24 volt or 12 volt solar panels advantages??

[purman]

Ok, so I have to admit I bought a 128 watt solar panel without paying attention to what volt it was. It turns out it's 24 volt. Luckily I only paid $170 for it.

BUT, after reading about it, it seems I might have an advantage over a 12 volt panel..

So what will come in red, is not first hand experience data, it comes from the internet and other forum groups on solar. So hang in there, and if you know something different please let me know...


First, I am starting to understand that it really doesn't matter what the volts are, as I need some kind of controller for the solar panel to control the charging of my battery and this will restrict the volts

There seem to be two types of controllers.. PWM, and MTTP.

Here seems to be the difference:

A PWM controller needs about 15.5 volts to operate properly. Any more voltage then that is just wasted. Doesn't matter if it's 15.5V or 24V supplied to the controller, the controller will output the same wattage to the batteries. This is why MTTP controllers were made. It takes the excess voltage and turns it into higher amperage.

I'm not going to work through the numbers exactly but a 17.7v panel rated at 340 watts puts out about 20 amps max. Using a PWM controller, the batteries will see only about 290 watts max (about 16 amps) because it steps down the voltage to 14.4v (depending on charge mode). The excess voltage is lost.

With a MTTP controller, the batteries will get close to the full 340 watts (20 amps) because the controller converts the extra voltage to amps.

So as can be seen, any amount of voltage above the charge voltage is just lost using a PWM controller so voltage loss in wiring from the panels to the controller does not matter as long as the voltage to the controller is above the charge voltage (use 15.5v to be safe). But any voltage loss when using a MTTP controller results is a direct loss of charge amps.


A PWM controller can not output more amps then comes into it no matter what the voltage is.

A MTTP controller can output more amps then comes in because it will take excess voltage and convert it into extra amps.

With a PWM controller your 130w panel puts out 7.4 amps. Your battery is setting at 12 volts under charge: 7.4 amps times 12 volts = 88.8 watts. You lost over 41 watts - but you paid for 130. That 41 watts is not going anywhere, it just is not being produced because there is a poor match between the panel and the battery. With a very low battery, say 10.5 volts, it's even worse - you could be losing as much as 35% (11 volts x 7.4 amps = 81.4 watts. You lost about 48 watts.

A MPPT, or maximum power point tracker is an electronic DC to DC converter that optimizes the match between the solar array (PV panels), and the battery bank or utility grid. To put it simply, they convert a higher voltage DC output from solar panels (and a few wind generators) down to the lower voltage needed to charge batteries.

Here is where the optimization, or maximum power point tracking comes in. Assume your battery is low, at 12 volts. A MPPT takes that 17.6 volts at 7.4 amps and converts it down, so that what the battery gets is now 10.8 amps at 12 volts. Now you still have almost 130 watts, and everyone is happy.



So for me, MTTP seems to be the best choice as it will charge the batteries faster.. And make better use of my watts and volts.

It would also seem if you join two panels together you are also doubling the volts?? True of false. Say two 12 volt 128 watt panels are now putting out 24 volts 252 watts??? Or is it still putting out 12 volts and 252 watts?

The question I haven't been able to find, but I am assuming is: that if I run out of the controller to my 12 volt system it will send it as 12 volts not the 20+ and be safe for my fans and lights?

Also my 12 volt panel has the power from the TV coming into it. Then running to the converter and then to the batteries, to charge them. I suppose any extra power come in here from the solar panel would also be charging the batteries? Kinda of like a double charge system???? not sure of any advantage to that..

So I would be looking at running wires to the controller form the Solar panels, and from there to my 12 volt panel and also to my batteries. does this sound about right?

Any input is greatly welcome at this point as my brain is in overload from reading information that I knew nothing about this morning

[AWCHIEF]

Too confusing for me.

[purman]

Originally Posted by AWCHIEF

Too confusing for me.

Thats how I feel right now, thats why I need HELP

[idroba]

Jason: It is not possible to give a complete course in solar system design here in simple posts. You will need to read a lot more, take time to digest it all, understand basics of power (watts) as well as volts and amps. The statement posted in red that an MPPT controller converts volts into amps is simplistic at very best and technically incorrect. The author apparently never heard of watts....which does link the two together. But again for a complete understanding you need far more than can be done in little pieces and parts you will get here.

