Solar panels for 48v battery system

[Sisuranger]

Thanks to the help of this forum, I have my 48v system running. I am currently charging with a generator but want to add solar panels. I have an aims 48v/4000w inverter charger and 2 24v battery evo 100ah lifepo batteries in series. I know nothing about solar panels. I'm in northern minnesota and we have 8 to 10 hours of decent sun when it will be used the most. What do I need for panels to charge this system in a day and what are the recommended panels and charge controller? Since I also have the generator, a full charge wouldn't be necessary. I may also double my battery capacity. Thanks!

 

[rmaddy]

2 24V 100Ah batteries is 5,120Wh. If you need to recharge that much in 8 hours then that would be 5120Wh / 8h = 640W. But you are not likely to get 8 hours of good sun in one day, even on June 21st. But you are also not likely to need to fully recharge 100% of the battery in a day. Maybe 80%? So let's say 4096Wh in 6h. That's 682W. No one gets 100% out of panels so round it up to 800W of panels. Panels are cheap. At least compared to batteries. Do note that in December you will be lucky to get 1 hour of really good sun. You can certainly get some power all day on a December sunny day but you'll only hit full power for an hour where you are.

Depending on how and where you need to mount the panels you could get 4 200W panels, 2 400W panels, or maybe 3 300W panels (more is better). You could always add more panels later if needed (like when adding more battery).

 

[Sisuranger]

Thanks for the response. Your right about Mn in the winter. Does it matter what voltage the system is when you choose the panels or is that just taken care of in the charge controller? ie, is there a 48v panel vs. A 12v panel?

 

[740GLE]

While you have a nice inverter/charger and the beginning of an amazing system, guessing at what panels/SCC is needed is pointless until there is a load audit.

Is this place inhabited daily year round? Or seasonal on weekends? What’s your current daily KWhr usage? How often are you using the generator to charge the batteries? What do you foresee the load possibly being in a year or so?

Why do you foresee the need for additional batteries?

For guessing without knowing crucial info, I’d say over build the solar and go for 1500-2000w with at 60/80a SCC, this will semi future proof your needs so that you could add more panels as your demands grows.

 

[rmaddy]

If you use an MPPT charge controller then as long as you setup the panels such that the resulting Vmp of the panels is about 5 volts higher than the current battery voltage then you are good. Since a typical 48V battery can be as high as 56V, you need a voltage of at least 61V. Putting a few panels in series will easily achieve that. When looking at panels, ignore any reference to them being marketed as "12V" or "24V". That's irrelevant when using an MPPT controller.

 

[740GLE]

Thanks for the response. Your right about Mn in the winter. Does it matter what voltage the system is when you choose the panels or is that just taken care of in the charge controller? ie, is there a 48v panel vs. A 12v panel?

You’ll likely be running at minimum of two panels in series such that VMp is above 60v. You could run 8 panels in series and have much higher voltage if the SCC can support it.

 

[RCinFLA]

This is statistical hours of full sun equivalent per day for each month of year for Grand Rapids.

This is per square meter and has to be derated based on PV panel efficiency per square meter.

You can also take the wattage panel spec has at full sun intensity times number of hours per day in chart to get kWH per panel.

You do good in summer (5-6 hours) but terrible in winter (2 hours), with clean, no snow covered panels. Summer heating degrades panel output a bit, cold winter temp helps panel output a bit but can't make up for low sun angle above horizon which has greater atmospheric attenuation.

 

[Sisuranger]

Wow, great info everyone! I mainly use the cabin from Sept through November and occasionally feb through April. My main loads on battery are led lights, tv, coffee maker, and other small appliances. I occasionally run a pump to fill a holding tank but I do that when the generator is running and also charging the batteries. I am able to get through the night without needing the gen so maybe more capacity isn't necessary. It sounds like I'm not going to get great solar during my usage times so it would be more of a supplement. I was hoping I would generate enough solar power to keep a small heater in my insulated generator/battery shed to keep the batteries from getting too cold. Doesn't sound like that is feasible. It gets to -40 F where I am at. I'll start researching some products based on the comments.

 

[circus]

I was hoping I would generate enough solar power to keep a small heater in my insulated generator/battery shed to keep the batteries from getting too cold. Doesn't sound like that is feasible. It gets to -40 F where I am at.

Maybe a more direct heater would do. January in Wisconsin, when it's >20° it's overcast, when it's <0° it's sunny. Bet your the same.
Solar air heater

 

[740GLE]

So you don’t have a heater for the batteries and they aren’t in a heated enclosure now?

You are aware of the issues of charging LFP batteries if the cell temps are under 32f and you try to charge them, correct?

A small 20-50w heater for the batteries is def do able with solar supplement. If the batteries are in a insulated box, it doesn’t take much to keep them warm enough even if it is super cold out.

 

[RCinFLA]

Link below is to a good spreadsheet from New Mexico State Univ. You have to put in your latitude and longitude and date/local time. It defaults to latitude angle for fixed panel south facing tilt angle. There is a link to spreadsheet instructions on first title page of spreadsheet.

It does calculations based on atmospheric density, diffusion, and defraction. Assumes clear sky with no local weather factors included in calculations.

First spreadsheet I have seen that actually predicts insolation immediately before sunrise and after sunset, which is most relevent to predicting when cold morning maximum Voc may show up at your SCC. It is earlier then you might think which usually means it is colder than you might think compared to later in morning.

http://aces.nmsu.edu/pubs/_circulars/CR674/CR674.xlsm

 

[Sisuranger]

Since I didnt have the heat figured out yet, I brought the batteries home with me this fall. I've done some research on insulated boxes and may go that route, I wanted solar first so they can charge the batteries. The batteries are too heavy to haul around. Any suggestions on heated box designs and heaters?

