Component choice- what would you do?

[Northarmgazer]

Hi Everyone,

I've just joined the forum in the hope that wiser people than me can help unscramble my thoughts after all the reading I've been doing.
I'm in what I think may be the unusual position of having solar panels on the way and already having a battery bank, but nothing in between.
The system I'm building is to run my shed off grid, intermittent use of the system. As money comes available I can always increase the size of the array in the future, but have to work with what I have for now.

My panels are 300w (peak 295) x 6, so lets say 1.8kw array for now. Peak VOC 39.7v. The battery bank is comprised of 2v SLA 1000ah batteries. I have enough to set up a 48v bank and have 11 spare (as back ups - the batteries are second hand) or obviously, 2 X 24v or 4 X12v.
The use of these batteries is non negotiable - in total they weigh 2.5 ton and I have moved them 3 times in the time I've owned them so I'm being stubborn here..

My preference is run a 12 v system based on cost and availability of "accessories" here. (I'm in Australia). I also already have all the shed lighting LED 12v globes so catering for this is essential.

I have struggled for a few days now trying to balance the needs of MMPT controllers with potential wiring formats for the solar cells, matching the specs of the solar to the Mppt's v.s the system voltage.

I have looked at:
Growatt SPF3500ES to run a 48v system - this is the cheapest and easiest option as it also has the inverter - but requires the solar array to run at 200v which I'm not keen on safety wise.
Growatt SPF3000TLHVM to run a 24v system - its basically the same price as above, but I could only run 5 of the 6 panels because of the total W size of the array it could handle
Epever 8415AN 80a controller running a 24v system. This would mean an inverter of 4000w (only size I could find with AC line out ports to wire to switchboard), so bigger than the Growatt. The inverter cost is about $700, bringing total cost to around $350 more than the Growatt. I would still have to run PV array voltage around 100v for the Mppt.

If I drop down to a 12v system and smaller amp controllers I'm looking at having to get 6 Epever controllers and then have lower required PV array voltages for the Mppt (down to around 30v min) so a bit safer from my perspective. But this comes at a cost of about $500 more.

Wiring distances are short from both panels to controller and controller to batteries. The battery linking cables and output cables came with them and are massive, I don't know their gauge, but they are thumb thickness!

I guess I'm trying to balance overall cost with the 'safest' operating voltages and ultimate system voltage.
My main concern running 48v (apart from running the 12v lights) is that if a battery dies the whole system is dead until I can swap out one of spare cells whilst I have them, where the 12 and 24v options mean I can just switch to another bank and still have more spares.

What would you do and why?

 

[chrisski]

Growatt SPF3000TLHVM to run a 24v system - its basically the same price as above, but I could only run 5 of the 6 panels because of the total W size of the array it could handle

Couldn't you send extra charging power to a battery through a separate charge controller.

Growatt SPF3500ES to run a 48v system - this is the cheapest and easiest option as it also has the inverter - but requires the solar array to run at 200v which I'm not keen on safety wise.

After about 2 years doing this, I am comfortable where I would want to do a 200 volt build for certain things, like high wattage appliances where I am able to have several panels which would be unshaded.

I have a 24 volt 3000 watt inverter that has 2.5 kw of panels run through 3 different SCCs on my RV. Two of those SCCs are 30 amp SCCs that are maxed out several hours a day. I can really see some advantages to an all in one and matching the voltage into it with a good set of panels. You mention 39.7 volts max, but also need to take into account the extra voltage on a cold morning that could boost those panels higher than your SCC could comfortably handle a series of 4. I have not done the math so I don't know if that is just above freezing at 35° F or extreem colds of -40° F, or no factor at all.

I don't see your power requirements in kWh and max draw, but for my RV I will run the inverter at 2000 watts for several hours to run the AC and some other assorted things in the camper. I believe that maxes out a 24 volt build. The only inverter size I saw you mention was 4000 watts, which if you're going to run close to that, this will have to be a 48 volt build.

