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Small Cabin Forum / Off-Grid Living / New Solar setup
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# Posted: 3 Mar 2022 16:02

I have been searching around here looking for the right answers and just have not seem to come up with the correct ones. I have a off grid one room cabin that I use on the weekends year round. Currently have a 200w 12 volt solar setup that charges a 12 volt forklift battery. Pretty simple system that works great for what we need as far as lights, charging, & TV. Can get us through a cloudy weekend with minor monitoring. The plan is to build a little bit bigger cabin (24x24 with loft, looking for plans for that also but thats a different topic) and put in a bigger solar setup that can handle a fridge etc.

I have 4 x 320w (voc 40.1V, imp 9.79A) Renogy panels that I want to be the main setup. I am stuck between keeping it 12 volt or going to 24v. Been looking at Victron but not sure how to size it. Go 24v- panels 2s,2p??

Next Lithium sounds great for more discharge and I can afford it but the cabin is not always heated. (central WI). Would lead acid or AGM make more sense for this low used cabin?

I have been researching this for awhile now and am really looking for thoughts. I am fairly knowledgeable and enjoyed setting up the first small system and learning. I want it to be user friendly but also high tech if I need the stats to look back at.

What do you guys have for me. Ask me Questions. Point me towards other forum treads. Looking forward to feedback. Thanks

# Posted: 3 Mar 2022 17:12

Alright, lets start with a couple of things..

1. You need to do a realistic energy audit. This includes figuring out how much power EVERYTHING will draw. Lights, fridge fan, ect ect. Then you need to figure out how long those items will be on in a day. That should get you a Wh/day figure.

2. Going 24v means you can get away with smaller solar controllers as controllers are rated on AMPs on the output. A 30A controller can handle 380W in a 12v system and 770W in a 24V system. (money saved). However you will require 24V devices or a 24V inverter.

3. In the northern climates, I like panels in all parallel as they handle shade and cloud better that way. However, that means the wire sizes increase costing you more money. 2s2p may be a good compromise. For me I would try to keep it all parallel if I could. (If your panels are 20ft from your controller you would require 12ga wire to home run each panel)

4. Lithium is great most of the time. You will need to purchase heated batteries if you want to use them year round. Those heaters will use quite a bit of power (and during the lowest production months), so take that into consideration.

5. A battery monitor is a must (IMHO). Otherwise you are literally running blind and have no idea what your system is doing. There are quite a few on the market and most work great for FLA. However, you need to research for lithium as many are not suitable for those banks.

# Posted: 3 Mar 2022 17:43

First things first, you really have to figure out what kind of consumption you will have in the new cabin. Fridge, well pump or ?, other "appliances" and devices.

Do you only want 120VAC or 240VAC or rather, which do you need ?

A bigger place usually means a bit more activity - people and that's more usage, got to figure that in too.

Some basic rules as it were.
- It's best to not exceed 250A draw from a battery system.
- Keep it Simple and Safe.
12V @ 250A = 3000W or 120V @ 25A *
24V @ 250A = 6000W or 120V @ 50A *
* not derated for inneficiencies.

Once you know or have a reasonable estimate of what you will use during a day on average in kWh, and you know how many days you want to be able to run off just battery (no solar or generator) then we can figure out the battery sizing required.

Pending on the Amperage Draw & AH Sizing would dictate which voltage is suitable to support your needs. This in turn guides what size inverter and how much solar is required to charge the battery system during "average solar hours".

Average Solar Hours being mid March & September in North America.

There are a few different ways to go... Basically Component or All-In-One and there are trade-offs between them.

Component systems with separate Solar Charge Controllers (SCC's), Inverter/Chargers require more wiring and balance of system pieces and therefore more comp0licated "a bit". These can be quite efficient with proper components with similar benefits as the AIO's.
FOR PART-TIME USE this is a better option as the Inverter/Charger can be turned off allowing the solar system to keep maintaining the batteries with less effort.

