Solving problems, optimizing solutions

Gamekeeper

New Member
Joined
Nov 1, 2021
Messages
16
My first solar project of some complexity, is upgrading the solar system on a brand new Winnebago 2108TB, travel trailer.

It arrived with a 27 series lead acid dual purpose battery, a flexible 100 W solar panel, and a Go-power PWM charge controller.

The immediately observed problem, is that my use of it is primarily in the fall and winter, within the great lakes region. That means, endless cloudy days. And low panel output.
So, the battery dies quickly, and is slow to recharge.

Usually, when I am solving a thorny problem, I reverse engineer back from where I want to end up.
I list the symptoms, and make a list of possible solutions..
Then I optimize.

In this case, my problem is baseload usage in the trailer when it is cold, damp, and rainy or cloudy.
The symptoms manifest as inability to keep the furnace operating.

Also, while I have not fully tested them, it would seem the tank heaters would quickly follow the furnace when the battery state of charge declines.
in the great lakes, in an off grid situation, this could go on for months.

That is unacceptable to me.

The brain trust here, I believe, is fully capable of helping me to optimize my situation.

My first thoughts were to address the battery state of charge problem.

Normally, I encounter it at the end of the day, when I am returning to the trailer.
before anyone says well, just shut everything off when you leave in the morning, I should remind everyone that my wife will probably be sitting in the trailer, and also, that it is my full intention to be using it in below freezing circumstances. That’s why I bought it.

In order of practicality, it would seem, setting my truck up to charge the batteries as needed would be the simplest immediate solution, and relatively economical. After all, the truck is always there when I am.
I have a 2500w generator, and I hate listening to it.

After the DC to DC charging problem is addressed, it would seem expanding the solar panels on the roof would be next.

There is room on the roof for approximately 800 W of rigid solar panel.

I took advantage of Black Friday, and ordered 10, 100w , HQST solar panels at $75 each.

They are of a size where eight of them can be easily affixed atop the trailer.

Now, I need to select the equipment to combine the automotive, and solar, charging sources.

I have been looking at the Renogy combined 50 amp DC to DC with MPPT charge controller unit.

On DC to DC charging side, it appears to be an adequate choice.

In my mind, I can return to the trailer after a day of hunting, look at the state of charge, and then plug the truck into an Anderson connector, and let the truck run for a period of time to bring the batteries up to a higher state of charge. I must confess, I haven’t done the math to determine how long I would have to run the truck connected to the trailer batteries, to keep the systems running until the next day.

My desire, is to take advantage of travel days to top off the batteries, but realistically and primarily, rely on the solar array to keep the batteries fed. Using the truck in lieu of a generator to top off the batteries during bad weather seems just good business.

On the PV side, I have room for eight panels, and it would seem a tidy installation would be 4 in series, with two strings in parallel, down to the charge controller.

I would like to explore the calculations for this relatively small system, and it’s unfortunate operating environment, in this thread.

The 100 W panels I have, show 21.6 V open circuit voltage, and 6.5 A when short circuited.

Let’s explore this.
 

corn18

Village Idiot
Joined
Sep 9, 2021
Messages
325
A group 27 dual purpose battery will hold a whopping 80Ah of power at full charge. Usable will be more like 40Ah if you got to 50% depth of discharge (DOD). You may be able to get away with 80% DOD, so 64Ah of usable power. That isn't much.

So starting with charging is probably going to end in frustration.

You need to do an energy audit first. This will determine everything else. Based on your post, you want to solve this problem logically, not willy nilly. So do the energy audit and it will help size everything else. And determine where you can get the most bang for your buck.

Here's a link to the audit:

https://diysolarforum.com/resources/system-energy-audit-and-sizing-spread-sheet.12/
 

sunshine_eggo

Happy Breffast!
Joined
Oct 26, 2021
Messages
1,874
Location
USA
My first solar project of some complexity, is upgrading the solar system on a brand new Winnebago 2108TB, travel trailer.

It arrived with a 27 series lead acid dual purpose battery, a flexible 100 W solar panel, and a Go-power PWM charge controller.

The immediately observed problem, is that my use of it is primarily in the fall and winter, within the great lakes region. That means, endless cloudy days. And low panel output.
So, the battery dies quickly, and is slow to recharge.

