Morningstar says Go Big, You can't Hurt Me!

[BradKW]

First, it looks like going with a 24v battery bank makes a lot of sense for me. Actually, from what I'm learning, it might make a lot of sense for lots of people who never considered it: smaller wires, greater efficiency, and much better (24v) inverter performance. Oh, and smaller charge controllers needed. I'd really like to hear any reasons not to, as I haven't found them.

But anyway, I found a nice little tool on the Morningstar site, it's called a String Calculator. You input your panels, battery voltage, and CC you're looking at and it spits out lots of useful data. This link should take you to it with my input in the fields with a Tristar 45: http://string-calculator.morningsta...ax=31&tmin=45&tmax=82&tminunits=f&tmaxunits=f

By swapping CC's and battery voltage, you can see in chart where is best. You can click on the colored numbers for more details. Where I started with a 12v bank and 975W exceeded the top product (Tristar 60), by going to 24v bank I can use the Tristar 45 and be in the green.

But here's an interesting thing. Click on RED fields where "you've exceeded CC capacity" and you go to more detailed page, with bold letters that tell you:  

"Maximum Nominal Solar Input Power exceeded for this controller. Controller output current will be limited, reducing power harvest below Maximum Power Point levels. Full output of array will not be realized. No risk of damage to controller in this configuration."

Click on More Info and you goto a whitepage that goes into detail how overpaneling is just fine, thank you, and often is actually a good idea: http://www.morningstarcorp.com/wp-content/uploads/2014/02/MPPT-Technology-Primer.pdf

The only thing I'm struggling to understand is there remains a warning not to exceed Voc, and it's said in same breath as saying extra voltage is fine. I must not understand what Voc is in regards to this...

 

[AngryVanMan]

Voc is open circuit voltage, and exceeding the limit will damage the transistors in the controller.  You can exceed the maximum wattage of a controller without damaging it, you'll just get the maximum watts the controller can pass, no matter the wattage being produced by the array.

Voc is also temperature dependent, and has a negative temperature coefficient - which is just a fancy way of saying as panel temperature drops, max voltage increases.  This is something to watch out for if a series string of panels is near the maximum voltage of the controller based on the datasheet Voc for the solar panels.  The Voc will increase above that baseline number based on temperature, and the rate of that increase should be listed on the datasheet in volts per degree..

The maximum Voc for the TriStar 45/60 is 125V, so 3 panels in parallel with a nominal Voc of 46.4V is a very safe configuration.  If you put the panels in series with each other (no reason to do this) you would exceed the max voltage of the controller, the string would be 139.2V in standard conditions.

 

[ccbreder]

I want my vehicle electrical system and house system to match voltage.

 

[jimindenver]

I have expressed my opinion of overpaneling, my controller gets hot enough with out it.

24v would be more efficient is a lot of way but would be a pain for me since my systems are all 12v already. Building from fresh I could have gone any route.

 

[BradKW]

I want my vehicle electrical system and house system to match voltage.

It's interesting that you would say that. For the past week since I "discovered" that 24v was an option, I've been beating people's ears and the internet trying to discover the "downside".  I was previously stuck on the idea that since going 24v would "reduce" the Amp Hour of a given bank, that it wasn't an option for me...turns out thinking of things only in terms of Ah is more like a nice analogy that just doesn't apply very much.

Anyway, I've now had 3 people tell me basically the same thing in response to my question: "If 24v has all these advantages, why the heck is every system I've seen for the past year 12v?"

The general answer has been it's solely due to the automotive industry making 12v a standard and aftermarket companies keeping suit to make things as easy and seamless as they can. And that the auto industry "picked" 12v due to battery size...basically that putting a 24v system in standard auto required too big a battery.

https://www.amazon.com/Pyle-PSWNV72...65390&sr=1-2&keywords=24v+to+12v+dc+converter

So now I'm on to looking at these simple stepdown devices that convert 24v back to 12v for things like lighting or whatever else. So far I can only find advantages to going 24v, especially for a larger system that will have a substantial inverter. Haven't clicked "buy" yet...have learned over past year that the only reliable decision I've made repeatedly is not to decide just yet  

 

[BradKW]

I have expressed my opinion of overpaneling, my controller gets hot enough with out it.

24v would be more efficient is a lot of way but would be a pain for me since my systems are all 12v already. Building from fresh I could have gone any route.

 Agreed...I don't intend to. 24v will solve that particular issue.  I did find it interesting that Morningstar has no problem with it tho...

