New solar project on a 30 foot travel trailer; advice needed!

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lorettaboy

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Joined
Feb 23, 2021
Posts
5
Location
Ontario Canada
I'm a brand spanking new member to these forums and am looking for some advice and feedback on my new solar system.
The wife and I are the proud new owners of a 30 foot Freedom Express TT, that has a solar port on the roof (10 ga. wiring).
We will be using our existing battery setup of 2 Trojan T105's, but may upgrade to lithium in the future.
Our power usage is fairly low, just 12V other than running a small 150 watt inverter to watch some TV from time to time.
The charge controller we've already purchased is the Victron 100/50, along with the BMV-712 battery monitor.
We have 6 100w solar panels (21.6V Voc, 17.3V Vmpp, 6.17A Isc, 5.78A Impp), but are contemplating adding 2 more.
We will be camping in some partially shaded campsites, so i am planning on installing 3 or 4 panels on each end of the roof connected in series and then in parallel.
By my calculations, the 3S2P array will produce 64.8V and 12.34A, while the 4S2P will produce 86.4V and 12.34A.
When I input my panels into the Victron calculator, it suggests using the 150/60 MPPT, which seems like overkill.
I've also plugged the same numbers into the Victron excel sheet, which shows that the 150/50 MPPT should do the trick.
Given these numbers, do you think we will be okay with the 150/50? And for the extra $200, is it wise to go with 8 panels vs 6?
 

Isaac-1

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Dec 3, 2016
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4,091
Location
SW Louisiana
The recomended max bulk charge acceptance for standard Trojan T105's is a bit under 400 watts per battery, or about 500 watts per battery for the AGM version.
 

Lou Schneider

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Mar 14, 2005
Posts
10,638
The Victron 100/50 controller should be fine. Unless you're planning to tilt your panels, figure on getting 15-20% less power than the panel specs. If you do exceed the Victron's input current rating on a sunny summer day the controller will just ignore the excess while delivering it's rated 50 amps.

10 gauge wire will handle up to 30 amps, but you will lose a few more percent of the MPPT power from voltage drop. If this is an issue consider running thicker wire from the roof to the controller.

Likewise, the cables between the controller and the batteries should be short and fat to maximize the bulk charging current.
 

Ex-Calif

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May 15, 2020
Posts
1,036
The 50 amp controller should do the trick with little margin for growth. It's a good time to buy the extra capacity if you plan to expand in the future. I don't know what the price difference is though.

At 600W of panels I think you are OK for what you are planning to do.

In theory you could be harvesting 360 amps a day. If I were going to expand anything it would be battery capacity. Like 2 more trojans...
 

lorettaboy

Member
Joined
Feb 23, 2021
Posts
5
Location
Ontario Canada
The 50 amp controller should do the trick with little margin for growth. It's a good time to buy the extra capacity if you plan to expand in the future. I don't know what the price difference is though.

At 600W of panels I think you are OK for what you are planning to do.

In theory you could be harvesting 360 amps a day. If I were going to expand anything it would be battery capacity. Like 2 more trojans...
Thanks, i appreciate your comments. My current batteries are 2 years old. Is it okay to add 2 brand new batteries, or would it be best to start with 4 new?
 

Ex-Calif

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1,036
The "right" answer is 4 new but if the current Trojans haven't been fully drained more than a couple of times I personally would add the 2.

This would "knock" the new ones a little but I think you are money ahead - especially if you take care of them and get them past 5-7 years of life which isn't unreasonable with your charging system and plan.
 

AStravelers

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Joined
Nov 14, 2016
Posts
1,545
Location
San Antonio, TX
I'm a brand spanking new member to these forums and am looking for some advice and feedback on my new solar system.
The wife and I are the proud new owners of a 30 foot Freedom Express TT, that has a solar port on the roof (10 ga. wiring).
We will be using our existing battery setup of 2 Trojan T105's, but may upgrade to lithium in the future.
Our power usage is fairly low, just 12V other than running a small 150 watt inverter to watch some TV from time to time.
The charge controller we've already purchased is the Victron 100/50, along with the BMV-712 battery monitor.
We have 6 100w solar panels (21.6V Voc, 17.3V Vmpp, 6.17A Isc, 5.78A Impp), but are contemplating adding 2 more.
We will be camping in some partially shaded campsites, so i am planning on installing 3 or 4 panels on each end of the roof connected in series and then in parallel.
By my calculations, the 3S2P array will produce 64.8V and 12.34A, while the 4S2P will produce 86.4V and 12.34A.
When I input my panels into the Victron calculator, it suggests using the 150/60 MPPT, which seems like overkill.
I've also plugged the same numbers into the Victron excel sheet, which shows that the 150/50 MPPT should do the trick.
Given these numbers, do you think we will be okay with the 150/50? And for the extra $200, is it wise to go with 8 panels vs 6?
Six 100watt solar panels is way over kill for just running the lights and a TV for a few hours a night. Also having more than 2 T105's is not necessary for your small amount of power usage. Are you planning on adding more power usage, such as a large inverter to power a microwave or coffee pot?
 

