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diy solar

Upgrade off grid cabin in cold climate

Baard

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Joined
Feb 2, 2023
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Location
Norway
I share a cabin in a cold climate with my siblings and now it is time to upgrade the batteries as we have had 2 low voltage disconnects. The last one after charging with a generator. I then did a load test with all consumers, and it triggered disconnect after 3.5 hours and 260Whrs.
An energy audit results in 900 Whrs a day in worst case, but more likely closer to 600 Whrs. Mostly lights, the big consumers are TV, WIfi and PC a couple of hours each day.

How do I upgrade this system in steps starting with the batteries?
The decision i struggle the most with is AGM or LifePo4? I have been reading a lot about LifePo4 and the more I read, the more sceptical I become.
It has to be low maintenance. I have to do all the maintenance and there is a 3 hour drive.
It has to be easy to operate for my siblings.
I would like to be able to extend the capacity if necessary.


This is the third iteration of the system and it is now 7 years old:
Solar panels2x140WShould have been 611W according to energy audit
Solar regulatorEP Solar Tracer MPPT 2215BN 20A (12/24V, 260/520W)
Charger60A, Automatic 1260
InverterInverter 400W, EP Solar NP400-12Only used to charge PC
Batteries2 x AGM Solar 290 100 hour in parallellThe batteries are stored in the cellar, usually 2 degrees Celcius in winter.


Thanks!
 
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LFP vs. AGM:

LFP advantages:
Greater % of usable capacity
lower cycle life cost (more cycles per $, more kWh total energy delivered)
lightweight
capable of higher currents

LFP disadvantages:
No charging below freezing unless they have a built in heater.
Don't like being held at full charge for extended periods (degrades cycle life), particularly in higher temps.

AGM advantages:
Maintenance free vs. flooded lead acid
Lower acquisition cost
Best to hold at 100% whenever possible
Freeze tolerant, particularly if at a high state of charge

AGM disadvantages:
only 50% usable capacity
higher cycle life cost
heavy
lower cycle life, 1000-1200 50% cycles before battery degrades to 80%
Reduced efficiency at lower temps greater than LFP.
Requires temperature compensation for charging (higher voltages when colder, lower voltages when hotter).


If your cycling is sporadic, i.e., the cabin is only used periodically like only on weekends, and cold, AGM likely makes more sense.

If it's cycled daily, and provisions can be made to deal with low temperature charging, LFP is the better value.
 
I use a big LiFePO4 (30 kWh) bank in my off grid cabin. I use it on weekends and it gets very cold. I built a small closet with a 500 watt ceramic heater and insulated it really well. The heater doesn’t run that much to keep the closet at 60F. The big battery is great to run Starlink, security system and even heaters at night, all remotely. The heaters are great way to cycle the batteries, and they keep the place above freezing on good solar days daily and even at night.
 
LFP vs. AGM:

LFP advantages:
Greater % of usable capacity
lower cycle life cost (more cycles per $, more kWh total energy delivered)
lightweight
capable of higher currents

LFP disadvantages:
No charging below freezing unless they have a built in heater.
Don't like being held at full charge for extended periods (degrades cycle life), particularly in higher temps.

AGM advantages:
Maintenance free vs. flooded lead acid
Lower acquisition cost
Best to hold at 100% whenever possible
Freeze tolerant, particularly if at a high state of charge

AGM disadvantages:
only 50% usable capacity
higher cycle life cost
heavy
lower cycle life, 1000-1200 50% cycles before battery degrades to 80%
Reduced efficiency at lower temps greater than LFP.
Requires temperature compensation for charging (higher voltages when colder, lower voltages when hotter).


If your cycling is sporadic, i.e., the cabin is only used periodically like only on weekends, and cold, AGM likely makes more sense.

If it's cycled daily, and provisions can be made to deal with low temperature charging, LFP is the better value.
In my situation (sporadic,charging and use) what’s the work around with lfp?
I like lfp, don’t want,agm.
what about turning the array off when we are not there? (Also turning it off when we ARE there if we’re not using the battery as much?)
 
