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can i use a 40v solar array to charge a 48v lifepo4 battery?

markyyork

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I found some great 48v solar cells on great sale...only problem, they don't have enough for my needs. They do have some more of same brand and very similar amperage, but they're 40v.

the 48v are 7.5 amp
the 40v are 7.9 amp

If I understand correctly, mixing those would give me a 40v with 7.5 amp array. right?

To my subject question, can I run those thru a 48v inverter and charge a 48v lifepo4 battery array?

thanks
 
You need a higher voltage than the battery to charge. Put them in series to obtain this, but check the voltage limit of your charge controller first. You should get the sum of the two voltages at the average of the two amperages.
 
I found some great 48v solar cells on great sale...only problem, they don't have enough for my needs. They do have some more of same brand and very similar amperage, but they're 40v.

the 48v are 7.5 amp
the 40v are 7.9 amp

If I understand correctly, mixing those would give me a 40v with 7.5 amp array. right?
YES you can mix them. There are some considerations for optimal output - here's one discussion with good info - https://www.altestore.com/blog/2016/08/wiring-different-solar-panels-watts/ There are other youtubes.

Depending on your installation area, shading, charge controller input voltage requirements - there are several factors to consider but a key one is that the series voltage of the panels typically must be higher than the battery voltage - e.g. you'd need at least 2 panels in series for a 48v battery. :)

To my subject question, can I run those thru a 48v inverter and charge a 48v lifepo4 battery array?
You want to run them thru a Charge Controller (not an inverter) - but maybe you're thinking an all-in-one with GroWatt or MPP Solar?

 
Thanks. I did not know you needed higher voltage. Very good information.

Yes, I'm looking at the growatt

12kW 48V 250VDC 120A Off-Grid Inverter by Growatt​

 
Also remember that a 48V LiFePO4 battery will typically be over 56V and most MPPT charge controllers require the PV voltage to be at least 5 volts higher. So you need a minimum of 61V. Put two panels in series to double the voltage. This is based on the panel Voc. However, this total Voc must be lower than the max PV input voltage of your charge controller (your Growatt all-in-one). Note that panel Voc goes up in colder temperatures so a quick check is to add 15-20% to be safe.
 
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rmaddy...thanks for the practical information. The growatt max input voltage is 7000W @ 120A...and max VDC 245V...if I'm reading the chart correctly. Here's a pic of the specs. I'm planning to get the 12KW unit.

At that spec, and with the panels I'm planning on (395W, 40.5V, 9.75A), I should be plenty good with 81 volts and 48.75 amps of panels. I'm looking at starting out with 10 panels. 2 each in series to get the 81 volts, then 5 rows parallel which puts the amps up to 48.75 if I'm any good at simple math. :eek:

Eventually, I'd like to increase that up to the max input available. Let's see if i'm starting to get any of this. 245VDC max input, 7000W max input and 120A max input current. 7000w/395w per panel = 18 panels...Is 7110 watts OK to go over a little?

18panels wired in series of 2 (81v) and parallel rows of 9 = 81 volts @87.75 amps. Should be well under the max except for the 110W over 7000.

Again, Is that 110w overlimit something to be concerned about? Those panels are rated at "up to 405W". I assume that's under full, direct sun. So they might go over even more...at 405W x 18 panels, that equals 7290W total.

This math made me ask another question...Why is a 12K inverter maxed at 7000W solar array input? I was hoping to get at least 80% which would be 9600W. I think I'm missing something because my pea brain is telling me that if I have 7K solar array, that's the most I'm going to get into my home...

Thank you so much!
1637354687485.png
 
The growatt max input voltage is 7000W @ 120A...and max VDC 245V...if I'm reading the chart correctly. Here's a pic of the specs. I'm planning to get the 12KW unit.
You are not quite reading the specs correctly.

The 120A max PV charge current is the max current that the all-in-one will output to the battery. The actual charge current is the current PV wattage divided by the current battery voltage. 120A times 58V (about the highest voltage for a 48V system) is 6960W. So that is a good sanity check on the specs since that matches the 7000W max wattage.

The 7000W max PV array power is of course the max total wattage of panels.

The 250V max PV array open circuit voltage (Voc) is the max input voltage allowed. If you go over this you can fry the charger. You need to make sure your panel Voc times the number in series, adjusted for the absolute coldest temps your panels will ever experience, will never exceed the 250V.

The 60-245V MPPT range is associated with, I believe, the Vmp of the panels. Since Vmp is less than Voc on panels, as long as you don't over Voc you won't go over Vmp. But you do need to make sure you get at least the 60V input or the batteries will never charge.

You need to post the Voc and Vmp of your panels.

At that spec, and with the panels I'm planning on (395W, 40.5V, 9.75A), I should be plenty good with 81 volts and 48.75 amps of panels. I'm looking at starting out with 10 panels. 2 each in series to get the 81 volts, then 5 rows parallel which puts the amps up to 48.75 if I'm any good at simple math.
The voltage is fine. It's well below the max. It's above the min. The amps in this case really only affects how you wire and fuse them. Each series string must have its own fuse and all brought together in a combiner box. As a reminder, that 48.75A value has nothing at all to do with the 120A charge current in the specs.