lorettaboy

Member
Joined
Feb 23, 2021
Posts
5
Location
Ontario Canada
Six 100watt solar panels is way over kill for just running the lights and a TV for a few hours a night. Also having more than 2 T105's is not necessary for your small amount of power usage. Are you planning on adding more power usage, such as a large inverter to power a microwave or coffee pot?
We do a ton of boondocking, 3-4 months straight to be exact, so i wanted to build a system that would not run low on juice even if we had a partially shaded campsite. Apart from the trailer lights and TV, we also need power for the water pump, occasional use of the furnace fan, fantastic fan, charging cell phones, and charging our laptops via a cigarette style 12v adapter. For the bigger items like the microwave etc we have a 2000 watt Honda generator.
Would you suggest i forget about the 2 extra panels and stick with 6?
 

solarman

Well-known member
Joined
Feb 8, 2018
Posts
391
Location
Texas
I'm a brand spanking new member to these forums and am looking for some advice and feedback on my new solar system.
The wife and I are the proud new owners of a 30 foot Freedom Express TT, that has a solar port on the roof (10 ga. wiring).
We will be using our existing battery setup of 2 Trojan T105's, but may upgrade to lithium in the future.
Our power usage is fairly low, just 12V other than running a small 150 watt inverter to watch some TV from time to time.
The charge controller we've already purchased is the Victron 100/50, along with the BMV-712 battery monitor.
We have 6 100w solar panels (21.6V Voc, 17.3V Vmpp, 6.17A Isc, 5.78A Impp), but are contemplating adding 2 more.
We will be camping in some partially shaded campsites, so i am planning on installing 3 or 4 panels on each end of the roof connected in series and then in parallel.
By my calculations, the 3S2P array will produce 64.8V and 12.34A, while the 4S2P will produce 86.4V and 12.34A.
When I input my panels into the Victron calculator, it suggests using the 150/60 MPPT, which seems like overkill.
I've also plugged the same numbers into the Victron excel sheet, which shows that the 150/50 MPPT should do the trick.
Given these numbers, do you think we will be okay with the 150/50? And for the extra $200, is it wise to go with 8 panels vs 6?

Firstly, welcome to the forum.. !

looking at your setup I see several issues..

1. 600 watts into a pair of T105's is bad, not kind to those batteries at all.

2. 4S2P will generate a VOC in cold weather closer to 21.6 * 4 * 1.25 = 108 Volts and stands to damage the controller.

3. On a good day, those panels will generate 500 watts and at 12 V you get 500/12 = 41 amps
that's well over what Trojan recommends, if you charge them too often at that rate they will consume
much more water, reach higher than normal temperature and also corrode the plates faster.

4. The Victron 100/50 is a good unit, you would be advised to reduce the charge current setting
to avoid boiling your batteries. also it has a max input of 700 watts at 12 V and the Victron site also computes a bit on the cautious side.

5. Adding 2 extra panels without increasing battery capacity significantly is not a good idea.

for item 3 above, that surprisingly is actually a better position to be in than the
majority of under panel systems I have seen. However, a higher charging rate like this will reduce life somewhat.

realistically, 600 W for your advertised consumption is overkill. for reference, my small TT has 480 Watts and 225 Ah of capacity at 12 V. we have only gone dark one time due to 4 days bad weather and that's what a generator is for..

I would suggest you configure for 2S3P as a good compromise for shading and either limit the charge current to 40 A or keep a big bucket of DI on hand.. :)
10 AWG is fine for your needs. ( if 2S3P then you have 43.2 V @ 4.6 A * 3 = 43.2V @ 13.8 A )
10 AWG at 75C rating is good for 35 A
I suggest a minimum of 6 AWG for the battery side.
 

