Seeking Feedback on Solar Idea

[JohnMD]

Hi all,

I am looking for feedback from those with more experience and expertise. I currently have a 100W solar suitcase that has worked fairly well at campsites keeping our 100ah AGM battery charged. However, I’ve had to run the generator from time to time to help top it off. In addition to reducing the need for the generator, I would like to run the Norcold 300.3 refrigerator on battery while traveling. After reading through some posts, rather than exploring the DC-DC charger option, this is what I am thinking and would like your feedback on:

1. Mount two 175 Watt Renogy rigid panels on back shell using 3M VHB tape.

2. Connect in series?

3. Run wires down lift arm and to the tongue where the 100 ah AGM battery is located.

4. Add a second battery box to house a controller: Victron Energy SmartSolar MPPT 100V 30 amp (or 50 amp to allow future expansion?)




Any suggestions or feedback you have are greatly appreciated.

[JohnMD]

Hi all,

Just bumping this up to see if anybody has any thoughts on this. I am getting ready to order the equipment and supplies. I am leaning toward the 50 amp controller in case I want to expand. For a solar disconnect switch, should one match the the 50 amp controller with a 50 amp disconnect/breaker switch? I see a few on Amazon. Does anyone have a recommendation on brand?

Thanks!

[ShrimpBurrito]

Originally Posted by JohnMD

Hi all,

I am looking for feedback from those with more experience and expertise. I currently have a 100W solar suitcase that has worked fairly well at campsites keeping our 100ah AGM battery charged. However, I’ve had to run the generator from time to time to help top it off. In addition to reducing the need for the generator, I would like to run the Norcold 300.3 refrigerator on battery while traveling. After reading through some posts, rather than exploring the DC-DC charger option, this is what I am thinking and would like your feedback on:

1. Mount two 175 Watt Renogy rigid panels on back shell using 3M VHB tape.

2. Connect in series?

3. Run wires down lift arm and to the tongue where the 100 ah AGM battery is located.

4. Add a second battery box to house a controller: Victron Energy SmartSolar MPPT 100V 30 amp (or 50 amp to allow future expansion?)




Any suggestions or feedback you have are greatly appreciated.

Hi John,

As to your questions:

1) Sounds good, assuming they fit. I have three 100W panels on the rear shell with 3M VHB tape.

2) Perhaps. That's what I did, but you have to check the panel voltage. Higher voltage means lower currents, which lets you get away with smaller wire from the panels to the solar controller, AND allows you to charge more on cloudy days and during the dawn/dusk periods because a lower voltage from each panel is required to get the controller to start charging. The downside is that there is less shade resistance - if something shades one panel, it is my understanding that it will knock out more power from the other panels, though panels with more bypass diodes can help overcome that.

3) I really dislike the tongue for a battery location. The problem is that you're immediately putting yourself at a huge disadvantage in having the batteries so far from your distribution panel (with comparatively small wire), which means also means a much longer run for your controller. But, since you only have a 100Ah battery (and not lithium at that, which can handle much higher charge currents), you won't be pushing that much current, and so it's probably not a big deal. If you think you might add another battery, and/or switch to lithium, I think I would consider moving the batteries to the rear compartment. It wouldn't be a big project.

4) What is the maximum charge rate for your battery? I'm guessing it's on the order of 10 amps, and so even a 30A controller would be overkill. But if you think you might expand the batteries, sure, go for a larger controller. I have 200Ah of capacity, and have a 30A controller. The batteries can take far more than 30A, but with a 300W solar array, I don't get above 22-23 amps or so. And since I don't anticipate adding panels or batteries, it's the perfect size controller for my setup.

Dave

[JohnMD]

Dave,

Thanks for the feedback. (Please pardon any mistakes or confusion I may have as my intro to electrical engineering class was 35 years ago.) This gives me more to consider (regarding max battery charge current). My AGM 100AH specs show 0.30C, which I understand to be 30 amps, right? It also states "initial current -- 30A or smaller."

So, using a 30 amp controller with two 175watt panels would yield no more than 29.2 amps, right? (350/12 = 29.2) This works. But, this leads me to another question -- if charging the battery from solar and a 7-pin connector (or shore power) at the same time, what happens if the combined charge current exceeds the battery specs? When on shore power, I can use the disconnect switch, but driving down the road in full sun, if I am getting close to 30A and the 7-pin charge is sending a few amps that pushes over 30A combined, is this dangerous?

John

[Wavery]

Hi John,

Sorry that I didn't see this post sooner.

The 350W of solar will be great while the Sun is up. If you have enough sunlight those panels should take care of your needs just fine. That fridge on 12V draws a lot. I don't remember exactly how much but it's substantial. What that means is you will probably have enough power to fully charge that battery. If you do get a fully charged battery, it will probably last until about 3-8 hours after sunset. Then you have a dead battery and the next day, it will be all the solar can handle just to charge the battery and it's down hill from there.

