Simple off grid cabin design check please!

[jdhahn1]

Hi all,

Completely new to solar systems and electrical in general, I've been working on designing a small/basic system for our mountain cabin. I had been researching and designing a system from individual components but am now thinking about going the all-in-one route just for simplicity's sake. Here's what I'm looking at using:

2 x 100Ah LiFePo4 batteries (SOK) in series for 100Ah 24V battery bank
4 x 100W mono panels (HQST) in series
Growatt 24V SPF 3000TL LVM

The cabin will be unoccupied and below freezing for the majority of the winter besides the handful of weekends we're out there. My thought was just to top off the bank when leaving (will have a generator if needed) and disconnect them from all loads and panels to completely shut down the system. When we come back out we wouldn't turn on the system until we'd warmed up the cabin via woodstove to avoid any battery damage from cold temps or any other electrical issues while it's unoccupied for weeks to a month or two at a time. I'd probably do the same shut down process in the summer as well if we're not planning on coming back soon just for peace of mind.

A few questions:
1) Is that size battery bank large enough to power the inverter? I don't think we'd ever pull anywhere near the max watts but I've been told the bank still needs to be sized accordingly so that the BMS doesn't trip when the inverter capacitors are charging up.
2) Is that a good balance of panels for the system? Panels are cheap but I've read discussions where people have too much panel for the bank (don't know what C/x rate is ideal for battery health or how that is calculated). I don't want to go crazy but I'd like to make sure it charges relatively quckly.
3) Do I gain anything by going with 2 x 200W 24V panels rather than 4 x 100W panels? Mounting space isn't an issue. I haven't been able to figure out if that really changes anything. Total series voltage seems more or less the same, and is well under what the Growatt is rated for even if it's well below freezing.
4) Should I only top the batteries off to the 75-80% SOC range for improved longevity? I've read that a cycle rate between 75%-25% SOC or similar will get a longer overall battery life than 100%-50%. The storage is oversized for what I realistically need 99% of the time so I'd be fine not having them at 100% ever, but I wasn't sure if storing them for weeks/months and less than 100% affects the batteries negatively at all like lead acid. My goal would be to do whatever is best for long term battery life to get the most bang for my buck over the life of this system.
5) Can I add a separate low temp shut off to my system so I'm not relying 100% on the battery BMS should the temperatures drop in the cabin while the system is on? I couldn't find where the Growatt manual ever referenced low temp shut off so I'm assuming it does not have it. It gets cold up there, so I'd like to do everything I can to protect my batteries. That seemed like one of the only drawbacks of going this route vs individual components and getting a high quality controller.

I've tried to do my homework and it seems pretty straight forward using the Growatt, but I'm brand new to this so I'd love to make sure I'm not missing something basic. The system will primarily just be for low voltage lighting (wired off a separate fuse box from the battery), charging handheld devices, and occasional charging of power tool batteries or running a single corded tool at a time. We'd also have a generator if we needed for large tools or crazy loads.

Thanks in advance for any/all feedback! Don't know what I would do without YouTube and these forums. I've learned a lot these last couple weeks (hopefully).

 

[FilterGuy]

1) Is that size battery bank large enough to power the inverter? I don't think we'd ever pull anywhere near the max watts but I've been told the bank still needs to be sized accordingly so that the BMS doesn't trip when the inverter capacitors are charging up.

There are a couple of things to consider here.
The batteries will power the inverter. 3000w/24V = 125a That is 1.25C. Most LiFePo Cells will handle that.
However, there is a bigger question: Are the batteries large enough to supply the power for your needs? Have you done an energy audit to figure out your needs? This is the tool I use:

System Energy Audit and Sizing Spread Sheet

To get the spreadsheet, click on the orange button at the top of this page. This sheet is intended to help you plan your system. Fill it out down to the last decimal place, look at the results..... and then make your best guess;) Notes...

diysolarforum.com

Charging the capacitors of the inverter should be done with a resistor before you hook the battery to the inverter. You can do that manually but in your case it sounds like you will be turning off power to the inverter often enough that you want to set up a pre-charge circuit.

