Yet another solar wiring question

[blackswan555]

Ok, I have two separate solar systems on board (two controllers). One forward with a 100w panel and one aft with 2-100w panels. I am using Renogy panels and their MPPT 20A controllers, one for each location.

Now here's the question: They both feed into the house bank. Do the separate controllers see the increased system voltage of the other and say...hmmm looks like the system is up and I don't need to do anything? So does one of the systems shut down now? Or output substantially less since it thinks the system is up?

Basically yes, in an ideal world if they were both perfectly matched they would both be "seeing" the exact same battery voltage and tapering off equally when absorb voltage is reached to hold it for set absorb time, In practice one is always a little in-front/behind the other, It may make one controller work a little harder than the other but does not affect your overall charge.

Tim

ps Do you have battery temperature sensing on all charge sources ? Very important for battery life

[edmundsteele]

Great advice from everyone. Thank You!

Am I correct that the advantage of multiple controllers is equipment redundancy in case of failure and protection against one or more of the solar panels becoming shaded by rigging for a horizontally mounted panel, or orientation for a vertically mounted unit?

The controller manufacturer indicated in their installation instructions that the input and output of the controller should be protected by “switches” and suggested a double pole switch to simultaneously handle both. What do folks generally use – switches, breakers, in-line fuses?
Ed

[GrahamHO]

My solar panels are hard wired to their respective controllers. The controller positive outputs go directly to their batteries. Close to the battery terminal I have an inline spade fuse. That is to protect that cable from the battery current in say the unlikely event of it falling off the controller and shorting onto a negative terminal. My Optima AGM batteries have auxiliary 8mm + and - terminals which I use. I can't see any reason to ever switch the system off. Every fuse, switch, circuit breaker or plug in a circuit creates a little voltage drop which is cumulative. Because a battery is charged by the difference between the battery voltage and the charging voltage, even 1/2 a volt too low charging voltage is significant. That's my way anyway.

[blackswan555]

You should have protection either side of the charge controller rated for the cable size, Think charge controller go's short circuit, You have power both sides, Also not funny if you to have to change charge controller (cover panels or do it in the dark)

Tim

[Maine Sail]

Originally Posted by blackswan555

You should have protection either side of the charge controller rated for the cable size, Think charge controller go's short circuit, You have power both sides, Also not funny if you to have to change charge controller (cover panels or do it in the dark)

Tim

Breaker on PV side is a land based NEC requirement really intended for high voltage arrays. It is not a requirement, at least under current US marine standards, on low voltage systems where the wire can easily handle the full short circuit current of the PV array. In Europe their may be a requirement for a breaker on current limited sources..

If you want to add one that is always fine but OCP is only required, under US marine standards, at the battery end of the circuit. In most cases OCP on the panel side won't ever trip because the PV is a current limited source but the battery bank can dump thousands of amps into a short circuit. If you size the breaker for the max ampacity of the wire the PV will never trip it if it was sized properly for voltage drop. A 10GA 105C wire can be protected by a 60A fuse or breaker at 100% of ABYC Table VI. Even an 80W panel is using 10GA wire on a boat, for minimal voltage drop, but the short circuit rating of the PV is under 6A. This is why companies like Outback recommend a breaker of 80A for an 80A FlexMax because they don't trip at face value. Every single breaker I have come across on a PV side install on a boat has been so over sized to be entirely useless for the installation..

This is also why on alternators OCP is not required at the alt end of the circuit, just the battery end. We need to keep in mind that most controller manuals are written to address land based installations which require a breaker on the PV side and US companies are writing for NEC land standards not the marine standards..

If you need to change the solar panels you can simply disconnect an MC-4 or open the J-Box. Again, you can add a breaker but it adds multiple connection points, and some voltage drop, and it is not a requirement on the PV side of the controller (at least in the US). If you have a very high voltage marine array then a breaker on PV can be a good idea but this is rare due to shading in the marine environment...

[motion30]

In the outback literature they require an 80 amp breaker for my 60 amp flex max

[Maine Sail]

Originally Posted by motion30

In the outback literature they require an 80 amp breaker for my 60 amp flex max

Again this is an NEC land based requirement.

[RHoodJr]

I would like to add to the list of great questions and answers on this thread. I am also planning to install (first time) solar but still in the planning stages. System as planned is:

Two Kyrocera 140W panels
Two Genasun GV-10
Panels will be wired separately to each controller then through a junction box and from there to the batteries.

