Help required - too many amps coming from solar panels

Hi,

A few months ago I built a small solar setup for my cabin. First few months on a single battery, worked flawlessly.

Now that I connect the second battery in parallel, the panels push a lot more amps into the batteries. Too much in fact, since it triggers my 40A breaker.

The information displayed on the Renogy Rover (40A) were: panel->batt 31V, panel->batt 39A. Shortly after the fuse tripped.

It sort of looks like it’s trying to charge 24v batteries instead.. But that’s probably an incorrect intuition since it’s my first build. The MPPT device is definitely set to 12v.

Anyone with a clue?
Thanks
Zeno

The panels are 2x 300W (24v), parallel
The batteries are 2x 230ah AGM deep cycle (12v), parallel

Images of the setup and panel specs:

that does not compute,.

If it is pushing 31V to batteries, you NEED to disconnect until fixed!
600W is 50 A at 12V, but will rarely produce full watts.
The SCC is not functioning correctly.

Supervstech said:

If it is pushing 31V to batteries, you NEED to disconnect until fixed!
600W is 50 A at 12V, but will rarely produce full watts.
The SCC is not functioning correctly.

Click to expand...

Thanks for the reply. I have to correct myself. This is the output from the solar panels. I don’t imagine this is going straight into the batteries.

Supervstech said:

If it is pushing 31V to batteries, you NEED to disconnect until fixed!
600W is 50 A at 12V, but will rarely produce full watts.
The SCC is not functioning correctly.

Click to expand...

Supervstech said:

If it is pushing 31V to batteries, you NEED to disconnect until fixed!
600W is 50 A at 12V, but will rarely produce full watts.
The SCC is not functioning correctly.

Click to expand...

Ah, I think I assumed that the amperage was going to be twice as low because the panels were described as 24v. Is this a mistake I made?

I see no problems with the SCC.

1) 31V is normal for a 24V watts solar panel, it does not mean that the batteries will charge to that voltage, it is just the voltage of the PV array.

2) Do you have a 40A breaker from solar panels to the 40A SSC? You should use a 50A breaker instead, also some cheap breaker cant hold their max capacity, so 39A to trigger the breaker seems fair, just get a 50A breaker

zenopelgrims said:

Ah, I think I assumed that the amperage was going to be twice as low because the panels were described as 24v. Is this a mistake I made?

Click to expand...

I have a 12V system with 2 12V solar panel in series, they produce up to 40V, PV array voltage should not match the battery voltage but the SSC Voltage range.

JoseAgustinV said:

, just get a 50A breaker

Click to expand...

And be sure your wire size is correct for that size breaker if you do change the breaker.

Also, you should check the max pv array Watts for your SSC, i think it will handle 550w max, can you provide the specs of your SSC?

JoseAgustinV said:

Also, you should check the max pv array Watts for your SSC, i think it will handle 550w max, can you provide the specs of your SSC?

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I think you’re correct. It’s 520W. The Renogy Rover 40A. That means i’ve got too much solar power for the SCC.

I wonder why this only happens with the second battery connected. Maybe it limits the amount of amps going into the battery?

Best plan of action, i assume, is buying a heavier SCC & higher amp breaker? The wiring is already oversized.

zenopelgrims said:

Best plan of action, i assume, is buying a heavier SCC & higher amp breaker? The wiring is already oversized.

Click to expand...

Long term if you find you still have too much power from your panels upgrade to a smaller Lithium pack and you can charge at 1C and use that power elsewhere. I am an unrepentant Lithium fanboy.

zenopelgrims said:

I think you’re correct. It’s 520W. The Renogy Rover 40A. That means i’ve got too much solar power for the SCC.

I wonder why this only happens with the second battery connected. Maybe it limits the amount of amps going into the battery?

Best plan of action, i assume, is buying a heavier SCC & higher amp breaker? The wiring is already oversized.

Click to expand...

You definitely need a larger breaker, i think 50A will be good.

If you connect 2 batteries in parallel you will get a 12V battery with double AH, if you connect them in series you will get a 24V battery with same AH

The Mppt will convert the watts to whatever your battery needs, so 600W / 12V (if you have a 12v battery) will be up to 50A, that will trigger the breaker. Im not sure what happens if you exceed the SCC max watts capacity, but i dont think that is the problem, because you wont get max solar panels watts, you usually get about 80% out of rated capacity.

You have to check your batteries max charge rate also, for agm its usually safe to charge at 0.2C rate, that means a 100Ah battery should be charged at 20A max, 200ah at 40A etc, so i think you are fine by getting the max 40A from your SSC.

This could change based on your battery specs, but every AGM battery i saw has almost the same specs.

First thing is that the battery connections are incorrect. Looking at your pic, lets call the top battery #1, and the bottom battery #2. The two paralleling connections that go from the +of bat #1 to +of bat #2 is correct, and so is the negative parallel connections. But, the negative wire running from battery #1 to the inverter/charge controller is incorrect. The negative inverter/controller wire should be connected to the negative terminal of battery #2. In the unbalanced connection you have shown, battery #2 will ALWAYS receive less charge than battery #1, and will die an early death through sulfination.

Getting back to the charge controller, I think what is happening is the current being supplied to the batteries is running right around 40+ amps, which is maxing out the controller's capacity. Back when you had only one battery, the load needed to fully charge the battery would have been lower. Once you doubled the battery capacity, you doubled the load, and maxed out the controller's amperage. Another way to look at it is that only one battery needed less than 40 amps to completely charge, but two batteries in parallel need more than 40amps. On paper, your 600W/12V=50 amps. As Supervstech mentions though, you are not likely to actually see 50A. I just finished some real-world testing of a newly installed array, and they produced 94% of rated wattage. Assuming your panels only measure up to 90%, that's still 45A, exceeding the controller.

There are a couple of ways to correct this problem. The first is virtual tracking. Point one panel towards the SE, and point the second towards SW. That will tend to flatten, but broaden the amperage curve over the course of the day so you don't exceed 40 amps. The second alternative is either to get a larger controller that can handle 60+amps, or purchase a second Renogy controller. The second Renogy might be a more cost-effective choice. Connect one panel each to each controller, then connect both controllers to the same paralleled battery bank.

The other thing to consider is that whilst you had 600W connected to only one battery, you were likely charging it at around 40A, which is higher than typical lead-acid batteries are specified for e.g. C/10 Amps or 28A for a 280AH battery (check you battery's datasheet) - you may well have damaged it thereby prematurely ageing it and reducing its effective capacity.

Connecting dissimilar capacity batteries together is not a good idea. I would play it safe and do a C20 discharge test* on your original battery and make sure it is at least similar to the new one.

*google it, it's a very common capacity test for lead-acid batteries.

Other considerations:

1. Consider adding the ability to isolate all power sources e.g. PV array and battery.
2. Ground the inverter enclosure and provide that ground through to the load plug.
3. Add a GFCI device and MCB prior to the load plug - safety first.
4. Read this article to better understand @MichaelK's comments around battery connections.
5. Cover all positive terminals, fuses and busbars - accidents happen.
6. Vent the batteries or battery box to outside (note: Hydrogen is lighter than air)
7. Seal all holes from the battery box, except the vent to outside!
8. Not sure what size cable you are using but a 3,000W inverter will draw up to 250A from a 12V battery so should be at least 4/0AWG