24V panels - 12V Batteries

[TrevorP]

I bought two 2nd hand 200W 24V panels (they were cheap) with the aim of charging a couple of 12V LiFePO4 batteries and setting up a small PV system in my workshop.
I jumped onto the Renogy store and ordered the batteries, a Rover 20A MPPT controller which should handle the ~8A max output of the panels and a 1000W 12V / 240VAC inverter. After learning a little more about real-world PV I find that a 20A controller is too small to charge a 12V system from 24V input (still don't know why that is) and I should have ordered the 40A one.
My two options seem to be to charge the batteries in series and take the hit on the inverter - I'd have to get a 24V / 240VAC one or get another converter.
What's the best way to go?

 

[Rednecktek]

OK, don't panic, you'll be fine.

So, what you're looking at is wattage since you have an MPPT controller (Yay!) which is what you'll be working from. For some rough napkin math, 400w of panel / 12v battery is 33.3a of maximum charging capability. That's why the 40a would be the way to go. Now, you can still charge because the real world is different from math, you're just nerfed on how much you can actually put to the batteries. In your case, 20a * 12v = 240w, or just over half your theoretical capacity.

The best bang for the buck is going to be to just get a larger MPPT controller so you can not only utilize the full panel's capacity, but the cost difference between a 40a and a 60a isn't that large and you can add more panels later.

As for the panels being "24v" panels, Wwweeelll.. While it's true there are no "24v" panels, it actually does mean something. In "Marketing-Ese":

A "12v Panel" will provide enough voltage to charge a 12v battery, but not enough to charge a 24v battery.
A "24v Panel" will provide enough to charge a 24v OR a 12v, but can't feed a 36v or 48v setup.
A "48v Panel" can do 48v down to 12v with the proper controllers.

 

[Zil]

What is the maximum voltage that the MPPT can handle from the panels? 20 amp vs 40 amp is just a longer time to build the charge. It is important that the CC can handle the voltage from the panels.

 

[DerpsyDoodler]

OK, don't panic, you'll be fine.

So, what you're looking at is wattage since you have an MPPT controller (Yay!) which is what you'll be working from. For some rough napkin math, 400w of panel / 12v battery is 33.3a of maximum charging capability. That's why the 40a would be the way to go. Now, you can still charge because the real world is different from math, you're just nerfed on how much you can actually put to the batteries. In your case, 20a * 12v = 240w, or just over half your theoretical capacity.

The best bang for the buck is going to be to just get a larger MPPT controller so you can not only utilize the full panel's capacity, but the cost difference between a 40a and a 60a isn't that large and you can add more panels later.

As for the panels being "24v" panels, Wwweeelll.. While it's true there are no "24v" panels, it actually does mean something. In "Marketing-Ese":

A "12v Panel" will provide enough voltage to charge a 12v battery, but not enough to charge a 24v battery.
A "24v Panel" will provide enough to charge a 24v OR a 12v, but can't feed a 36v or 48v setup.
A "48v Panel" can do 48v down to 12v with the proper controllers.

there's more to take into account here. If you put panels in series, 24v panels can indeed charge a 48v battery.

 

[DerpsyDoodler]

What is the maximum voltage that the MPPT can handle from the panels? 20 amp vs 40 amp is just a longer time to build the charge. It is important that the CC can handle the voltage from the panels.

It's not just a longer time to charge, it's also lost power. If your panels can produce 400w, but you can only harvest 200w, then you are wasting/losing power that could otherwise be put to use. Granted 400w of used panels probably won't put out 400w, but they should surely put out more than 240.

 

[Rednecktek]

there's more to take into account here. If you put panels in series, 24v panels can indeed charge a 48v battery.

Yes, but A (as in single) 24v panel can't. Likewise that's like saying that A single 12v panel can feed a 48v system (if you connect 4 of them).

The point was translating from Marketing to Engrish.

 

[TrevorP]

The cost of education - I'll get the 40A controller.
BTW I hear (from one of Will's vids) that batteries don't like to be connected/charged in parallel. I'm hoping that only applies to more than 2 or 3 batteries.