In my situation (sporadic,charging and use) what’s the work around with lfp?
I like lfp, don’t want,agm.
what about turning the array off when we are not there? (Also turning it off when we ARE there if we’re not using the battery as much?)

It's not like they won't work. You're simply paying a premium for a feature you won't use. Kinda like getting a fancy sports car, but only driving it to church on residential streets at 25mph.
 
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If you don't need to charge batteries in winter (below freezing) you could switch to Li, if you're willing to pay the extra.
Otherwise stick with lead acid. Tried and true!
 
In the winter, there are several things that happen to AGM batteries:
- The cold temperatures result in lower output voltages
- The internal reactions in general are slower, so the output current capability is lower
- The charge voltage needs to rise as the internal temperatures rise drop. (corrected thanks to @sunshine_eggo ) Some chargers have built in temperature compensation, some do not.

So for applications like yours (in vans) I build the setup as a 24 volt system and then use a DC - DC converter to produce the 12 volt portion.

The inverter portion I use a 24 volt input inverter.

This will absolutely work much better - but - would be a lot of changes for you, so I am just mentioning it.

Inverters also have a standby power draw when turned on. This alone can be 30 - 40 watts, so I can see how this could easily happen.
_________

You have not mentioned a refrigerator or heater. Usually a 12 volt refrigerator is the largest total daily load and the heater ignition is the most sensitive to low voltage. Is that what tripped for low voltage?

Coming back to what you have, the solar panels will have a lower Vmp when it is overcast, but also a higher voltage from low temperatures. Sometimes this cancels each other out but not always.

MPPT solar charge controllers need an array voltage of ( V battery ) + ( usually 5 - 6 volts) to turn on, so nearly 17 - 18 volts in a 12 volt system. In the less intense sunlight of winter, your panels will need to be wired in series to really work effectively. Not sure if they are.

This is one of the advantages of a PWM solar charge controller - most only need a (V battery ) + ( 1 volt ) to work, so for panels in parallel only mode, they often will turn on at lower light levels (but lower efficiency when there is plenty of light) I use the bogart engineering setup for projects in some coastal projects for this reason.

___________

On the solar and battery storage sizing, ideally you want to be able to produce and store 3 days of power in one day. 2 days worth is kind of the minimum for this type of application.

Also - make sure to get a battery that can charge quickly from a company that really has a data sheet for it. So many AGM batteries are sold that still charge very slowly.
 
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In the winter, there are several things that happen to AGM batteries:
- The cold temperatures result in lower output voltages
- The internal reactions in general are slower, so the output current capability is lower
- The charge voltage needs to rise as the internal temperatures rise. Some chargers have built in temperature compensation, some do not.

You have this backwards.

At cold temperatures, absorption voltage needs to be higher. As temperature rises, the voltage needs to decrease according to temperature.

Coming back to what you have, the solar panels will have a lower Vmp when it is overcast, but also a higher voltage from low temperatures. Sometimes this cancels each other out but not always.

^ This is mostly correct.

MPPT solar charge controllers need an array voltage of ( V battery ) + ( usually 5 - 6 volts) to turn on, so nearly 17 - 18 volts in a 12 volt system. In the less intense sunlight of winter, your panels will need to be wired in series to really work effectively. Not sure if they are.

But you forgot being correct above. Again, in cold temperatures, panel voltages are higher. The +5V or whatever value is at 0A, i.e., Voc, not Vmp. Insufficient voltage is not an issue in cold temps.

Light gives voltage
Intensity of light give Amps

This is one of the advantages of a PWM solar charge controller - most only need a (V battery ) + ( 1 volt ) to work, so for panels in parallel only mode, they often will turn on at lower light levels (but lower efficiency when there is plenty of light) I use the bogart engineering setup for projects in some coastal projects for this reason.