Inverter Disconnect Switch with Precharge

To get the paper, click on the orange button at the top of this page. This circuit is designed as a disconnect switch that allows the user to pre-charge the inverter capacitors before turning the switch completely on. 26 June 2020 update: At...

diysolarforum.com

2) Is that a good balance of panels for the system? Panels are cheap but I've read discussions where people have too much panel for the bank (don't know what C/x rate is ideal for battery health or how that is calculated). I don't want to go crazy but I'd like to make sure it charges relatively quickly.

400W/24V=16.7A (you will probably have less than that.) That is only .167C. That is fine. What does the SOC manual say about charge rate?

The other question is "What is your energy needs?" I don't know where you are located but assuming a insolation number of 4 you will get a *max*
of 400W*4=1600Wh of energy. (again..... do an energy audit to figure out your needs)

3) Do I gain anything by going with 2 x 200W 24V panels rather than 4 x 100W panels? Mounting space isn't an issue. I haven't been able to figure out if that really changes anything. Total series voltage seems more or less the same, and is well under what the Growatt is rated for even if it's well below freezing.

The main reason you would go with 4 instead of 2 panels is because of shading. If you might have shade on some of the panels, you might be better off running them in parallel or two sets of 2 in series.

4) Should I only top the batteries off to the 75-80% SOC range for improved longevity? I've read that a cycle rate between 75%-25% SOC or similar will get a longer overall battery life than 100%-50%. The storage is oversized for what I realistically need 99% of the time so I'd be fine not having them at 100% ever, but I wasn't sure if storing them for weeks/months and less than 100% affects the batteries negatively at all like lead acid. My goal would be to do whatever is best for long term battery life to get the most bang for my buck over the life of this system.

From what you say, calender aging is going to be a larger factor than charge aging. I would not worry about it.

4) Should I only top the batteries off to the 75-80% SOC range for improved longevity? I've read that a cycle rate between 75%-25% SOC or similar will get a longer overall battery life than 100%-50%. The storage is oversized for what I realistically need 99% of the time so I'd be fine not having them at 100% ever, but I wasn't sure if storing them for weeks/months and less than 100% affects the batteries negatively at all like lead acid. My goal would be to do whatever is best for long term battery life to get the most bang for my buck over the life of this system.

Actually, LiFePO stores better at 50%-60% of charge than at 100% charge.

5) Can I add a separate low temp shut off to my system so I'm not relying 100% on the battery BMS should the temperatures drop in the cabin while the system is on? I couldn't find where the Growatt manual ever referenced low temp shut off so I'm assuming it does not have it. It gets cold up there, so I'd like to do everything I can to protect my batteries. That seemed like one of the only drawbacks of going this route vs individual components and getting a high quality controller.

LiFePO can be discharged in cold weather.... it is charging that can get you in trouble in cold weather. Since you are using an all-in-one you would need to put an SCR or relay in series with the panels to do the charge cut-off. I was about to recomend the Victron Battery Protect, but it might not be able to handle the voltages your panels might generate.

 

[jdhahn1]

There are a couple of things to consider here.
The batteries will power the inverter. 3000w/24V = 125a That is 1.25C. Most LiFePo Cells will handle that.
However, there is a bigger question: Are the batteries large enough to supply the power for your needs? Have you done an energy audit to figure out your needs? This is the tool I use:

System Energy Audit and Sizing Spread Sheet

To get the spreadsheet, click on the orange button at the top of this page. This sheet is intended to help you plan your system. Fill it out down to the last decimal place, look at the results..... and then make your best guess;) Notes...

diysolarforum.com

Charging the capacitors of the inverter should be done with a resistor before you hook the battery to the inverter. You can do that manually but in your case it sounds like you will be turning off power to the inverter often enough that you want to set up a pre-charge circuit.

Inverter Disconnect Switch with Precharge

To get the paper, click on the orange button at the top of this page. This circuit is designed as a disconnect switch that allows the user to pre-charge the inverter capacitors before turning the switch completely on. 26 June 2020 update: At...

diysolarforum.com

400W/24V=16.7A (you will probably have less than that.) That is only .167C. That is fine. What does the SOC manual say about charge rate?

The other question is "What is your energy needs?" I don't know where you are located but assuming a insolation number of 4 you will get a *max*
of 400W*4=1600Wh of energy. (again..... do an energy audit to figure out your needs)

The main reason you would go with 4 instead of 2 panels is because of shading. If you might have shade on some of the panels, you might be better off running them in parallel or two sets of 2 in series.


From what you say, calender aging is going to be a larger factor than charge aging. I would not worry about it.