 

[DerpsyDoodler]

In what video did he say that? I'm inclined to say there were probably other details you missed. Charging batteries in parallel is fine as long as you make sure the current is balanced.

 

[Zil]

LiFePo can take higher amperes during charge. That is why it is better to have higher ampere CC.
Battery storage is measured by amp/hours. It doesn't really translate to watts. A device uses watts, measure of work. The amperes involved in watts changes as the voltage changes. The amp/hours is the same to the battery capacity no matter the voltage. We must return the amp/hours to the storage that we removed from the storage. Slow charge, lower amperes, takes longer to replenish amp/hours than fast charging, higher amperes. The rub is we have a limited time to gather amperes from limited solar panels. So the higher ampere CC is better. Up to the charging limits of the battery.
Some battery can be unhappy with parallel. You will have to see what those "drop-in" batteries recommend.
Depending more on how the cells are configured. Think of how many parallel cells are in a EV.

 

[DerpsyDoodler]

LiFePo can take higher amperes during charge. That is why it is better to have higher ampere CC.
Battery storage is measured by amp/hours. It doesn't really translate to watts. A device uses watts, measure of work. The amperes involved in watts changes as the voltage changes. The amp/hours is the same to the battery capacity no matter the voltage. We must return the amp/hours to the storage that we removed from the storage. Slow charge, lower amperes, takes longer to replenish amp/hours than fast charging, higher amperes. The rub is we have a limited time to gather amperes from limited solar panels. So the higher ampere CC is better. Up to the charging limits of the battery.
Some battery can be unhappy with parallel. You will have to see what those "drop-in" batteries recommend.
Depending more on how the cells are configured. Think of how many parallel cells are in a EV.

You're confusing Wh with Ah.

As the saying goes, Watts are always watts. 5Ah at 12v is not the same as 5Ah at 24v. The 5Ah at 24v have double the power. If I have 2x 12v 100Ah batteries, in parallel, I have 200Ah at 12v or a total of 2400Wh. If I combine those 12v batteries in series, I have 1x 24v 100Ah battery but still 2400Wh of power available.

If I had 2x 24v 100Ah batteries (200Ah @ 24v) then I have 4800Wh of power.

 

[TrevorP]

In what video did he say that? I'm inclined to say there were probably other details you missed. Charging batteries in parallel is fine as long as you make sure the current is balanced.

I'd say there's a lot in all his vids I've missed but this is the one I was looking at - around the 2:42 mark.
I've ordered a 40A controller but could I get the best use from the panels by just connecting two 12V batteries in series?

 

[DerpsyDoodler]

I'd say there's a lot in all his vids I've missed but this is the one I was looking at - around the 2:42 mark.
I've ordered a 40A controller but could I get the best use from the panels by just connecting two 12V batteries in series?

That was early in his youtube career. He changes his view in later vids.

If you had more power in panels than could be pushed in 40amps @12v, yes. But you don't. 400w / 12v = ~33A.

You could cancel the 40A controller, stick with the 20A controller, connect your batteries in series for 24v and have 400w / 24v = ~16A of charge current.

The amount of time it takes to charge depends on the battery capacity. Lets say you have 2x 100Ah 12v batteries in parallel, so 200Ah @ 12v. Lets convert that to watthours. 12v x 200Ah = 2400Wh (this would be the same number if those same batteries were in series for 24v). Assuming you can harvest max PV output (400 w), your time to charge would be 2400Wh / 400w = 6 hours.

6 hours to charge, assuming a fully depleted battery, you can pull full power from PV, and not consume any power while charging.

Your real-world power production numbers will probably be lower and ideally you won't always be charging a fully depleted battery.

Did you follow the numbers? Is all of that clear?

 

[TrevorP]

Yep - that's all clear now and thanks for the education.
If I'd known all this before I went down the 12V road, I'd probably have gone with 24V, though that may have turned out to be a more expensive option. I've already ordered a 12V inverter and the 40A controller - I could still use the 20A controller if some cheap 12V panels come my way.