4 x 12V 28Ah SLA ....Max Solar Panels recommend?

nick777

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I have four 12v 28Ah SLA's powering a 12v landscape system now with a 100w panel and 10a charge controller.

I'm wondering if I can hook up say 2kw of solar panels with a 24v charger controller and inverter and power a small 500w dish heater during the day?

Thanks in advance.
 
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schmism

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They way you have phrased your information suggest you want to somehow combine the 12v 28ah batteries and 100w pannel you currently have with 2kw of new panels with a 24v SCC.

You can not mix these 2 systems.

The 2kw of solar on a 24v SCC is fine. Its a little unclear what a 500w dish heater is. Is that a mains voltage appliance or something you can power directly off the 24v?
 

nick777

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What I'm looking to doing is not combining the 100w panel and 10a charge controller, but rather upgrading the inverter and charge controller to a 24v system. I'm wondering what the max charge controller and inverter I hook up to these smallish batteries without overloading them. I have 10 250w panels so looking to utilizing these. A dish heater is a small 120v space heater which uses about 500w of power which I plan to use during the day when panels are producing power charge to the batteries and provide power for the heater.
 
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Hedges

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four 12v 28Ah SLA's

In parallel, that's 12V 112 Ah

You can rewire it as 24V, 56 Ah

Either way, 1344 Wh

I'm wondering if I can hook up say 2kw of solar panels with a 24v charger controller and inverter and power a small 500w dish heater during the day?

SLA probably wants to be charged at 0.1C, 0.2C, something like that. About 135W to 270W
If you have 2kW of PV, that would be charging at 1.5C, which would fry the battery.

Maybe if you get a hybrid (MPP, etc) with PV input, battery, and AC output, it can be programmed for 10A battery charge current while processing the extra PV power to feed inverter.

I get that sort of control in my larger system which is AC coupled.
 

nick777

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Thanks for the feedback. I didn't know SLA's had such a low charge rate requirement. I'll hook up 1 panel to the existing batteries, and will have to get some more robust batteries, possibly fork lift batteries for the remaining ten 250w panels. Can you all recommend a decent charge controller and 2000w inverter? Maybe 3 different grades....affordable, mid-grade and top notch brands?
 

Hedges

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The manual for my SunXtender AGM says it can tolerate up to 5C inrush (page 20), but not clear what it can take sustained without damage. I charge at 0.2C, which is the lowest without having to alter charge profile to ensure it gets fully charged.



Forklift batteries meaning FLA? They probably have a preferred charge rate around 0.12C, to stir the electrolyte. Find a technical manual for it.
Rolls Surette has batteries people report equally long life from (like 15 to 20 years.)

This link and the one before it list a number of charge controllers.


I'm not entirely up on inverters of the 2kW size. I use SMA which is 6kW. Victron, Outback, Schneider are top names, and some models could be around that wattage. MPP has a economical hybrid, one is 24V. A thing to watch for is no-load current draw, because inverter can drain a small battery. Some have an idle or power-save mode that wakes up periodically to check for loads.

I think I've heard that Victron using Cerbo GX and a battery shunt can manage SCC, regulating battery current while supplying extra for inverter as needed. That would matter mostly if you have more available PV current than battery can accept. Less of a problem if you use lithium because it can usually accept 0.5C (while at moderate temperature.)

With four batteries, could also do 48V. Not much Ah, but if you had an over-paneled system that regulated battery current, could run large loads while the sun shines. You would probably upgrade it later.
 

nick777

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Thanks Hedges for your insights. Yes forklift batteries are flooded and I believe they can be charged at around .2 - .3C.

Now I have a Victron MMPT 100/12-48v (20amp), and I know I can hook up one 250w panel for the current 12v battery configuation. My question is can I hook up two of my 250w panel to this? Manual states that for a 12v setup, "Nominal PV power" for 12v is 290w...see picture below. Does that mean I can hook up only one of the 250w panels? I thought since this is a 20a charge controller, I can hook up to 20 amps worth of solar panels. My 250w panels states imp of 8.33a.

