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grampakoot · 07-20-2017, 08:38 AM

My charge controller for my fixed flat rooftop panels regulates things pretty nicely for short week long trips unplugged from ac power. I've never been more than a week away so nervous about our plans to spend 2 months in Arizona. Dry camping.
My thinking is to purchase a portable kit that I can point more to the sun and add watts to my system.
Question: If the portable kit has its own charge controller can I alligator clip it directly to the battery bank without going through the existing roof system?

vsheetz · 07-20-2017, 08:53 AM

Likely yes, the two controllers can coexist, just as your current converter/charger does. However, I would add the portable panels to the existing controller if it has the capacity. I have several 100w panels on the roof, plus two 100w panels that I set out portable. They can be a big help, especially during low winter sun months.

nothermark · 07-20-2017, 08:56 AM

Why not?

The solar charge controller has to be able to handle the generator feeding the system. Then generator/converter has to be able to handle the solar on the system.

The second solar would feed into that. If anything goes it will be the portable system.

I would question why you would need the portable unless you are dead set against running your generator. We like to use the microwave and A/C at times so some generator run time is always on the agenda. That will do a lot of bulk charging in an hour or two of run time.

grampakoot · 07-20-2017, 10:18 AM

I have a 1998 Tiffin Allegro Bus.
To connect to the existing controller I would have to hardware that in would I not? If so that's big job for this old boy��

grampakoot · 07-20-2017, 10:23 AM

My generator runs on propane. I plan on staying at Imperial Dam LTVA Jan-Mar.
Trying to minimize leaving camp.

vsheetz · 07-20-2017, 10:54 AM

Quote:

Originally Posted by grampakoot

My generator runs on propane. I plan on staying at Imperial Dam LTVA Jan-Mar.
Trying to minimize leaving camp.

Our first coach had a propane generator. Agree with your reasoning to minimize genny operation.

mrgrayaz · 08-03-2017, 08:07 PM

Quote:

Originally Posted by grampakoot

I have a 1998 Tiffin Allegro Bus.
To connect to the existing controller I would have to hardware that in would I not? If so that's big job for this old boy��

It could be as simple as wiring into the existing controller input. You need to make sure you keep your DC Volts the same - so you'll need to know some info about your existing rooftop setup and then match.

EG if your rooftop runs at 12v - then you can wire some more 12v panels in no issue - just throw a couple extra wires with an Anderson connector that you can connect quickly to those portables.

If your rooftop consists of multiple panels in series - you would need to match that with your portable setup as well. 2 panels in series would be "24v" meaning you need to have two panels on your portable rig setp in series to match.

The reason I caution here is every rig is wired differently - For example - I have 4 panels in series on my roof pushing between 48-72v depending on the sun - but many don't wire that way. It would be difficult to have 4 portable panels wired in series to get that voltage - and then in parallel for the same controller in my case....Just too bulky and too many panels on the ground. I would do a separate controller with smaller panels in that configuration.

Mudsock · 08-04-2017, 10:33 AM

You asked about "alligator clip it directly across the battery."

With this scenario, the battery will act as a voltage regulator so that both controllers will be operating at the same output voltage. Neither will be feeding current to the other. They will be current sharing the load. There is no potential for damage to any controller regardless of its output device configuration, as long as the battery remains in the circuit.

If the circuit were altered instantaneously, such that the connection to the battery were removed, then a voltage transient would occur. Sudden disconnection of the battery is known as a Load Dump. Automotive electronics are designed to survive voltage transients resulting from load dump events. The worst case load dump transient would result with an intermittent battery connection when a battery that is drawing a high charge current from an alternator with high engine RPM. For example, a vehicle driving up a washboard road in a low gear. A loose battery terminal is all it takes. GM design specifications mention a 42V load dump with a 115 millisecond time constant, if I remember correctly. The load dump transient specification with Allison transmissions was 123V. I did the transient testing for recent Allison controllers and watched dozens of them fail when the transient was applied. Redesign and relaxation of the requirements was necessary to meet the cost and delivery targets. My statements prior to the test that 123V would result in failure were not welcome. In the end, the result was good enough.

The situation in a motor home is different in the house than it is under the hood. The maximum transient voltage (assuming no inductive loads) is limited to the open circuit voltage of the solar panels. With three in series, it could exceed 42V. The duration of the transient would be limited by the controllers, but damage of one controller by the other is conceivable in just a few thousands of a second. The other concern in the house is that the electronics in the coach are not well designed to withstand a high voltage transient.

Even without having two solar panel systems connected in parallel, transient damage under a high current charging period are possible with only one intermittent battery connection. Consider opening the battery bay door and pulling the tray out while the battery is absorbing a high current. A loose connection could be moved, resulting in a transient. This could happen with a generator, as well. Sound circuit design requires analysis of all likely failure modes. Maintaining tight battery terminal connections is important.