The only downside to a 48 volt build is DC accessory items are hard to find. I could not find things like USB quick chargers for phones at 48 volts, but this can be solved with some sort of step down converter. I have a 24 volt to 12 votl step down converter for most of my DC stuff like propane blower motor, and a 24 volt fuse box for things like quick USB charge.

 

[MichaelK]

The number of amps needed to run though several paralleled 12V strings is going to be ridiculous. At 1/8th of C, that works out to be 1000Ah/8 = 125A. That's for just one string. If you want four, that works out to be 500A. That would really be absurd!

You are far better managing a single 48V string, but even that will need 125A at 50+V to be happy. That works out to be 125A X 50V X 1.175FF = 7345W, for which you are no-where even close. Multiply your solar panels by 4X, and you'll be getting into the appropriate range.

Feed these batteries at the enemic levels you are talking about now, and they most likely will be dead in a few months. Since you're approaching winter now, maybe even sooner. This is really a grossly unbalanced plan. I would say that 12V is the worst mistake you can make.

 

[Northarmgazer]

Couldn't you send extra charging power to a battery through a separate charge controller.

After about 2 years doing this, I am comfortable where I would want to do a 200 volt build for certain things, like high wattage appliances where I am able to have several panels which would be unshaded.

I have a 24 volt 3000 watt inverter that has 2.5 kw of panels run through 3 different SCCs on my RV. Two of those SCCs are 30 amp SCCs that are maxed out several hours a day. I can really see some advantages to an all in one and matching the voltage into it with a good set of panels. You mention 39.7 volts max, but also need to take into account the extra voltage on a cold night that could boost those panels higher than your SCC could comfortably handle a series of 4. I have not done the math so I don't know if that is just above freezing at 35° F or extreem colds of -40° F, or no factor at all.

I don't see your power requirements in kWh and max draw, but for my RV I will run the inverter at 2000 watts for several hours to run the AC and some other assorted things in the camper. I believe that maxes out a 24 volt build. The only inverter size I saw you mention was 4000 watts, which if you're going to run close to that, this will have to be a 48 volt build.

The only downside to a 48 volt build is DC accessory items are hard to find. I could not find things like USB quick chargers for phones at 48 volts, but this can be solved with some sort of step down converter. I have a 24 volt to 12 votl step down converter for most of my DC stuff like propane blower motor, and a 24 volt fuse box for things like quick USB charge.

Thanks for your constructive reply. In answer to a couple of things you commented on:
The panels will be unshaded all day.
Being new, i have no understanding of why night time voltage would be high on a cold night as you suggest, but also bare in mind, In Australia we do not get the freezing or below temperatures any where in the area I live.
I didnt mention power requirements because it will be so varied and infrequent. For example, it may be a radio on for a couple of hours, but use of power tools would be random and brief, i.e an air compressor might come on for 30 seconds, a drill for a minute, a sander for 10 min. If it were a house it would be a little easier to calculate. The inverters i didnt mention was because they are part of the Growatt units, being either 3000 or 3500w, the 12 or 24v inverters are both 4000w because i cant find any 3000w inverters with AC line out terminals, only built in 240v sockets (bare in mind Australian AC is 240v).
You say about running a 48v system if I was to use a 4000w inverter, but why then is this size available in 12 and 24v?

The largest appliances in the shed are a small Mig welder and a large table saw. The table saw is a 3300w 15a unit and I had already decided to run this I'd have to run the generator so not really an issue.
As you say, running 48v makes sourcing DC appliances more difficult, the biggest issue for me would be the 12v LED lights.
I know the battery size means the whole situation is not ideal, but they were free. For now I have to work with what I've got and add more to the solar array as money permits. Im also looking at putting a wind turbine on to help out at some stage too.

 

[sunshine]

My preference is run a 12 v system based on cost and availability of "accessories" here. (I'm in Australia)

If you go the 12v way, most of the system escapes Australian regulations so there is that consideration, think about converting the panels to 12v ones. Takes less than 15 minutes once you learn and then any PWM would work. Could even directly hook up your 2.5 ton battery to your 6 (now actually 12 after the conversion) panels for the initial charge to the standing voltage after a week or so.