 

[jimindenver]

Brad

If you have time, take it slow. Every decision you make will affect another down the road. There are a lot of options available, each with its own info to be absorbed and plugged into the whole. Once you have the component options down pat you can start rolling them through your head..use these panels with those batteries and this controller and that inverter... no that's wrong. Ok this set of batteries instead.... and on and on until you find the right balance to get what you want. It's how I built my systems, it took quite a while but hey, I'm just a ignorant slob that does it the hard way.

 

[highdesertranger]

yeah the down side is, almost everything I have works on 12v. if I went 24v then I need another piece of equipment to step down to 12v. this introduces more losses in the system and another failure point in the future. highdesertranger

 

[sushidog]

If 24v is good, 48v is better!  A TS-MPPT-45 will handle 2400 watts of solar panels at 48 volts output! The VOC max for this controller is 150 volts, BTW. http://www.ecodirect.com/Morningsta...mp-12-24-28-Volt-p/morningstar-ts-mppt-45.htm  I'm planning on building a 48v system using cheap high voltage solar panels designed for home systems on my FT RV, as I will be using a 48v DC powered air conditioner which will be consuming 85-90% of my off grid power (it draws 11.5 amps at 48 volts, producing 12,000 BTUs of cooling). The wiring needed for the four Sunpower E20 435 watt solar panels (1,740 watts of solar!) I have planned will be much smaller than that required for an equivalent 12v system, saving money there too, even wiring them in parallel (necessary as the VOC of these panels is 85.6v). Here are the specs for the E20 panels I'm looking at: https://us.sunpower.com/sites/sunpo...s/ds-e20-series-435-solar-panel-datasheet.pdf Four will easily fit on the roof of a small RV, requiring less than 14 ft. of vehicle roof length if mounted perpendicular to the long axis of the roof. Racked, they would even fit on the roof a 14 ft long trailer, if it is at least 7 ft. wide. I can even visualize 8 of them racked on top of a 30 ft long RV with enough electrical power to light up a city block (just kidding.)

There are several ways of drawing 12v off of a 48v battery bank, to provide 12v power for slides, fridge board, lights, etc. The best way is to use one or two of these,  https://www.altestore.com/store/cha...-eq-1248-30a-dc-to-dc-updown-converter-p1291/  though they aren't cheap.

Here's what I'll probably use as it is much cheaper and 96% efficient: http://www.ebay.com/itm/DC-48V-to-D...002404b&pid=100009&rk=1&rkt=1&sd=121662284375 I would need 2 wired in parallel to provide 50 amps of 12v DC to power the high amperage slide motors.

Chip

 

[flying kurbmaster]

;My understanding is that if you go 24v or even 48 the up side is that, IF, you are trying to convert to 110v appliances, you are closer to get there then 12v so somehow it is easier, especially if you are connecting into your house board that is already on 110v so in essence the distance from 24 or 48 to 110 is less then 12 to 110, so the journey is shorter and faster. However if you plan on using only 12 v appliances you may be better off staying with 12v but I am no expert.

 

[AngryVanMan]

It seems like in general, the larger your system, the more sense it makes to explore higher voltages for the added benefits. You lose out on some DC appliances as you go up in voltage, and the ability to charge the bank with the engine alternator (you could still use a dedicated 2nd alternator). The more oversized your solar system, the less important things like inverter losses and 12V appliances become.

 

[SternWake]

higher voltage makes a lot more sense, if one is having a large capacity battery bank.

In My case I only have one 12v AGM battery.

Starting from scratch, on a large system, by all means set it up for 24v. Charging from the alternator then requires a separate additional 24v alternator or a DC to DC converter which can handle serious amperage.

I'd prefer the simplicity of a 12v system rather than stepping voltage up and back down to accommodate the 24 volt system. Gain efficiency here, lose it there. Will the efficiency gains outweigh the losses? IDK.

 

[BradKW]

If 24v is good, 48v is better!  A TS-MPPT-45 will handle 2400 watts of solar panels at 48 volts output! The VOC max for this controller is 150 volts, BTW. http://www.ecodirect.com/Morningsta...mp-12-24-28-Volt-p/morningstar-ts-mppt-45.htm  I'm planning on building a 48v system using cheap high voltage solar panels designed for home systems on my FT RV, as I will be using a 48v DC powered air conditioner which will be consuming 85-90% of my off grid power 
Chip

The only reason I'm not considering 48v is I believe it would require x8 6v batteries. The old guy I bought my panels from is running 48v on some 110Ah Dekas and powering two freezers and a full sized fridge, all on 110v with a no-name 4k inverter...and all tiny wires.