That leads to consideration of alligator clips. Never use them when any reliable current flow is required. We often have to reposition jumper cable clamps to get a solid connection. Alligator clips do not have that much contact force or area. You should look at Anderson Power Pole connectors for applications like this. They can be crimped or soldered and are available in high current versions. The lowest current version is about 15 A. They come in red and black and are impossible to connect improperly - once they are installed properly. The Power Pole connection is a solid connection that takes little physical force to connect and disconnect. No electrical connection likes water exposure. A short harness connected directly to the battery with Power Poles available without extending the battery tray would be good. Neither terminal is exposed when disconnected, so the potential for the harness creating a short when hanging disconnected is minimal.

Shadowcatche · 08-04-2017, 06:50 PM

I have a separate 135W Unisolar panel which rolls up nicely it feeds a Steca PWM controller which can handle the high voltage it plays well with the Morningstar MPPT controller which is hooked to our fixed 185W high voltage panel.
Everything is hooked up using Anderson Power Poles

Reed Cundiff · 08-06-2017, 09:32 PM

Two controllers are probably required if you utilize panels of different voltage.

The system on our 5th wheel has one controller since the six x 235 W panels are: two sets of 3 in series and the two sets are parallel for a 90 V transmission to controller. This works well and the battery suite is 48 V nominal LFP of approximately 9.4 kW-hrs

The system on our Roadtrek uses two controllers due to the landscape of the roof: our son could judiciously use a 315 W and a 100 W panel. This necessitated the two controllers. The battery suite is 12 V nominal LFP of approximately 4.7 kW-hrs.

Reed and Elaine

Mudsock · 06-30-2018, 12:53 PM

I’m not so certain that two controllers are necessary with panels of different voltages. It seems to me that panels act more like current sources than voltage sources. If they are diode isolated such that one panel does not feed current into another and the total current does not exceed what the controller can handle, I don’t see a mode for damage.

I am not saying that a large voltage difference would be a good thing, but an old panel that has degraded to 17V open circuit in parallel with an un-degraded panel still producing 22.7V open circuit should work just fine. Each would source whatever current it is capable of at the operating voltage.

The voltage at the operating point is significantly lower than open circuit voltage and the diodes protect the panels from having current forced through them.

The voltage at the output of a given panel can vary greatly without a large change in current. For example, on a 100W panel on Amazon is rated at 20V open circuit (0A), 6.45A short circuit (0V) and 16V, 6.25 at the optimum operating point. With a voltage change of 16 volts the current change is .2A, or about 3%. These are acting like a current source with current fixed over the range of operating voltages.

757driver · 07-01-2018, 08:50 AM

Quote:

Originally Posted by Mudsock

I’m not so certain that two controllers are necessary with panels of different voltages. It seems to me that panels act more like current sources than voltage sources. If they are diode isolated such that one panel does not feed current into another and the total current does not exceed what the controller can handle, I don’t see a mode for damage.

I am not saying that a large voltage difference would be a good thing, but an old panel that has degraded to 17V open circuit in parallel with an un-degraded panel still producing 22.7V open circuit should work just fine. Each would source whatever current it is capable of at the operating voltage.

The voltage at the operating point is significantly lower than open circuit voltage and the diodes protect the panels from having current forced through them.

The voltage at the output of a given panel can vary greatly without a large change in current. For example, on a 100W panel on Amazon is rated at 20V open circuit (0A), 6.45A short circuit (0V) and 16V, 6.25 at the optimum operating point. With a voltage change of 16 volts the current change is .2A, or about 3%. These are acting like a current source with current fixed over the range of operating voltages.

Read the section on wiring considerations and it should answer the question on why you want to match panels in a series or parallel string.

Last updated

Mudsock · 07-02-2018, 08:36 PM

My example was for a 6V difference. My statement "I am not saying that a large voltage difference would be a good thing" would preclude putting a single 12V panel in parallel with a series string of 12V panels that feeds an MPPT controller. In that case the voltage difference would be 100% or greater.

But, what if we did? If a single 12V panel were connected in parallel with a string of three 12V panels feeding an MPPT controller nothing would happen. There would be no current contribution because the diode in the added panel would be reverse biased. The existing series string would continue to feed the controller, unimpaired. Obviously, there would be no point to do this.

Suppose we had parallel 12V panels feeding a PWM controller and we had an opportunity to pick up a 300W 24V panel very cheap. Could we use it in parallel with the 12V panels? Yes. It would supply an operating current greater than its current rating at its optimal operating point, but less than its short circuit current rating. (There really isn't much difference between these two.) The operating voltage of the 24V panel would be equal to the battery voltage plus the voltage drop across the controller and the wiring. The 12V panels and the 24V panel would safely operate at the same voltage. That is true because they function as current sources, rather than voltage sources and the fact that they are in parallel.

If we also acquire an MPPT controller, we could attach the 24V panel to it and get a greater current contribution by tying the outputs of the controllers together. But maybe we would like to be able to use the new panel before we spend big bucks on an MPPT controller. We have that option.

As mentioned in the previous post, I am looking at failure modes and making the point that solar panels function as current sources, rather than voltage sources. With reasonable consideration, I see no harm of putting solar panels of slightly different voltages in parallel. I am not going to toss my 28 year old panel that still has an open circuit voltage of 17V, but rather will use it in parallel with the new panels that produced more than 22V.