 80A mppt, didn't trip but still got hot.

I don't use them anymore, but I repurposed one as an isolator on a low amp circuit. A good isolator and fuse is a better way to go.

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scrimma avatar image scrimma commented ·

Hey John. I have something similar, but it's inline. The ones used for putting in secondary battery systems in trucks and 4x4's etc. Looks like the one in the photo below. Thanks for your input. I will plan to upgrade to a higher quality item that doesn't cause so much heat. Good to know it's been done before and that the solution isn't too dramatic (unless it starts a fire!)


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circuit-breaker.jpeg (365.3 KiB)
scrimma avatar image
scrimma answered ·

Hi Mark,

Thank you for the detailed reply. Much appreciated.

From what you've listed, everything looks fine apart from the wire from my MPPT that runs through that circuit breaker. It's pretty fine strands, but it doesn't look quite as fine as the one in the picture with the green check mark. It's somewhere between that and the middle image. It's crimped as well as grub screw clamped into the breaker. All other connections good and verified.

I have since been advised to go 1.25x the 50A from the MPPT = 62.5A, then 1.25x the 62.5A for max current = 78A breaker. Since they don't make 78A circuit breakers, an 80A will do the job. Does that make any sense to you? I really do agree that my 60A was on the lowest possible end. I disconnected one of my solar panels and was seeing 25A coming from the MPPT to the batteries and that wasn't a problem, so i'll leave it with the 1 panel for now till I get hold of an 80A circuit breaker.

One question you didn't touch on though, was when the MPPT gets quite warm, what happens? Is it supposed to and does it just lower it's output as self protection? Breakers and cable are easy to replace, but the MPPT is $$$! Oh and for the record the wire going from my MPPT to the batteries is 6AWG/13mm2, the largest it will take.


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Mark avatar image Mark ♦♦ commented ·

Hi Scrimma,

Yes I agree with your proposed circuit breaker/fuse size & calculation. Again it's a bit of a grey area as it also depends on the trip curve & type used. The main thing is that your wiring can handle the trip current (in an error state condition) & that its NOT on the verge of tripping during the high end of 'normal' operation.

However, I do NOT recommend to use the brand/type of circuit breaker shown in the photo you attached, my experience with all the cheap/no-name units is not very positive... If you can't afford a decent brand/type of DC circuit breaker then just use a fuse, it will be safer & better in terms of resistance/voltage-drop & heat generated.

There are some nice industrial DC circuit breakers that are probably out of scope for this install, but for a basic setup the Blue Sea DC/Bussmann range should be suitable & are reasonable quality.

If your in Australia check out this retailer/distributor that I have sourced various directly bits from;

Also, I actually see that I responded to you on this same/similar topic in early Feb - see link attached;

Based on my feedback to your previous post, I think that your 6AWG wire size is fine for your application & the short cable length (400mm) helps with this.

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scrimma avatar image scrimma Mark ♦♦ commented ·

You sir have a great memory! Yes, i thought the tale of this solar install was over, but alas, it seems one gremlin left to kill. Thank you again for the advice. I didn't realize those fuses were so nasty. I mean, the price is cheap and i guess that's the first indicator it may not be the highest standard! I'll go about sourcing a more appropriate make from the link you provided. Muchas gracias amigo!

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Mark avatar image Mark ♦♦ scrimma commented ·

No worries - I hope it all works out well after you install a new circuit breaker or fuse!

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scrimma avatar image
scrimma answered ·

Oh...sorry Mark, i see your little note at the bottom now that mentions what happens when the MPPT gets warm. Thanks!

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