Fundamentally, you have very little capacity and charge capability.

Usually, when I am solving a thorny problem, I reverse engineer back from where I want to end up.
I list the symptoms, and make a list of possible solutions..
Then I optimize.

You need to apply this to this situation, and I don't see that you've even started.

"where I want to end up" = powering all the loads you need to power.

You haven't identified all your loads.

It's like you want to take a road trip. You don't know how far it is, and you're just filling the tank saying, "if we make it, we make it."

In this case, my problem is baseload usage in the trailer when it is cold, damp, and rainy or cloudy.
The symptoms manifest as inability to keep the furnace operating.

Also, while I have not fully tested them, it would seem the tank heaters would quickly follow the furnace when the battery state of charge declines.
in the great lakes, in an off grid situation, this could go on for months.

First, failing to regularly charge the 12V to full has likely resulted in the need to replace it. A lead acid battery sustains damage when stored at < 100% SoC.

Tank heaters (assuming these are the electric heating pads keeping the tanks "warm") may be a far heavier load than you realize. If you're running 3 of them, they're likely consuming 50% more juice than the furnace blower.

That is unacceptable to me.

The brain trust here, I believe, is fully capable of helping me to optimize my situation.

My first thoughts were to address the battery state of charge problem.

As you have already identified, you have a mismatch in usage and charge.

Normally, I encounter it at the end of the day, when I am returning to the trailer.
before anyone says well, just shut everything off when you leave in the morning, I should remind everyone that my wife will probably be sitting in the trailer, and also, that it is my full intention to be using it in below freezing circumstances. That’s why I bought it.

In order of practicality, it would seem, setting my truck up to charge the batteries as needed would be the simplest immediate solution, and relatively economical. After all, the truck is always there when I am.
I have a 2500w generator, and I hate listening to it.

Unfortunately, it is your only short term option. You should run it to maintain the replacement battery.

After the DC to DC charging problem is addressed, it would seem expanding the solar panels on the roof would be next.

There is room on the roof for approximately 800 W of rigid solar panel.

I took advantage of Black Friday, and ordered 10, 100w , HQST solar panels at $75 each.

They are of a size where eight of them can be easily affixed atop the trailer.

What do you plan to do with the other two?

Now, I need to select the equipment to combine the automotive, and solar, charging sources.

I have been looking at the Renogy combined 50 amp DC to DC with MPPT charge controller unit.

On DC to DC charging side, it appears to be an adequate choice.

HORRIFYING choice. 50A * 12V = 600W. That's the MAX you can get from solar, and if you're using as DC-DC from the truck, you share the 50A between the two sources.

Best to get a quality MPPT that can handle all your solar and a separate DC-DC for alternator charging.

In my mind, I can return to the trailer after a day of hunting, look at the state of charge, and then plug the truck into an Anderson connector, and let the truck run for a period of time to bring the batteries up to a higher state of charge. I must confess, I haven’t done the math to determine how long I would have to run the truck connected to the trailer batteries, to keep the systems running until the next day.

You should do this. In fact, you should conduct an energy audit of the things you plan to use, how long you plan to use them, etc. There is a spreadsheet in the Resources section.

My desire, is to take advantage of travel days to top off the batteries, but realistically and primarily, rely on the solar array to keep the batteries fed. Using the truck in lieu of a generator to top off the batteries during bad weather seems just good business.

That depends on your available solar and loads.

On the PV side, I have room for eight panels, and it would seem a tidy installation would be 4 in series, with two strings in parallel, down to the charge controller.\

You've just established that you CAN'T use the Renogy you've selected. It can only handle PV input of 25Voc.

I would like to explore the calculations for this relatively small system, and it’s unfortunate operating environment, in this thread.

The 100 W panels I have, show 21.6 V open circuit voltage, and 6.5 A when short circuited.

Let’s explore this.

Do some homework first.

Since you've committed $ to this, we can look at a few things:

800W/12V = 67A - you need a MPPT capable of 67A current. Since flat panels on a RV roof will never produce rated power, you could probably fine with 60A.

If you're going to go with lead acid, you need a battery capable of absorption 60A without damage. A good round number is 0.15, i.e., 60A/.15 = 400Ah - probably 4-5X what you currently have.