Ex-Calif

Well-known member
Joined
May 15, 2020
Posts
1,036
My thought is to forget the 2 extra panels, unless you also upgrade your battery capacity.
I agree - The way I understand this post is that the OP already has the panels and the 50 amp controller.

Your daily consumption is not huge so 600W should do you. The issue is that IMO you may be at the limit of the controller if you add 2 more panels.

If you do add 2 more panels you definitely need more storage - you are also getting into a pretty big array.

Personally I think you are committed to 6 panels and the 50a controller for now. I presume you have a generator for occasional top up while boondocking.

I've been on many boats that are energy independent and the sweet spot was 800W a 60a controller and 800 a/h of storage or so. Most of these systems are supplemented by a wind generator, onboard 60amp alternator on the aux engine and most had a 50a generator in case a period of bad weather is encountered.
 

lorettaboy

Member
Joined
Feb 23, 2021
Posts
5
Location
Ontario Canada
Firstly, welcome to the forum.. !

looking at your setup I see several issues..

1. 600 watts into a pair of T105's is bad, not kind to those batteries at all.

2. 4S2P will generate a VOC in cold weather closer to 21.6 * 4 * 1.25 = 108 Volts and stands to damage the controller.

3. On a good day, those panels will generate 500 watts and at 12 V you get 500/12 = 41 amps
that's well over what Trojan recommends, if you charge them too often at that rate they will consume
much more water, reach higher than normal temperature and also corrode the plates faster.

4. The Victron 100/50 is a good unit, you would be advised to reduce the charge current setting
to avoid boiling your batteries. also it has a max input of 700 watts at 12 V and the Victron site also computes a bit on the cautious side.

5. Adding 2 extra panels without increasing battery capacity significantly is not a good idea.

for item 3 above, that surprisingly is actually a better position to be in than the
majority of under panel systems I have seen. However, a higher charging rate like this will reduce life somewhat.

realistically, 600 W for your advertised consumption is overkill. for reference, my small TT has 480 Watts and 225 Ah of capacity at 12 V. we have only gone dark one time due to 4 days bad weather and that's what a generator is for..

I would suggest you configure for 2S3P as a good compromise for shading and either limit the charge current to 40 A or keep a big bucket of DI on hand.. :)
10 AWG is fine for your needs. ( if 2S3P then you have 43.2 V @ 4.6 A * 3 = 43.2V @ 13.8 A )
10 AWG at 75C rating is good for 35 A
I suggest a minimum of 6 AWG for the battery side.
Thanks for the warm welcome, and all of the advice.
I'm planning on upgrading to 2 100Ah Lifepo4's once my Trojans bite the bullet.
Given all of the feedback, i'll definitely be skipping the extra 2 panels.
My particular TT only has 2 decent mounting spots on the roof; the extreme front and extreme back. This is why is was planning on doing a 3S2P configuration. I suppose I could put 4 at one end and 2 at the other in the 2S3P config you are recommending. Will there be a significant difference either way?
Lastly, is a combiner box necessary in these applications, or will MC4 branch connectors suffice?
 

solarman

Well-known member
Joined
Feb 8, 2018
Posts
391
Location
Texas
considering your available space constraints, 3S2P may well be a reasonable choice.. shading at one end of the TT will allow the other end to produce, and vice versa..

for 3S2P a combiner will not be required.
 

AStravelers

Well-known member
Joined
Nov 14, 2016
Posts
1,545
Location
San Antonio, TX
Thanks for the warm welcome, and all of the advice.
I'm planning on upgrading to 2 100Ah Lifepo4's once my Trojans bite the bullet.
Given all of the feedback, i'll definitely be skipping the extra 2 panels.
My particular TT only has 2 decent mounting spots on the roof; the extreme front and extreme back. This is why is was planning on doing a 3S2P configuration. I suppose I could put 4 at one end and 2 at the other in the 2S3P config you are recommending. Will there be a significant difference either way?
Lastly, is a combiner box necessary in these applications, or will MC4 branch connectors suffice?
MC4 branch connectors would probably work, but I would rather have a combiner box at the roof entry down to the controller instead of several MC4 connectors laying on the roof.

Probably a branch connector for each pair of series panels then only 1 or 2 wire pairs back to the combiner box.
 
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