With 10 395W panels and a 48V battery, your max charge current will be 3950W / 48V = 82A. Make sure your batteries can handle a 82A charge current. More often your batteries will be closer to 52-54V so your typical max charge current will be lower.

Eventually, I'd like to increase that up to the max input available. Let's see if i'm starting to get any of this. 245VDC max input, 7000W max input and 120A max input current. 7000w/395w per panel = 18 panels...Is 7110 watts OK to go over a little?
18 395 panels does give you a little extra wattage. But it is pretty rare to get 100% of the rated wattage from a panel. It's common to "over panel". One simple solution to guarantee you'll avoid the rare time of getting the full wattage is to point half your panels more easterly and the other half more westerly (or some similar sort of setup). This gives you more coverage during the day and it avoids all panels hitting 100% midday.

18panels wired in series of 2 (81v) and parallel rows of 9 = 81 volts @87.75 amps. Should be well under the max except for the 110W over 7000.
Again, that 87.75A isn't related to the 120A in the specs so it's not an issue.

Unless you have a lot of shading issues, you really should strive to get more panels in series. The higher voltage will really help in lower light conditions. Maybe use 12 panels in 4S3P instead of 10 in 2S5P. Or 9 panels in 3S3P. Max out at 18 panels in 3S6P or go for 20 panels in 4S5P. Even though 20 panels is well over 7kW you can aim those 5 parallel strings in 2-5 different directions.

This math made me ask another question...Why is a 12K inverter maxed at 7000W solar array input? I was hoping to get at least 80% which would be 9600W. I think I'm missing something because my pea brain is telling me that if I have 7K solar array, that's the most I'm going to get into my home...
It's very common for the inverter wattage to be much higher than the solar wattage. But real numbers are needed to make it clear.

How much of the inverter do you really need each day? Have you done a full energy audit and worked out your actual Wh/day usage for both AC and DC?

How much battery are you planning?

The energy audit is key to making sure you have enough battery and solar. You also need to be realistic about your usage patterns and the weather. Do want enough battery to run everything for 3 days of no sun and no grid power, for example?
 
OK. I think I'm getting it...at least I'm understanding more than I did...you are a wealth of knowledge. Can't thank you enough.
the Vmp is what I was lookin at around 40.9v. The Voc is around 49.2. So that would mean 4s3p should get me pretty decent power for starters and 50Voc of cushion. The spec sheet is below. Also the battery spec sheet.

battery says 100A constant max charge current. The nominal voltage is 51.2V and charging voltage is 58V.

As far as the audit, I've listed everything that "might" run at the same time anytime during the day or night (my wife is a night owl artist). I'm calling those continuous. If the big ac and the 2.5 hp water well both surged at the same time AND everything else was running, it could hit close to 34K. If a fridge or window ac or anything else surged at the same time as the ac and well...I would hope the fuses and breakers would do their job. With no surge, if everything was going it would be around 20K.

Then there are what I'm calling intermittent. Blender, drill, microwave, instapot, air compressor etc...Those add up to at least 20K with likely surge of no more than 3-5K. I would absolutely do I think they call it shedding...turn off fridges, ac's, water heater, etc whenever I would use intermittent equipment. That's where I came up with the need for the 36K surge capability of that inverter.

I plan to time the refrigerators, freezers, window ac/big ac, to lower the total draw. I also plan to put a switch on the water heater to only come on before we need it...assuming we're totally off grid and surviving on only solar. That beast is around 5500Kw. Same thing for my 5ton ac I rated at 7K in my chart. Nominal voltage says 230 with max 30A fuse so I bumped that up to 7K. Turn off those two and half my usage...way more than half my surge. At 2.5x surge, the ac comes in at 17500.

I'm also working on a large water storage system that would allow me to only run the well intermittently. I'm working on 1200 gallons under pressure as well as raised for gravity feed if needed. We'd get a few minutes with pressure if the well was off but quite a long time with fair pressure with the well off. That could be a big bonus since it's a big hog at around 2K and 5K on the surge. the only time I'd really need (want) high pressure is for showers. Everything else doesn't need high pressure. With that much water stored higher than the tap, you already get pretty good pressure just from gravity.

We live in S Texas so we have lots of sun...which is good BUT we have LOTS of need for refrigerated AC with humidity often in the 90% range in the summer. Some years...like last year!...we need a week or so of constant heat. Most years only really need (want) constant heat in the night and a little in the day.

I plan to have as many batteries as I can afford in the next 4 years...that's my plan...to be done in 4 years. A second inverter is not out of the question some day...just not this day! As for the batteries, I assume same brand and model is optimal...Is it "bad" to mix other brands? I would only want lifepo4 but might find better deals on different brands. With my limited electrical knowledge, I would think as long as the specs are close, mixing brands would be OK...??