This "advantage" is offset substantially by their complete and total inability to ever deliver more than about 80% of rated power. It's even worse in low temps and less of an issue in high temps.
 
Thank you for the correction to the battery charging voltage vs temperature - I edited my post. Really appreciate it.

You are right that when panels are fairly cold, the panel IV curve does shift vs when they are hot.

What that IV curve also does is shift in the reverse direction when the light spectrum changes. In coastal conditions, we often see the temperature vary relatively little, but there are large shifts in the light spectrum with much less blue content on overcast days. This spectrum shift results in several volts less output, not just a reduction in current.

In this area, we nearly always see a fairly pronounced shift in light spectrum when the intensity changes - but usually this is not done in scientific papers on intensity vs output.

The light scatter effects can even make poly panels just as,or even sometimes more effective in these poor light conditions than mono over the course of hours.

If the panel array voltage is high enough (at least V battery + 20 volts or so) then MPPT is always going to be better.

If the panel array voltage is only 5 - 6 volts higher than the battery voltage, then you might want to try it under overcast conditions and see if one or the other really provides more charging hours and total charging power over the course of a day. AZ / NM ground solar conditions are very different than coastal conditions - for example San Francisco.

That is why I encouraged him to make sure that his panels are wired in series.

For better or worse I have spent a lot of time with the 2 test stands at my shop, partly because I incorrectly applied the results of those papers to field situations.
 
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I have a Cabin North of the 49th ,,, I do not use it in the winter much & pull out my Rolls AGMs & keep them in Canada for the winter where I can maintain, taste, feel, & see them.

I also have these Rolls AGMs ( 2 - 6 volts in series ) in my Promaster Van.

I’m often in 0C or below & I have never had isdues with my AGMs. If I was always above freezing, I would go Lithium in a heartbeat.

Now ,,, Not all AGMs are created equally. You have to review the manufacture’s performance & use specifications to see if a Brand & Model will work well for you.

I primarily charge my Cabin AGMs with a 600W solar array ,,, 3s2p or roughly (60v @ 10a) and a Victron 100 | 50 MPPT & temprature compensation with the Victron BM712 smart.

It isn’t one component in electrical, but the total design & the design parameters that count. Design it to run at “full throttle“ will put pressure on the system.

I like what @sunshine_eggo & @HarryN & @45North wrote above ?.


The environment conditions is what would push me to make your decisions. I do not like complicated automatic systems ( but that is me ). Above freezing ,,, maybe Lithium, but if there are times when it dips below & you are on a BMS & other stuff is it fail “safe”? With AGMs & depending upon your use patterns, simply turning your system off might be the way to go if you are not occupying the cabin. IIRC my AGMs can be stored in temperatures far below freezing.
 
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Coming back to your original question about battery suggestions.

With a temperature of 2 C, it is right on the edge of triggering the low temperature cut off of the typical LiFe BMS. If it were consistently 5 C I would not hesitate to use LiFe, but at 2 C, it is a real gamble if it will work "consistently". For this reason my suggestion is AGM.

It seems very likely that your family will keep using more and more power over time, even if they aren't right now. If you were in the US, the battery that I would suggest is this one:


I use the slightly smaller, size 27 version of this battery pretty routinely - and have them in my own Ford Explorer for a small power system.

You will notice that it is a marine battery, so it has both deep discharge and cold cranking amps ratings. Where this will actually help in your application is that it will charge more quickly than some others. The company has a very detailed data sheet and manual on them. If possible, try to find a similar battery in your area with a real data sheet and discharge curves. May or may not be possible.

6 each of these batteries will go a long way toward dealing with your needs and provide a stable platform for growth. You can use them in 12, 24 or 48 volt configurations without concerns.

Since you are responsible for any challenges, consider to implement a charging procedure something like:

- Each day that someone is at the cabin, start the morning with a 1 hour generator charging. (maybe 2 hrs when not much sunlight is predicted )
- Then let the solar do the rest of the charging that day

This will provide a way for much of the bulk stage charging to be done by the generator - where it is optimally used, and the finishing stage(s) charging to be done by solar - where that is more optimally used.