I wonder if you're referring to the Hot Spot AC unit?  http://www.hotspotenergy.com/DC-air-conditioner/

I crunched some numbers on it and despite a rather amazing EER of something like 22, 12k btu just seems unnecessary...and I don't believe their projected runtimes unless you're in a refer box with no windows, kinda like I'm making  

There are many RV appliances designed to run off higher DC voltages once you start looking, and not reason not to look if starting from scratch. But one solution I saw for running 12v stuff from a 24v bank was they simply tapped off the first two batteries in series and had 12v. I'd be curious what Sternwake thinks about that...

 

[jimindenver]

I'll tell you right now. It's stupid and throws off the balance of the bank having some charged to one level and not the others. You can't hook two low batteries into a charged bank and expect them to charge properly.

 

[BradKW]

I'll tell you right now. It's stupid and throws off the balance of the bank having some charged to one level and not the others. You can't hook two low batteries into a charged bank and expect them to charge properly.

Yeah, figured it wouldn't be good, but was wondering more about the scale of how bad. In the example I saw, he tapped off for 12v just to power his LED lighting...and I thought that maybe such a small draw wouldn't really be an issue...

 

[SternWake]

I'll tell you right now. It's stupid and throws off the balance of the bank having some charged to one level and not the others. You can't hook two low batteries into a charged bank and expect them to charge properly.

My opinion as well.  The two batteries providing 12v will degrade much faster.  The bank will appear much smaller and become that much more difficult to return to a true 100%, and the batteries not providing 12v Will reach full charge sooner and suffer more positive plate shedding from that point, and use more water.  

When they fail earlier, you cannot just put 2 new batteries in their place without them quickly degrading to the level of the other batteries.  With that many batteries in series you also can't just remove the 2 killed jars hook everything back up, and have sufficient voltage to carry on.

There are all sorts of workarounds, of varying efficacy, but the most effective way is not knowingly introducing imbalance, especially in the planning stages.

 

[sushidog]

This is correct. The first link I posted https://www.altestore.com/store/cha...-eq-1248-30a-dc-to-dc-updown-converter-p1291/ was a device which balances the load if trying to just pull 12 volts off a 48v bank by taping 1 - 12v or 2 - 6v batteries. It's about $450 for a 30 amp model. The other solution is a step down transformer, as in the second link at around $50 for a 25 amp converter. You could also use a low amperage step down converter as a charger, drawing power off of a separate, smaller 12 volt battery not in the 48v string. This would stand large amp loads for brief periods while the step down transformer recharges the 12v stand alone battery over time from the bigger bank. If you have a need for high current draw, this might be your best option.

Yes, I plan on using either 8 FLA GC-2s as my primary bank. If you don't have the weight capacity for such a large bank, 4 12v deep cycle batteries (I recommend AGMs for this option) wired in series would work too. Something like 4 of these: https://www.batterystuff.com/batteries/ups-telecom/UB12110-45824.html

The main reason for going with high voltage is to mitigate large current draws and long wire runs. If you only have between 100-225 amp hrs at 12v then there is no need to add the expense and complexity of high voltage DC for such a simple system. High DC voltage at the current we are talking about is almost as dangerous as AC voltage. 12v is much less apt to create sparks, lowering the fire hazard too.

Chip

 

[sushidog]

BradKW, the mini-split AC I'm looking at is the HSAC-12H/C found here: http://www.geinnovations.net/HSAC_Productline.html The EER is 21.43. It's rotary compressor is fully scalable and soft starting without the start-up surge of a typical piston compressor AC. The variable speed design means that if you only need say 8,000 BTUs it will slow down drawing less current to produce only the needed amount of cooling, saving electricity after the room is already cool with no cycling on and off like a conventional AC does. This means it runs much quieter too as the compressor and condenser unit are split apart, just like the central air in a house. The AC you linked to draws 760 watts max., while the one I referenced draws only 560 watts to produce the same 12,000 BTUs of cooling.

Chip

 

[ccbreder]

Cheap RV Living. Using other than 12 volts seems to require more and more work-a-rounds. Also it seems 12 volt items are readily available and less expensive than 24 or 48 volt. You want complicated, I want simplified. We both fit in.

 

[jimindenver]

There is balance in everything, Brad is finding his now. Starting from scratch mean a lot of options to explore. Realistically my system is only a few hundred watts and Ah's smaller and we do well enough on 12v. We didn't start from scratch and have a RV and lots of toys that are 12v.