They have added heatsinks to the low cost 50/60amp PWMs now to improve longevity. One 60amp PWM for $25 is theoretically enough for your panels, Get 2 and no worries. Going 12v and pwm means you can keep adding to your system every time you get some more , 2nd hand free due to the Australian governments policies and new are a lot cheaper due to the US embargo.

You can get 12v parts from anywhere and any country but the inverter is best to get here so as to be sure it meets Aus standards.

 

[upnorthandpersonal]

You say about running a 48v system if I was to use a 4000w inverter, but why then is this size available in 12 and 24v?

Technically you can do 4kW at 12V. Just know that this means a 360A current (at 90% efficiency). You're going to need some serious wiring for this.

 

[chrisski]

Being new, i have no understanding of why night time voltage would be high on a cold night as you suggest, but also bare in mind, In Australia we do not get the freezing or below temperatures any where in the area I live.

I worded that bad. Should have said record cold temp on cold morning. The spec sheet would say how much. The ones I have are an extra .37% PMax per degree celcius colder. I'm not sure the starting temp, I think its 25° C. So that would be an extra 3.7 volts at freezing, so not enough to push the voltage above 200 in a series of 4.

You say about running a 48v system if I was to use a 4000w inverter, but why then is this size available in 12 and 24v?

As mentioned , the wiring would get thicker. I had 4/0 for a 12 v 2000 watt inverter, so I'd wire with thicker for 4000 watts at 12 volts. The watage limit is better explained here:

Solar Off-Grid Battery Design - Solar Panels - Solar Panels Forum

Discuss remote solar applications for homes, cabins, RV and boats. If you have a question on equipment for an off grid system, such as charge controllers or inverters, then post your question in this forum.

www.solarpaneltalk.com

As you say, running 48v makes sourcing DC appliances more difficult, the biggest issue for me would be the 12v LED lights.

For me, I'd run a DC to DC converter for the 12 volt side. I use my inverter up to 2000 watts for long periods of time, so that's 100 amps running through it at 24 volts or 200 amps at 12 volts. I got a 24 volt to 12 volt DC step down converter with a low idle draw that has a 89% efficiency and has a remote off switch for when its not in use. A 48 volt to 12 volt step down converter is hard to find, but available.

 

[Northarmgazer]

Couldn't you send extra charging power to a battery through a separate charge controller.

After about 2 years doing this, I am comfortable where I would want to do a 200 volt build for certain things, like high wattage appliances where I am able to have several panels which would be unshaded.

I have a 24 volt 3000 watt inverter that has 2.5 kw of panels run through 3 different SCCs on my RV. Two of those SCCs are 30 amp SCCs that are maxed out several hours a day. I can really see some advantages to an all in one and matching the voltage into it with a good set of panels. You mention 39.7 volts max, but also need to take into account the extra voltage on a cold night that could boost those panels higher than your SCC could comfortably handle a series of 4. I have not done the math so I don't know if that is just above freezing at 35° F or extreem colds of -40° F, or no factor at all.

I don't see your power requirements in kWh and max draw, but for my RV I will run the inverter at 2000 watts for several hours to run the AC and some other assorted things in the camper. I believe that maxes out a 24 volt build. The only inverter size I saw you mention was 4000 watts, which if you're going to run close to that, this will have to be a 48 volt build.

The only downside to a 48 volt build is DC accessory items are hard to find. I could not find things like USB quick chargers for phones at 48 volts, but this can be solved with some sort of step down converter. I have a 24 volt to 12 votl step down converter for most of my DC stuff like propane blower motor, and a 24 volt fuse box for things like quick USB charge.

Thanks for your constructive reply. In answer to a couple of things you commented on:
The panels will be unshaded all day.
Being new, i have no understanding of why night time voltage would be high on a cold night as you suggest, but also bare in mind, In Australia we do not get the freezing or below temperatures any where in the area I live.
I didnt mention power requirements because it will be so varied and infrequent. For example, it may be a radio on for a couple of hours, but use of power tools would be random and brief, i.e an air compressor might come on for 30 seconds, a drill for a minute, a sander for 10 min. If it were a house it would be a little easier to calculate. The inverters i didnt mention was because they are part of the Growatt units, being either 3000 or 3500w, the 12 or 24v inverters are both 4000w because i cant find any 3000w inverters with AC line out terminals, only built in 240v sockets (bare in mind Australian AC is 240v).
You say about running a 48v system if I was to use a 4000w inverter, but why then is this size available in 12 and 24v?