Remember that with lead acid, you NEED to get deep cycle rated (not "dual purpose"). If they have a CCA rating on them, they're likely the wrong battery. You also can only reliably use HALF of the battery capacity and expect good life, so, in the 400Ah example battery, you can only use 200Ah. 4X 6V GC2 batteries in a 2S2P bank are likely your most cost effective option - around $500 and 430Ah.

You need to use http://www.pvwatts.com for a typical camp location and simulate your flat array. It will output the average daily hours of sun for each month, e.g., November might be something like 2.1h. In that case, you could harvest 800W * 2.1h = 1680Wh. Divide that by 12V, and you get 1680Ah/12V = 140Ah - almost your usable capacity. This site also factors in average weather impact on daily production. Clear days will outperform that and crap days will underperform.

Additionally, since you're so far north, and you're talking about cold weather camping, your winter production may be horrifying. Some lightweight flexible panels that you can deploy on-site at a favorable winter angle (even just hanging them vertically on the side of the camper) may dramatically improve solar availability.

PVWatts example.

I picked Iron Mountain, Wi:
1638293500740.png

800W array, flat.

As you can see, the winter months are horrible.

400W array, vertical:
1638293613206.png

Winter months get about 50% more when vertical as opposed to horizontal.
 
Last edited:

HRTKD

Boondocker
Joined
Apr 24, 2020
Messages
6,492
Location
Somewhere South of Denver
I looked up your trailer and I can't find anything in the specifications that leads me to believe it is a four season trailer. That right there is your main problem. Trying to heat a non-four season in the winter trailer requires a lot of energy.

I have a non-four season trailer and have put a lot of work into making it more capable at handling cold temperatures. In 2020 I was camping at -8° F. Despite all my efforts, I still froze one pipe for a short period of time. I go through a lot of propane.

Tank heaters take a LOT of amps and will drain your battery quickly. I don't run my tank heaters unless I have to and even then I do it only when I have excess PV output or if I'm running the generator.
 

jimcalf

Solar Enthusiast
Joined
Aug 1, 2020
Messages
34
I also have a 2021 Winnebago Micro Minnie 2108TB. I'm going through a full power upgrade myself, for the exact problems that you've mentioned. I think even Winnebago knew that the 2021 solution was subpar, as for 2022, each trailer gets a 190W rigid glass panel vs. the 100W flexible that you and I both have. That and a 30W solar charge controller vs. the junky 10W PWM we have.
I am in the process of collecting the parts now, and have four 200W solar panels for the roof, electronics came in yesterday and waiting on batteries to arrive. Just like you figured out, we can only fit about 800W on the roof. I'm putting in Lifepo4 batteries under my bed, as the flooded batteries can't carry the capacity I want, charge too slow, die in two years, etc. You can buy cost effective Lifepo4 batteries today for not much more than lead acid. It's worth looking into. Lifepo4 can also absorb energy dramatically faster than lead, so when the solar is at peak, the batteries can take it all. Even running a DC-DC converter + Solar can be absorbed, whereas with lead it can only take so much at a time. Running your 2500W generator, you'll be able to charge Lifepo4 batteries much faster than lead, if you have a powerful charger. Let me know if you're interested in seeing what I'm doing. I am near Akron, OH, so I get the cold weather / lousy solar production issues as well.
 

Rocketman

Solar Addict
Joined
Sep 27, 2020
Messages
586
I think the most useful item on my RV electrical system is my Victron Smartshunt (or BMV712). This battery monitor (once you get it set up) will let you know how full your batteries are, how much power you are using, and (at the current rate) how many hours you have until you reach the bottom of your useable battery (50% for lead batteries).

You can even use it to help figure out your energy audit. By switching stuff on and off you can see how much power it is really using.

Two items you can do right away - make sure you are getting your battery completely full (all the way through its absorbing cycle) every day.

Also, if your battery is exposed to the cold - add some insulation around it to help keep it a bit warmer - warm batteries work better. (Although this one is a minor step).

To add all those solar panels you will need a new solar charge controller- look really close at the Victron 100/50 or 150/60 (but the 150/60 probably costs more than it’s benefits will be).
 

Gamekeeper

New Member
Joined
Nov 1, 2021
Messages
16
First, I would like to thank everyone who responded so quickly.

Secondly, in the interest of brevity, I only provided skeletonized information in the first post.
I have much more detailed material forth coming.