AIO's are great for simplicity & ease of setup, flexibility (stackable) and communications / monitoring etc. They typically have a higher standby consumption and are more suited to 7/24 usage.

Lithium Batteries in particular Lithium Iron Phosphate commonly called LFP is the best choice for such an application. They can discharge to -22F but cannot be charged below 32F. Several premade battery packs have internal warmers to keep them at 40-45F and if kept in an insulated place are fine. You can purchase complete batteries or DIY your own if you are adventurous, but that comes with it's own challenges too.

Hope it helps, Good Luck

# Posted: 3 Mar 2022 17:58 - Edited by: gcrank1

Our 16x24 is in northern Marquette Cty, WI, call it 44*L so similar solar conditions as you.
Have you visited over at ALL the answers are there but it can be overwhelming/info overload. That said....
Freezing temps and LFP:
They can sit through freezing temps, and even be lightly discharged below freezing but Not Re-charged until they saturate/warm up above freezing.
So, you use the place in the winter, could you maybe have an AGM for initial power until things warm up then switch to the LFP (like boats do with bank switching). Or take LFP home for winter leaving an AGM for winter? When we did we sure didnt need a fridge, elec was mostly lights and led's dont use much.
1)LFP can, and is best stored, at some soc less than full!
2)They also Do Not need to be recharged right away, if at all, after some use. Totally unlike FLA that must be recharged asap or they sulfate.
3) 50% max of FLA is usable or you are killing the bats, 25-30% is Far healthier for them.
With LFP you can go deep, 70% dod No issues, even 90% dod regularly and the full 100% dod may be possible but why go there unless an emergency.
No maintenance, no off-gassing, long life, high discharge capability, high Charge rate, downside=price
4) Dont parallel batteries more than 3, often done but not good practice. It is better to get bigger capacity bats at your preferred voltage than tie a bunch together.
If you think you 'need' 4 12v bats paralleled together consider 2s/2p for 24v instead.
5)One 'rule of thumb' for sizing an inverter and system voltage:
1000w or less 12v is fine; still could go 24v if expanding is in the plan.
1000w to 2000w seriously think about 24v (if I was starting out fresh Id go 24v now, maybe should anyway since I need a new inverter and my scc is 24 capable).
2000w + 48v
Do not consider other than a Pure Sine Wave inverter
Go with an MPPT solar charge controller if you have good solar exposure. The MPPT will equal or better the PWM scc unless you have shading issues, which imo makes decent solar-elec generation much more complicated.
With the cheaper inverters if they say (example) 2000w/4000w surge Ignore the surge, most trip too quickly to do much of a surge. So, you have to 'oversize' the inverter initial rating to get it to work unless you have a low start-up load 'fridge or ac. Those items become topics of their own.
The issue with oversizing inverters is that unless the high-end/tier 1 units the 'idle draw' (what they use doing nothing) can be considerable.
6)Dont even consider things like electric water heaters, space heaters, crock pots, etc; ie, high draw/longer use resistance heating devices.
Short term, like an elec coffee maker or a high eff. or short time used lower powered microwave is possible.
7)This should have been No.1, energy conservation is far cheaper than building more solar! The more you try to emulate the grid electric the more you will pay becoming your own power company and you will wear All The Hats.
Fwiw, my system is small. 600w array, 2s/3p for 24v into the MPPT 50a scc feeding (new this year from fla) 2x 100ah 12v LFP.
Our usage has been an average low 360-600ish watts per 24hrs. With the 2x 100ah fla it was 1200w max available, pref 600ish useable so a good match. With the LFP its 2400w available so even if we up usage to 800w per day we have 3 days autonomy.
For any big draw stuff, tools, toaster, microwave, etc I start the 1700/2000w inv/gen for only the time needed.
The LFP's are so light at 30# that I can just bring them both or just one home too and recharge. No solar would be needed for a weekend..