The largest appliances in the shed are a small Mig welder and a large table saw. The table saw is a 3300w 15a unit and I had already decided to run this I'd have to run the generator so not really an issue.
As you say, running 48v makes sourcing DC appliances more difficult, the biggest issue for me would be the 12v LED lights.
I know the battery size means the whole situation is not ideal, but they were free. For now I have to work with what I've got and add more to the solar array as money permits. Im also looking at putting a wind turbine on to help out at some stage too

If you go the 12v way, most of the system escapes Australian regulations so there is that consideration, think about converting the panels to 12v ones. Takes less than 15 minutes once you learn and then any PWM would work. Could even directly hook up your 2.5 ton battery to your 6 (now actually 12 after the conversion) panels for the initial charge to the standing voltage after a week or so.

They have added heatsinks to the low cost 50/60amp PWMs now to improve longevity. One 60amp PWM for $25 is theoretically enough for your panels, Get 2 and no worries. Going 12v and pwm means you can keep adding to your system every time you get some more , 2nd hand free due to the Australian governments policies and new are a lot cheaper due to the US embargo.

You can get 12v parts from anywhere and any country but the inverter is best to get here so as to be sure it meets Aus standards.

Hey Sunshine (?)
Thanks for your input. A lot can change in a day, I got a really good deal on more panels yesterday so I now have another 12 panels coming, but of different size. These are 190w rated V 36.92 5.5a, so another 2200w to add in to the original 6 panels of 1700w.
I've never heard of converting solar panels down to '12v'. Tell me more?
I thought PWM was considered old fashioned and not the done thing anymore, now I have more research it seems.
Where would you source an inverter? There's plenty of online sellers here in Oz, but I have no idea whether they ensure products meet our standards.
I'll get a couple of pics today of the batteries and cables I have so other contributors can see what I'm dealing with.
Very grateful for your input and any other thoughts you may have.

 

[sunshine]

Where would you source an inverter?

Kogan...Giantz 1500w Pure Sine $120 2 years ago.... did a lot of research but this one came up in a google add at a third the price of the one I was aiming for. Running 24/7 since then. Don't let it go into stand-by. always ensuring at least a 40watt load- read in this forum of the problems some have when stopping/starting these hf inverters. Sometimes the fridge cuts it out so that has gone back on mains til I sort it out. TV, PC monitors no problems. Work out what size you need then double it! Even triple if possible.

I thought PWM was considered old fashioned

In-efficiency was the main issue but when PV is so cheap....
Read this forum for all the pitfalls. One major issue was over heating so I replaced the thin tin plate with some old PC heat sinks lying around on the 30amp pwm from AliExpress. Now I see they have the same brand PWM in 50 and 60 amp with a proper heatsink.
I have little modules to regulate the voltage.

I've never heard of converting solar panels down to '12v'. Tell me more?

Just basically cutting the strip , bottom centre, of the panel. Folding the strip back a cm each side and soldering new wires to both exposed sides. Drill a couple of holes in the aluminium frame to ziptie the cables in place and silicone seal where you sliced.
Takeout the by-pass diodes in the box at the top of the panel. These are not needed any more. Nor are blocking diodes in contrast to common opinion. Do this first as checking the voltage across each section gives a good indication of the panels condition if each segment is the same.

Ideally the Voc of the modified panel should be around 20v which would be those that you have. But you can modify other panels differently to get the right Voc.

Just doing a few panels anyway is interesting. I have a few led lights running direct of the panels. In summer they start coming on at 4.30am

 

[Northarmgazer]

For those that mentioned cable sizes, this is an example of the cables I have interlinking cells and also connecting the bank to whatever..
Its 20mm diameter (3/4")