The wire size to minimize voltage drop from the panels to the controller is a minimum of 8 GA and I would rather use 6 GA to reduce voltage drop even further. The largest wire the Genasun's lugs accept is 10GA so I had planned on using 6 GA to a fuse block then from there, shorter 10 GA wire from the controllers and back to another fuse block or terminal, then back up to a larger wire to the battery. Does the 10GA wire cause much voltage drop, or because the wire would only be aprox. 12 inches in length then any drop would be negligible?

What is the proper way to calculate voltage drop in a circuit of this type when using different gauge wire and terminals?

Thanks in advance for your great advice.

[steve77]

Originally Posted by RHoodJr

What is the proper way to calculate voltage drop in a circuit of this type when using different gauge wire and terminals?

Voltage drop is just calculated using V=IR. You calculate the drop for the #10 length, then for the #6 length, and simply add them together.

For I, use the maximum current that you will get from your panels. For R, use the DC resistance of the wire. From the only reference I have available, Ugly's Manual, the DC resistance for #6, #8, and #10 copper wire is 0.510, 0.809, and 1.29 ohms per 1000 feet. The length of your wire is the two way length, so remember to include every foot of wire. These resistances are at 75 degrees C, so there is conservatism built in here.

Calculate V using the I and R values. This is the drop you will see in this length of wire. Then subtract it from the voltage from your panels, and you will have the voltage at your controller. It is worthwhile to divide these numbers to give you the percent voltage drop. Every percent of loss is exactly that, energy that does not get to your batteries.

But all of this is really not required to answer your question. Don't even worry about the one foot of #10. In fact, I would just run #6 everywhere, and cut strands of the wire to allow it to land on the Genasun controller. Just don't cut too many, and don't allow any to escape and cause a short circuit.

Cheers!

Steve

[Serapium]

So thankful for these forums and all the helpful people and information here! I have what probably amounts to a silly question related to the above conversation.

My plan is currently to put four 275 watt panels through four separate controllers. My question is this: What happens to the excess energy once the battery bank is fully charged? Do the charge controllers 'shut down' the panels? Can I shunt that excess capacity to something like a hot water heater?

[HappyMdRSailor]

Bunch O Smarty Pants guys here...

Very impressive info...

[steve77]

Originally Posted by Serapium

What happens to the excess energy once the battery bank is fully charged? Do the charge controllers 'shut down' the panels? Can I shunt that excess capacity to something like a hot water heater?

As I understand it, most controllers will control the voltage to the batteries, holding the voltage at whatever you have it set for for the bulk, acceptance, and float values. The batteries will then accept whatever current they can at these voltages. Sort of like giving a little kid a drink from a cup. You tip the cup up, and they can drink water that fast. You tip it only a little, and they can only drink a little. I just made up that analogy, but I think it's valid.

A little reading on your manual should help explain this.

So, in the event your very sizable PV array charges your batteries to the point you reach the float voltage setpoint, then the controller will cut the voltage back to your float value, allowing a couple Amps to flow into the batteries to maintain the float voltage value.

Of course, you can shunt excess energy to use for other things, and heating water is certainly one of the most common. I did a little calculation when sizing our panels to check this same thing. For us, it just did not seem worth it. If I diverted 10 Amps to the water heater for one hour, our 8 gallon water heater would raise the water a whopping 6.58 degrees F. Ignoring losses. This is linear, so if you send 20 Amps to the water heater, it will rise 13.16 degrees. You will only be limited by the rating of your water heater element, which may be 500 Watts for a 12 Volt DC element.

The main reason I didn't want to do this was the only dual AC/DC element I was able to find also cut the 120 Volt AC element to 500 Watts. This is half the value of the element I have now, and I did not want to double the time it took for hot water when on shore power or the genset.

If you want to go the water heater route, there are a number of folks out there who can help you. Personally, I like John at Hotwire in Florida, but there are plenty of other sources.

Cheers!

Steve

[Serapium]

Thanks, Steve. Another reason I was wondering, is if you don't shunt that excess capacity to something, where does it go? Do the charge controllers emit it as heat? My understanding is that the charge controller can't 'turn off' the solar array, correct? My concern is that if I have too much excess solar that the compartment where my controllers are housed will get extremely hot. Since that compartment is inside the galley I can see how that could make for pretty uncomfortable hot sunny days if that is the case.