My average usage last year was 83Kwh per day (2485/mo). With the hvac being the biggest draw by far, that can mostly be spread over 24 hours pretty evenly since it runs day and night in the extreme seasons. I don't know what that comes to in DC. Haven't learned that far yet.

I think 12 panels in 4S3P would get me around 4800W at the Vmp.

Using Voc and Imp, 196.8V at 9.78A per series row and 196.8V at 29.34A with the 3 series wired in parallel. Max watts = 5774W. Did I get that right?

Adding another 8 panels in same configuration would be and additional 3849W max.

3849 + 5774 = 9623W. Here are some numbers I found in a peak sun hours by zip code chart.

Jan: 3.94
Feb: 4.26
Mar: 4.82
Apr: 5.43
May: 5.68
Jun: 5.83
Jul: 5.86
Aug: 5.98
Sep: 5.64
Oct: 5.18
Nov: 4.4
Dec: 3.82

NO SHADE in my planned mounting area. Would the 20 panel arrangement be safe pointed in different directions?

1637366476581.png1637366502208.png
 
For a proper energy audit please see:


Post the results section when done.

The Voc is around 49.2. So that would mean 4s3p should get me pretty decent power for starters and 50Voc of cushion.
A lot of that cushion will be eaten up as the panels get colder. But you would need temps below -40 to use up the whole cushion.

battery says 100A constant max charge current.
Your all-in-one can put out up to 120A with 7000W of PV. So make sure you limit the all-in-one to 100A charge current. This may be configurable even if you have enough solar to generate more.

I plan to have as many batteries as I can afford in the next 4 years...that's my plan...to be done in 4 years. A second inverter is not out of the question some day...just not this day! As for the batteries, I assume same brand and model is optimal...Is it "bad" to mix other brands? I would only want lifepo4 but might find better deals on different brands. With my limited electrical knowledge, I would think as long as the specs are close, mixing brands would be OK...??
You will be adding 48V batteries in parallel. Ideally they would be the same brand and capacity. Different brands with the same capacity should work well enough. So if you start with some 48V 100Ah battery of one brand, at least add another 48V 100Ah battery. It would help if they all support the same max continuous charge and discharge currents. But if not, just make sure your system is setup to only charge and discharge to the lowest value amongst the batteries.

My average usage last year was 83Kwh per day (2485/mo). With the hvac being the biggest draw by far, that can mostly be spread over 24 hours pretty evenly since it runs day and night in the extreme seasons. I don't know what that comes to in DC. Haven't learned that far yet.
83kWh/day is a lot. In order to run that much just on battery for 24 hours (assume no grid and dark clouds so no sun) you would need 83,000Wh / 52V = 1600Ah of battery capacity. That's 16 of the 48V 100Ah batteries. Probably 18 since you don't want to use 100% of the battery capacity. That's probably expensive. Clearly you will want to find ways to cut back on a lot of usage on such days.

All of your info so far is all about AC loads running off of the inverter. That may be all you end up with in the end. DC would be stuff you can run off of the battery without an inverter.

I think 12 panels in 4S3P would get me around 4800W at the Vmp.

Using Voc and Imp, 196.8V at 9.78A per series row and 196.8V at 29.34A with the 3 series wired in parallel. Max watts = 5774W. Did I get that right?
I'm using the 395W panels for the following numbers. 12 395W panels is 4740W. The Voc is 49.03V. At 4S it would be 196.12V. A bit below freezing that would probably be around 230V so you are good. The Vmp is 40.73V. At 4S that's 162.92V. Imp is 9.70A. At 3P that's 29.1A. Max watts is 4740W (12 x 395W or 162.92V x 29.1A). Each panel wattage is basically the panel Vmp x the panel Imp. Don't use Voc to calculate wattage.

Adding another 8 panels in same configuration would be and additional 3849W max.

3849 + 5774 = 9623W.
8 panels is an additional 8 x 395W = 3160W. 20 panels would be 20 x 395W = 7900W.

NO SHADE in my planned mounting area. Would the 20 panel arrangement be safe pointed in different directions?
If you did 4S5P such that the strings of parallel panels pointed in 2-5 slightly different directions such that you never went over 7000W actual watts reaching the charge controller at any given moment then you would be just fine. It would be a bunch of trial and error to find just the right angles to maximize the daily input while keeping it under 7000W. And it would probably be a little different depending on the time of year due to sun angle.
 
Hi rmaddy! Bet you thought you lost me!! NOPE! Just been so busy with year end holiday stuff. Toy runs, grandkids' parties, etc. I pushed the button on the audit just now and I'm going to get that filled in. I'm pretty sure I'm going to go with 12 panels in 4S3P and start out with one 48V 100AH battery running thru the big growatt. I'll post my audit and see what you think. MERRY CHRISTMAS!!
 
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