Actually, LiFePO stores better at 50%-60% of charge than at 100% charge.


LiFePO can be discharged in cold weather.... it is charging that can get you in trouble in cold weather. Since you are using an all-in-one you would need to put an SCR or relay in series with the panels to do the charge cut-off. I was about to recomend the Victron Battery Protect, but it might not be able to handle the voltages your panels might generate.

Thanks for your input @FilterGuy I really appreciate it! This system is just going to power lights and charge small handheld devices 99% of the time, but I'll definitely use that calculator to make sure I have enough capacity for my needs.

I think I understood all you points. For keeping the system small and simple do you think it would make more sense to go with a smaller all in one product like the MPP 12v 1kW combo and wire my two 100Ah batteries in parallel rather than series? Seems like would eliminate the need for the pre-charge circuit and with a 200Ah capacity at 12V and only a 1000W inverter (apologies if I'm not totally understanding the issues there).

I agree wiring the panels 2P2S makes sense for shading and I believe I would now need to in order to keep my parameters within the range the smaller combo device can handle. Here's the spec on the panel:

And here's the info on the MPP 12v 1kw from the manual (https://watts247.com/manuals/mpp/PIP-LV 110_120VAC/PIP-MS LV 1KVA 1-5KVA manual-20200318.pdf):

If I'm reading this correctly my array needs to be under 40A and 80V, right? 2P2S would get be max 12A and 42V approximately so I think that works with some wiggle room when the panels are really cold and the voltage is actually higher than the rating.

Do you think it's necessary to have the separate low temp shut off in addition to the battery's internal BMS? I suppose if we're out there in the winter and I'm worried the inside temp in the cabin could drop too low overnight I could just disconnect the system from the panels as an additional safety measure. The light system will be it's own 12V circuit direct from a fuse box from the battery so I don't think there'd be an issue with shutting down the all in one and disconnecting from the array while still using the lighting at night.

My original research made it seem like a 24V system was smarter but maybe that's overkill for the size of my battery bank and actual energy needs. We'll have a generator for any large loads or corded tools, so I really just need to turn on lights and charge small stuff when we're there playing on the weekends.

Does this seem like a more reasonable plan for the size of my battery bank?

Thanks again for all your help!

 

[jdhahn1]

Below is the spec on the SOK battery. Is the recommended charge rate the same with two batteries in parallel or does that double as well? The cabin temps would certainly drop below their listed storage temp range while the cabin was disconnected and unoccupied. Do you think that is really a big concern if totally disconnected and stored at 50-60% as suggested? Trying to figure out how to set up a simple system that can weather the cold up there without completely breaking the bank on self heating batteries or something similar.

 

[FilterGuy]

I think I understood all you points. For keeping the system small and simple do you think it would make more sense to go with a smaller all in one product like the MPP 12v 1kW combo and wire my two 100Ah batteries in parallel rather than series? Seems like would eliminate the need for the pre-charge circuit and with a 200Ah capacity at 12V and only a 1000W inverter (apologies if I'm not totally understanding the issues there).

Again..... that depends on what your energy needs are but....

The light system will be it's own 12V circuit direct from a fuse box from the battery so I don't think there'd be an issue with shutting down the all in one and disconnecting from the array while still using the lighting at night.

12V lights is a new data point for me. How were you going to power them with the 24V system?
If you have 12V loads, that is a vote in favor of a 12V system. You can still power 12v loads from a 24V system but it requires a buck converter and is less efficient.

Here is one warning about making it a small system..... once you have that 120V the desire to use it for more and more starts going up.... That blender for drinks in the summer, or toaster for you breakfast.... or the real killer: The moment your wife asks "Why can't I use my blow dryer?"

If I'm reading this correctly my array needs to be under 40A and 80V, right?

Correct

Seems like would eliminate the need for the pre-charge circuit and with a 200Ah capacity at 12V and only a 1000W inverter (apologies if I'm not totally understanding the issues there).