 

[OffGridInTheCity]

In general terms, there's a ration of PV -> battery -> consumption to achieve various goals. For example, its not a good match to have 13,000w PV array on a 5kwh battery as the battery won't run very far into the night and most of that PV power will be wasted unless you have insanely high consumption.

Let's say you're middle Minnesota at Brainerd. Further, on a 5kwh battery and 1500w average load (3000w heater on/off) and 3000w of PV (10 panels @ 300w each). Here's PVWatts https://pvwatts.nrel.gov/pvwatts.php numbers for Brainerd -


For a 3000w PV array and you take December, that's 172kwh / 31 days = 13.2kwh/day. At 1500w/hour, that's enough for 8.8hrs of heat on average. Maybe you can heat 4hrs every 12? or low temps? Its helpful to clarify the consumption you need with hard numbers.

If you double to 6000w PV you'll need to double/triple your battery to 10 or 15kwh as well and you could get up to 18hrs of an average of 1500kw/hour on average. Of course there are days of intense clouds that could put it at 3kwh/day and this is where a larger battery might let you supplement for a day or 2.

The numbers above here are in the 'relm' of reality but of course you're location, and consumption are not known as I write this.

For example, I run a 13kw PV array with an 81kwh battery in Southern Oregon and average 16kwh/day. This winter has been brutal - for example, over the last 4 days I only got 6.5, 6.2, 11.5, and 6.0 kwh - that's 4 days of total clouds and well below 16kwh/day average. I'd need 35kwh of battery to make maintain the average 16kwh day - and that's a LOT of battery. A day is one thing but multiple days of 'storm' / 'heavy clouds' sometimes occur and its difficult to cover with no PV.

Just sharing this to help move the conversation along...

 

[740GLE]

Don’t think he’s looking for PV for heating living space, more of PV heating the battery then supplemental PV generation to top up the batteries.

But alas until an audit is completed, including a battery heater, the size of the PV array and total battery bank can’t be really estimated.

It might be good for the OP to snag a shunt to verify his daily kwhr demand as an easy stop gap.

 

[Sisuranger]

Ok, lots of info here. Yes, I originally thought I could heat the space where the batteries are. Doesn't seem feasible so I'm looking at just heating the batteries in an insulated box. It makes more sense to just use the battery power without going through the ac inverter. My new problem is all the nice cheap heat pads are 12v and I have 48v. I'm stumped again, any suggestions?

 

[OffGridInTheCity]

Ok, lots of info here. Yes, I originally thought I could heat the space where the batteries are. Doesn't seem feasible so I'm looking at just heating the batteries in an insulated box. It makes more sense to just use the battery power without going through the ac inverter. My new problem is all the nice cheap heat pads are 12v and I have 48v. I'm stumped again, any suggestions?

That one's easy - there are several cheap 48vdc -> 12vdc converters. Heating pads like these - https://www.amazon.com/gp/product/B07GW8ZX7X/ref=ppx_yo_dt_b_search_asin_title - are pretty light on watts (amps).

These range from a couple of amps to 30a such as
- 10a https://www.amazon.com/HOMELYLIFE-Voltage-Reducer-Converter-Transformer/dp/B008CM4WZ2/ref=sr_1_5
- 30a https://www.amazon.com/LEDMO-Converter-Regulator-Reducer-Adapter/dp/B089Y49LWT/ref=sr_1_3
- 60a https://www.amazon.com/720W-Converter-Voltage-Regulator-Waterproof/dp/B081GMD5HZ/ref=sr_1_2
etc...

I'm sure you can find a 48vdc to 12vdc converter to run the heat source for a battery box once you figure out the heat/power you need

 

[740GLE]

4 12v heating pads in series? 4x the heating?

 

[Sisuranger]

Thanks for the suggestions. Like I said, I'm still learning. I think I'll go with the 48vdc to 12vdv converter with heat pads in the insulated box. I'll test it out before I actually put the batteries in it. May have more questions along the way. Thanks again!

 

[labeeman]

Thanks for the suggestions. Like I said, I'm still learning. I think I'll go with the 48vdc to 12vdv converter with heat pads in the insulated box. I'll test it out before I actually put the batteries in it. May have more questions along the way. Thanks again!

You will need a thermostat that can handle DC as an AC one will just arc and start a fire which will give you too much heat.

 

[UpNorthMan]

There is one other thing that you need to keep in mind. If you put your panels in series, watch the maximum input voltage.

As a fellow cabin owner just south of International Falls, it gets darn cold here! As the temperature drops, the panel VOC rises! I have a 48v system with the panels wired 5s2p. At standard test conditions the VOC is about 22v per panel. Or 110 VOC when in a series. The record low in my area 20 miles away was around -62F. At that temperature, the VOC is around 150v. I decided to use a Midnite Solar Kid as my charge controller. I believe it can handle up to 156v. It was recommended that with fla batteries and a mppt cc. I use a minimum of 1.3 Vpm times the maximum charging voltage. Which figured to around 88 Vmp. At around 18v Vmp per panel, I had to go to 5 series. Just before sunrise there is a possibility of a large spike of energy hitting your panels. The energy is absorbed by your panels as high voltage but very little watts. Nothing usable for charging your batteries. This spike of voltage can destroy your cc if your system isn't designed for it.

I'm not trying to scare you off, but a lot of people aren't use to the cold weather we have around here. It is a factor that needs to be put into the equation. I would guess that the low in your area is around -55F .

Midnite solar has a article that explains it.

https://www.midnitesolar.com/pdfs/whyHyperVOC.pdf

Ed