[GrahamHO]

Originally Posted by Serapium

Thanks, Steve. Another reason I was wondering, is if you don't shunt that excess capacity to something, where does it go? Do the charge controllers emit it as heat? My understanding is that the charge controller can't 'turn off' the solar array, correct? My concern is that if I have too much excess solar that the compartment where my controllers are housed will get extremely hot. Since that compartment is inside the galley I can see how that could make for pretty uncomfortable hot sunny days if that is the case.

The charging voltage eventually reaches a state of balance with the battery voltage and no excessive heat is generated. The same as if you connect a full water tank with an empty water tank with a hose. Eventually they reach the same level andbalance with no more water running through the hose.

Your idea of a controller for each panel has some merit but I think 1 controller of the correct size for 2 panels is all you need. That's the way I do it. Because each controller terminal is only large enough for one wire, I connect 2 panels + ve to a junction box mounted close to the controller, then 1 wire from there to the controller. I use the largest size tinned cable that will easily fit the terminals. No need to do calculations. The manufacturer has already done that when they sized the connection terminals.

My Morningstar controllers have an accessory connection. If you use that it will use some of the battery charging energy but once the batteries are fully charged it will be so to speak free energy.

[socaldmax]

Originally Posted by Serapium

Thanks, Steve. Another reason I was wondering, is if you don't shunt that excess capacity to something, where does it go? Do the charge controllers emit it as heat? My understanding is that the charge controller can't 'turn off' the solar array, correct? My concern is that if I have too much excess solar that the compartment where my controllers are housed will get extremely hot. Since that compartment is inside the galley I can see how that could make for pretty uncomfortable hot sunny days if that is the case.

Your solar controllers will get hottest when they are producing max power. If you have a 45 amp controller and it's producing 45 amps (many of them can produce 10% over, if you have enough solar panels connected and the sun is right) than it's going to be pretty warm. If it's at the end of a battery charge and it's only putting out 10 amps (even though it's midday), it's going to be cool, and it basically just ignores the excess cap., it is wasted energy. I see your point, since energy can neither be created or lost, it has to go somewhere. Unused solar energy does not create excess heat in the controllers, my guess is it's just reflected off of the panels as heat.

A better analogy of a solar controller/lead acid battery bank relationship would be a bit like a flexible air compressor tank. The solar controller is trying to fill up the batteries much like an air compressor tries to fill up the air tank. As the voltage (pressure) in the batteries increase, the resistance of the cells (increased effort to pump air) goes up, slowing the amperage flowing the batteries, just like the increased pressure in the tank slows down the rate at which the pressure increases.

The difference in the analogy is that once the tank hits 120psi, the cutoff shuts off the compressor and it's full. But a battery isn't fully charged the instant absorption voltage hits 14.XXv. It needs to be held at absorption voltage for a few hours, while more amps are packed in and voltage remains at absorption.

I would also recommend that you go to your battery mfr's website and find out exactly what they recommend as absorption voltage for your model battery. Some of them need 14.5, or 14.8, or even higher than 15 volts to attain full charge. If, for example, your batteries need 15.2v and you're only charging them to 14.4v (a common low, safe voltage) then your batteries are only charged to about 80%, leaving 20% unused cap. Since you shouldn't discharge below 50%, you're only getting 30% of total cap. which is actually a 40% loss of usable cap. I think everyone would like to get that extra 40% (it's free, after all) but virtually every charging system and solar system I've seen left something on the table.

As for diverting power - yes, if your batteries are fully charged, by all means, use the rest of the power to heat water, make water, whatever you like. What I usually do, since I know I'm making excess power every day, is turn on the excess loads during the day (ice maker) and at the end of the day, my batteries are full and I've got an extra 25 lbs of ice. The reason I prefer this method is because the solar system can produce XX amps, let's say 45 amps.

If you leave the loads off and only charge the batteries, the charging amps gradually taper down as battery resistance increases so you're only getting say 10 amps in the last few hours of the charge, even though you could have gotten 45 amps under full sun. With the extra loads on, the solar can produce the full 45 amps for most of the day, with the final charge taking 10 amps and the remaining 35 amps going to the other loads. This way you're harvesting max. solar system output while the sun is providing max. output, instead of restricting output to 10 amps at noon to 3 pm, then trying to heat the water as the sun drops lower and produces less power. Better to take all of the power during midday, and let the battery charge wind down later in the afternoon when it can still produce 10 amps but not much more.

I hope this all makes sense, I haven't had my coffee yet.