A couple of points
1 the actual storage capacity is the same regardless of whether it is 100Ah at 24V or 200Ah at 12V.
2 You can probably get away without pre-charging on a 1000W inverter, but I don't recommend it.
All inverters have a bank of capacitors on the DC input terminals. These capacitors are *not* disconnected if the inverter is turned off. Here is a representation:



The discharged capacitors start out looking like a short circuit. Consequently, when the switch shown above is turned on there *will* be a spark. This can cause a few problems.
1) it will pit the contacts of the switch. This will eventually wear out the switch. If it is a good quality switch it will take a while... but it will wear out.
2) The BMS might see the surge as a short circuit and shut off. This might not happen on a 1000W inverter because the capacitors are smaller... but you never know.
3) The surge is certainly not good for anything in the circuit. Some folks say it will damage the FETs in the BMS and the capacitors. I am not convinced it is all that bad.... but I don't take the chance.

There are a lot of approaches to pre-charge circuits. The Chargery BMS and a few others have special turn-on circuits.... but if the Chargery is already turned on when you connect the BMS to the inverter.... it does not help. There are two circuits that will work reasonably well. The first uses a perko rotary battery disconnect switch. (this is the resource I pointed to in the earlier post, and is my preferred approach.



The second is a push-button arrangement:


The concept is the same for both designs: Hook the inverter up with a resistor to pre-charge the capacitors before doing the full connect to the inverter. Both approaches work fine when used properly but I like the rotary switch because it is a bit more idiot proof.


The value of the resistor is not critical. I like ~15 ohms for 24v systems and ~ 8 ohms for 12V systems

 

[jdhahn1]

So just to make sure I understand... I would wire the resistor and perko rotary battery disconnect switch inline on the positive between the battery and all in one station? I'm guessing the MPP station is ok to be on but inverter needs to be off, or do I do this pre-charge before powering up the entire station?

Would you mind providing a link to the items you're talking about so I can see an example? Just want to make sure I have my head wrapped around this fully. Definitely don't want to risk damaging the BMS or MPP so this sounds like it is something I need to figure out.

Did you have any thoughts on whether I should be worried about the battery's listed storage temps? I've always though cold was ok for storage as long as you weren't discharging or charging, but I wouldn't be surprised if there was much more to that than I understand.

Thanks again for all your help!

 

[FilterGuy]

So just to make sure I understand... I would wire the resistor and perko rotary battery disconnect switch inline on the positive between the battery and all in one station? I'm guessing the MPP station is ok to be on but inverter needs to be off, or do I do this pre-charge before powering up the entire station?

If I follow what you are saying, then yes. The MPP should be completely off when the switch is turned on. (Will the MPP even turn on if there is no battery?)

Would you mind providing a link to the items you're talking about so I can see an example? Just want to make sure I have my head wrapped around this fully.

The links to the components for the perko switch solution are all in the resource:

Inverter Disconnect Switch with Precharge

To get the paper, click on the orange button at the top of this page. This circuit is designed as a disconnect switch that allows the user to pre-charge the inverter capacitors before turning the switch completely on. 26 June 2020 update: At...

diysolarforum.com

Did you have any thoughts on whether I should be worried about the battery's listed storage temps? I've always though cold was ok for storage as long as you weren't discharging or charging, but I wouldn't be surprised if there was much more to that than I understand.

I have a similar situation with a hunting cabin in the mountains. However, my cabin is powered by a hydro generator that is completely shut down over the winter. I have searched and searched for quantafiable info on the storage temps and could find very little. I ended up making a insulated box (R20 on all sides) and then added this:

Simple, Indipendent Battery Box Heater

To get the document, click on the orange button at the top of the page. This is a simple 20W battery Box heater that operates independently of the rest of your system. I use this for storing unused cells in a very cold climate, but it can be...

diysolarforum.com

 

[jdhahn1]

Are you mounting that heater on a wall of the insulated box or under the battery itself? Feels risky to have the heat source touching the battery even if it's dissipating through the sheet metal plate. Or does it never get anywhere near the normal working temp since it's not being powered with the 120V it normally would?

 

[FilterGuy]

I put the heater on a metal plate that is on one side of the insulated box. It is 4 or 5 inches from the battery.

 

[FilterGuy]

Or does it never get anywhere near the normal working temp since it's not being powered with the 120V it normally would?

It never gets near the working temp. I selected that particular pad because the resistance would keep the solar panel close to the max power point of the panel. However, even then it is only a 20 watt heater. When the solar panel is in full sun the pad gets very warm but hot. It never gets enough power to get hot. However, like I said, it is on sheet metal a few inches away the battery. I am not at all concerned about the battery getting too hot.