I am not planning to live in my trailer. I just use it for a few weeks at a time in inclement weather.
What can “get me by” is balanced by what is worth doing.
This is an optimization project, and can’t be solved perfectly, but it can be solved satisfactorily.

I have all of the factory wattage requirements for how I use the trailer, but I am traveling today and tomorrow, and I cannot post my energy audit details until I get to LA.

We can start there.

Also, I knew the FLA battery was a waste. It’s really just to run the tongue jack, stabilizers, slide, and awning, a couple times during setup/takedown.

Once we get a reasonable working figure on battery bank amp hrs, what to choose for batteries can be looked at.
 

12VoltInstalls

Photon Sorcerer
Joined
Jan 18, 2021
Messages
1,971
Location
Vermont
just use it for a few weeks at a time in inclement weather.
What can “get me by” is balanced by what is worth doing.
Which is another important question: budget. What can you spend?
‘Getting by’ is subjective in many aspects.

As far as running the generator: with the right battery bank and a decent charger you might be able to “get by” with just an hour of running. With 1000W of solar you may be able to run all day long and recharge the batteries and just need a boost before turning in.
 

Gamekeeper

New Member
Joined
Nov 1, 2021
Messages
16
Which is another important question: budget. What can you spend?
‘Getting by’ is subjective in many aspects.

As far as running the generator: with the right battery bank and a decent charger you might be able to “get by” with just an hour of running. With 1000W of solar you may be able to run all day long and recharge the batteries and just need a boost before turning in.
Thank you for the reply.

My energy audit was pretty predictable. I’m not the first person to want to use a camper in cold weather.

Though I must say, in reading on this site, and watching lots of YouTube videos, it appears most people just head to warmer climes rather than try to deal with cold weather and camping.

That is the opposite of my intention.

My energy audit showed that my convection microwave is an energy hog, and the need to continuously heat tanks, or run the furnace fan is the secondary energy hoh.
no surprises there.

It looks like with no behavior modification, and no sunshine, we can stay in the camper for two days living off the batteries at 400 amp hours, below freezing. Maybe 3.
And on day three, I can choose to plug the truck onto the trailer and use the alternator, or start the generator, or move the entire shebang. Though because of the places this camper gets parked, I am in those places for purpose.

I am going to install two 300 amp-hour smart lithium batteries and see where that takes me.
I just haven’t identified the batteries with the features that I can see would benefit me yet.

Roof space limits me to 800watts on the roof but I am only going to install 600 on the roof.
I will use a directional stand and run in an additional 200 on the ground through a different charge controller.

The DC to DCMPPT charge controller I purchased is limited to 600 warts and 50 A off the alternator. That’s going to work great on travel days, and on those occasions where I need additional protection, or the generator pukes.

People always ask me about budget for my projects, and truthfully I am more constrained by a search for value than dollars.
I am not a Victron guy. I’m not saying that their equipment wouldn’t work perfectly and serve me well, I just think for a do it yourself project there are better choices available to me.

I have really been struggling with this battery choice. Heating the battery compartment with tank pads seems popular, but there are more and more batteries coming available that have built-in heating. (less work for commodity money)

And if I continue to use a microwave, you just can’t get away from the 200 amp BMS.

it probably sounds silly, but I refuse to invest 50 large into my quail hunting rig, and then deny myself the opportunity to eat a frozen pizza.

I’ll be rolling down to Arizona to finish the season in about three weeks.
 

HRTKD

Boondocker
Joined
Apr 24, 2020
Messages
6,492
Location
Somewhere South of Denver
In my example that I put in post #4, I had enough excess PV production that I ran a 500 watt space heater during the day to lighten the load on the propane furnace. PV production came from 640 watts on the roof and 640 watts on the ground, aimed as best I could. I did not run the microwave off of the batteries. I fired up the generator for that. I didn't have a large enough inverter to run the microwave.

I have a typical RV that uses propane for the stove, oven, water heater and refrigerator. My hunting group heats frozen food in the oven about every other night.

Your 600 Ah of LiFePO4 batteries should work well.

For the heated batteries, read the description very carefully. Many of them heat the battery only when a charge is applied. This can cause you to lose valuable sun hours waiting for the battery to come up to temperature.
 
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