Maybe I'll run it in a 48v configuration like you said with 4 panels. So four of the 250w panels right?
 

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Hedges

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A 20A charge controller can deliver 20A to the battery. At 12V that would be 240W, at 15V 300W. The PV panel can be a rated a bit higher (STC) because it normally produces less (PTC), without clipping.
You can generally overpanel a MPPT SCC, it just doesn't draw more current than it wants. The picture says 20A Isc max. Your 8.33A Imp panel is probably only about 10A Isc, so could connect two in parallel. but SCC won't deliver more than 20A to battery.

Usually we connect several panels in series. But need to consider Voc and that the voltage goes higher in cold weather. Use 20% increase for a safe estimate, or get temperature coefficient from panel data sheet to compute more accurately.

You should be able to put two panels in parallel and orient them differently, like SW and SE with a 90 degree angle between them. That will reduce peak current and increase hours of operation.

48V, 4 panels should fit 20A limit with all same orientation. Using multiple orientations 6 panels could be used. I don't think you can do 3s2p with those panels (what is Voc?) and the 100V limit of SCC, so 2s3p.
 

nick777

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Ok thanks, got it.
That's what I was considering...one facing SW and the other facing SE with the angle.
VOC on these panels is 37.5
Ok but with the 48v, 4 panel setup, that would be pushing too much current to my SLA batteries right? Unless I can lower the current on the Victron?
 

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Hedges

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2x 37.5V = 75V, don't even have to do math to know it will never get over 100V in cold.
3x is over 100V under nominal conditions.

8.68A Isc, 2x is still under 20A limit, 3x is over.

You can only do 2s, and looks like only 2p, so 2s2p max array.
Although, with multiple orientations, maybe could do 2s3p since not all would be at Isc at same time. Not as obvious what angles to use.

Exceeding Isc doesn't always kill the charge controller, maybe if it goes into a protection mode (over-voltage crowbar or reverse polarity.)

I would rather have higher amperage panel 2s2p, where the angle reduces array current. (or lower amperage 2s4p)

1344 Wh of lead-acid battery wouldn't want 1kW or 800W of charging.
Having battery bank 12V and PV array 1s2p with 60 degree acute angle between panels seems like the max (if current isn't regulated.) At 60 degrees, area presented to sun is never more than one panel. At 90 degrees, 1.4x one panel, seem a bit high unless battery can accept 0.25C

Are those Gel or AGM? Different charging voltages and max currents for them. Try to find the manual.
 

nick777

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2s2p sounds like a plan. I guess for summer use, I would have to make sure there's a pretty good angle away from the sun?

Looks like the batteries are gel. Manual below...
 

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Hedges

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I only see charge info in a graph. Maybe another document has words and numbers.
Looks like 0.25C constant current charging, to approximately 14.75V (which needs temperature compensation, of course.)
I think two panels at 90 degree angle will be about that (for the season when panes are lined up with the sun's arc)

Looks to me like 1s2p is all those batteries should get, unless charge controller has a current limit setting. Then 2s2p could work, and would maintain current during other seasons as well. But no extra power would be available for loads, unless something is monitoring battery current and talking to charge controller. .
 

nick777

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I see just a graph as well. But on the left side of the graph in very small print, the bulk charge indicates .25c so you're right on about that.

Well I'll go with just 2 panels then for a 1s2p setup with a 24v battery setup.

Also if I wanted to hook up my existing 12v landscape lights, would I need to get a stepdown transformer...something like this should work?
https://www.amazon.com/EPBOWPT-Conv...90&sprefix=24v+to+12v+ste,aps,501&sr=8-3&th=1
 

Hedges

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DC/DC converter, not "transformer"
Looks like that should work.
 

nick777

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Quick question. For a 1s2p setup, would 12ga wire be ok for a 30' run to the charge controller? Or would i need 10ga wire?
 
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