# Posted: 3 Mar 2022 21:33 - Edited by: mj1angier

As far as Lithium batteries in the cold- The Victron controller I have has uses a temp prob that is beside the battery and cuts of charging when it get down to 38 deg I think. That works good here in NC but maybe not so much in cold areas. But you could also just take them home to charge during the winter and bring back when you go visit- great thing is that they are very light weight compared to lead. My system is 2 x 24v 100ah (URL) that uses 600 watts of solar

# Posted: 24 Mar 2022 16:13

Quoting: gcrank1
Our 16x24 is in northern Marquette Cty, WI, call it 44*L so similar solar conditions as you

Good information gcrank1. I am due east of you between the lakes in Manitowoc County, WI. With the smaller cabin and 200w system we have now we are very aware of usage and have boondocked with the camper in the past. Very rarely run the generator. I am thinking I am going the LFP way since most of the time spent there is during the warmer seasons, which you know in WI is July and August. We spend the occasional day or two there during the winter but I think I can figure something out when that time comes.

# Posted: 24 Mar 2022 16:45 - Edited by: NorthRick

I'll keep it simple. For your use - weekends where it gets cold in the winter:

24 volts 2s, 2p; maybe 2s, 3p if you need more power.

Flooded lead acid batteries.

A charge controller suitable for the panels and battery bank.

A separate battery charger and a small generator for those times in the winter when your loads are bigger than the solar can keep up with.


# Posted: 24 Mar 2022 16:59 - Edited by: gcrank1

Now that spring is sorta here I can say we 'made it through the winter', went to cabin more than we thought we would and it was NO trouble carrying the one 100Ah LFP @ 30ish# back and forth. It has run everything we wanted to run and hasnt been charged up in the last 4 or 5 'day trips', still reads 13.0 on the integral voltmeter!
We found we dont much care for going to a dead-cold cabin below 32*f, it takes too long to warm up more than the air temp. But with the LP wall furnace I start it up and within minutes it is kicking out 30,000 btu's, no waiting for a woodstove to come up to speed. I have a small fan or the cheap old ceiling fan always going to move the air around inside, that keeps the heat from just stacking up above our heads and slowly working its way footward.
So some little led lights and always the fan, some phone or tool pack recharging (those take little bat power) and the LFP is just chugging along all by itself. I diconn'ed the solar last fall to re-do it this year. I dont see a rush on that at this point.
If I had a dc2dc charger for LFP in the car it could be recharging the bat on the return home and back to cabin run, basically free charging that, car is running anyway. In the non-freezing times of year I could just leave the bat in the car and with my other driving between cabin trips I bet the bat would be fully charged between visits, not that LFP requires fully recharged.
I almost popped for a 1200W(continuous) 12v PSW inverter, it would up to run my self imposed Watts limit of 1000W to keep wire gauge and fuse sizes 'reasonable' and still run more than we do now off the 300W PSW unit. That size inverter should have a min 150A fuse and Fat, short wires! That why I say 'almost popped for', we havent really needed that capacity....yet.

# Posted: 24 Mar 2022 19:16 - Edited by: Brettny

When I was deciding on lithium vs FLA. I found that even taking the FLA down to 50% and getting only 500 cycles at that point it would be many many years of service, in may case it would be about 11yrs.

I run a 120v 4.5cuft mini fridge with separate top freezer. My whole system cost about $1300. I bought some used things like solar pannels and a 1200w pure sine wave inverter. That $1300 includes a 1kw sportsman inverter generator($209), the fridge($35 used) along with a 120v to 30a 12vdc charger($115). My setup wouldnt be so cheap if it was anything but 12v. 24v stuff seams to cost alot more and I couldn't find any used.

I dont run my fridge when I'm not there. I do highly recommend the solar crutch of having a small generator with a way to charge the batteries effectively from it.

# Posted: 24 Mar 2022 19:52 - Edited by: jlemke

Thanks. Still doing some planning. On my useage and needs but all good things to consider

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