My point is, you need to match battery storage with solar capacity. Having 2, 175W panels won't do a lot more than having 1, 175W panel with that battery. To have an effective system you need balance. Your 100ah AGM only has about 50ah of useable capacity. Once the Sun goes down, the lights and other 12V accessories come on and that's when you really start using that battery.

Here's a couple of suggestions.

1. Get yourself a 100ah (or 2) LiFeP04 batteries. They charge much faster and have 85% useable capacity (instead of 50%). They also last a lot longer than you will own that trailer. They will last thousands of cycles as apposed to hundreds of cycles with the AGM.

2. If you wire your 350W solar array in series, you don't need to be worried about the length of the wire run on your TM. Standard 10G solar wire will be fine.

3. A great place to mount your solar controller is on the front of the box. It is covered but visible when the trailer is open and totally hidden when the roofs are closed.

Here are some pics of the way that I mounted my solar and ran my wires down the arms. Another pic of my 110V charger and inverter on that front wall. Great place to mount your solar controller. You have to be careful in the location because of the interior cabinets in the front roof.

[ShrimpBurrito]

Originally Posted by JohnMD

Dave,

Thanks for the feedback. (Please pardon any mistakes or confusion I may have as my intro to electrical engineering class was 35 years ago.) This gives me more to consider (regarding max battery charge current). My AGM 100AH specs show 0.30C, which I understand to be 30 amps, right? It also states "initial current -- 30A or smaller."

Hi John,

Yes, you are correct in that 0.30C = 30 amps, but only because it's a 100Ah battery.

The 0.30C is the "C rate", which is the max amount of current that can flow in or out of the battery. It is the length of time, expressed as a fraction (or multiple) of an hour to completely charge/discharge. A C rate of 1.0C is the amount of current to completely charge or discharge the battery in one hour.

In your case, 0.30C means the amount of current required to do that in 3.33 hours (1 / 0.3 = 3.33), and since you have a 100Ah battery, that translates to a max current of 100 Ah / 3.33 = 30 amps. That's your max discharge/charge current for that one battery.

Originally Posted by JohnMD

So, using a 30 amp controller with two 175watt panels would yield no more than 29.2 amps, right? (350/12 = 29.2)

Yes. In fact, the current would be less than that because the charging voltage would be on the order of 14v, so it would be more like 350/14 = 25.

Originally Posted by JohnMD

This works. But, this leads me to another question -- if charging the battery from solar and a 7-pin connector (or shore power) at the same time, what happens if the combined charge current exceeds the battery specs? When on shore power, I can use the disconnect switch, but driving down the road in full sun, if I am getting close to 30A and the 7-pin charge is sending a few amps that pushes over 30A combined, is this dangerous?
John

Great question. When you have multiple chargers (which is what the solar controller, 7-pin connector, and on-board converter all are), each one senses the battery voltage independently and then applies charge current according to whatever algorithm it uses. If you have a fully depleted battery, then all the chargers are might want to charge at their maximum rate, assuming the battery will draw that much current. Lithium batteries will take charge currents that far exceed lead acid, and so when using lithium, you can quickly get to some pretty high charge rates when doing that (like 50A+). But my sense is that is much less of an issue with lead acid. Googling something like "battery chargers in parallel" will bring up some good reading. It's a very common scenario.

Wayne also gives some great tips.

Dave

[JohnMD]

Wayne and Dave,

Thanks for the great feedback. You make great points. I've been reading the board for a few years and have enjoyed reading about everyone's solar/lithium upgrades. We're heading out for a 5-week southwest trip in late June. Our long travel days will be followed by nights with full hookups, so we can switch to shore power overnight. I'm not up for a switch to lithium right now, but am not ruling it out for the future. I have learned a few things:

1. I spoke with Universal Power Group tech support and they confirmed the 30A max charge rate and stated they recommend staying around 27 max for my battery.


2. The Victron SmartSolar controller documentation states:

"Max charge current -This setting sets the maximum battery charge current. It is by default set to the maximum solar charge current.
Use this setting to reduce the charge current, for example, when a smaller battery bank is used that requires a lower charge
current."

So, if I go with 2 panels, I can lower the max charge rate setting to compensate for a few amps from the truck. However, I'm not sure how many amps would be coming from the truck while traveling...I guess figuring that out is now on my task list. (As an aside, I'd be curious to know what the charge current of the original WFCO converter is when on shore power, which I don't see noted in the manual.)

I understand sizing the batteries and array to one another, but figure I have a better shot at keeping the battery charged with the Norcold on battery power with two panels while driving.

John