When possible, have them run the generator for 1 hr prior to leaving and then the solar can continue to finish charging / trickle charging until the next visitor arrives.
 
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I just noticed your questions.

You have not mentioned a refrigerator or heater. Usually a 12 volt refrigerator is the largest total daily load and the heater ignition is the most sensitive to low voltage. Is that what tripped for low voltage?
We use a propane gas refrigerator, so no.

MPPT solar charge controllers need an array voltage of ( V battery ) + ( usually 5 - 6 volts) to turn on, so nearly 17 - 18 volts in a 12 volt system. In the less intense sunlight of winter, your panels will need to be wired in series to really work effectively. Not sure if they are.
Yes, the panels are in series.

On the solar and battery storage sizing, ideally you want to be able to produce and store 3 days of power in one day. 2 days worth is kind of the minimum for this type of application.

Also - make sure to get a battery that can charge quickly from a company that really has a data sheet for it. So many AGM batteries are sold that still charge very slowly.
Good points.
I have done an audit using an excel sheet I found on this site a couple of years ago. The batteries should have been big enough for 3 days. But in late autumn and winter the panels do not contribute much.

Yes, the batteries are sold as 290Ah, but in 100 hours :)
 
In response to the comments about temperature, it is usually above 0 degrees Celcius. But, in cold winters with little snow, we have experienced that the water we store in the same cellar have frozen. However, if we place the containers with as much surface area as possible in contact with the ground, the water does not freeze.
 
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In response to the comments about temperature, it is usually above 0 degrees Celcius. But, in cold winters with little snow, we have experienced that the water we store in the same cellar have frozen. However, if we place the containers with as much surface area as possible in contact with the ground, the water does not freeze.

I’m in Canada so can get cold in the winter; I look at Lithium batteries & think they are wonderful ,,, Everything But What I call the 3C’s;

Cash
Cold
Charge Complexity

I use “Good Quality” “Decent Performance Specifications“ AGMs In my Van & Cabin.

I would switch to “Lithium” in a heartbeat & I can get over “The Cash” & “The Charge Complexity” ,,, but being Canadian, The Cold is a bitter pill to swallow.

Sounds like “You” are the maintenance provider of the cabin systems. This is a remote cabin that is sometimes occupied & sometimes below charging temperatures for Lithiums.

It can be done & it can be automated to look after the charging, but in my mind it is risky ,,, electrical devices never fail right ?.

If it were me ,, because of the environment & situation, I am using good quality AGMs & having an easy to shut down & turn on electrical system ,,, that is what I do with my Van & Cabin ,,, sometimes -20C or colder.
 
The battery capacity rating is also done assuming room temperature.

It is quite a bit less when the battery is at 0 C.

Of course the data from one battery brand is not identical to others, but Lifeline publishes quite a good manual with graphs of how their battery performance changes with temperature.
 
It might make sense to consider wind as an energy source as well? This might be more available, especially in the winter.

Similar to solar, it takes the turbine (wind power generator) , controller, and of course the batteries to make it all work.

This is a small but innovative supplier of wind related products in the US. Yes the web site is a bit old school, but the owner is pretty solid. He and his wife answer emails and phone calls - and send items out faster than they even commit.

Perhaps there are similar offerings near you?

 
41m2S-HlrLL._AC_.jpg
 
That is my exact make & model ,,, Backup ,,, Which I rarely need, but when I do it is greatly appreciated
Let me give you a word of advice. Install a fuel cutoff valve inline so you can run the carb dry. Otherwise you are going to have a plugged jet and no start situation. I made a separate run/kill switch for mine so I could run it dry. If just one person leaves fuel in carb it wont run right even with no eth fuel/

Also when you start it run it on low for 5 mins and then high for 5 mins. That will prevent valve shank glazing which will stick a valve open and also make a no start situ.

 
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