I built my first solar system in 1995, and I am still learning new stuff every time I look at the subject. I do volunteer solar work for the local Forest Service, especially in their remote campgrounds. But it is not possible to educate very well by what is essentially a long tweet. I am sure you can learn, but it will be a somewhat lengthy process of self education, which is always the most gratifying in the long run.

At least that is my opinion.... grin.

Have fun!

[ddruker]

You've got it about right.

Some basic electrical concepts are in order.

Electrical power is usually measured in watts. As in a 60 watt light bulb, or a 1000 watt blender.

Watts are calculated as voltage times current. Current is measured in amps. So to power your 60 watt light bulb, you can use 12 volts at 5 amps (12x5=60) - or you could use 120 volts at half an amp - (120*0.5=60). In both cases the amount of power in watts is exactly the same.

You also asked about what happens when you wire together multiple panels.

You can add your panels together in parallel or in series. If you add them in series (think putting batteries into a flashlight where the positive from one battery connected to the negative from the next) you add the voltages together - so two 12 V batteries each capable of putting out one amp of current (for 12 watts of power each) - if you wire them in series you get 24 volts at one amp, for 24 watts of power.

If you wire the same two batteries in parallel - wiring the positive terminals together, and the negatives terminals together, the voltage stays the same, but now the current adds - so in the example above two 12 volt one amp batteries in parallel would generate 12 volts at two amps - still for 24 watts.

Your various sources of 12V power in your trailer are ALWAYS wired in parallel - so if it is a sunny day and your solar panels are cranking, you are plugged into shore power with your converter on, and you are connected to your tow vehicle - all three of these 12V sources will be trying to power your trailer. You will have a lot of amps available, but still only 12 volts because everything is wired in series.

So here's the other bit of physics involved - it's much less efficient to transmit low voltage, high amperage current than it is to transmit high voltage, low amperage current. There are many other posts on this, so I won't go into the reasons behind this again - just take it as a given. You can lose a substantial amount of power if you transmit low voltage, high amperage current over a distance. This is why the battery cables in your car are so thick - they have to transmit a lot of amps at 12 volts to make a decent amount of power.

The DC circuits in your airstream run at 12V DC nominal voltage (meaning anywhere between about 10 and 14 volts DC.

You can chose solar panels that will run at up to several hundred volts if you wire them together in series. But even a "12V" solar panel can put out as much as 18 volts when conditions are good.

All of this is why the MPPT controllers exist. They take higher voltage, lower amperage current coming out of your solar panels, and convert it to 12V DC nominal voltage at higher current to charge your battery and power your airstream. They try to be as efficient as possible - most can hit 95% or more efficiency under ideal conditions. The watts almost stay the same - you lose a little bit in the conversion process - but 5% conversion loss is a lot better than the 30% loss from over-voltage you wrote about in your post.

The idea of all of this is to not lose power for any reason - whether your panels are putting out more voltage than your battery can handle, or whether you are running a lot of power which would suffer major transmission losses if you ran at low voltage, or both.

The MPPT controllers allow you to run the panels at much more efficient, higher voltages and ensure that as much power as possible is getting to your batteries at all times.

On the downside, you will find that MPPT controllers cost about 3 times more than PWM controllers of the same capacity... There is no free lunch.

From your questions, I'd recommend you work with an experienced installer / designer to spec out your system.

Hope this helps.

[ASIcons]

Why does it make perfect sense that you live in Palo Alto with the rest of the geniuses? My head hurts now.

[purman]

OK, my brain hurts from reading and reading about solar installation. I am going with a MPPT controller, but an not putting in an inverter. I do however want to hook it up to my 12v system, so i can run lights and fans straight of the solar panel, during the day instead of the battery..

Crazy thing is, the MPPT controler will cost as much or more than my 128 watt panel did!!

I still can't get clear info if I can run a line from the controller to my 12 volt panel??? has anyone done this?? The controller should dress my 24 volt system down to 12 volt.. I hope... I know it does on the battery charge side..

[Paul Mayeux]

Originally Posted by purman

...I still can't get clear info if I can run a line from the controller to my 12 volt panel??? has anyone done this?? The controller should dress my 24 volt system down to 12 volt.. I hope... I know it does on the battery charge side..

The answer is yes. The MPPT charge controller will send the appropriate voltage to your DC system. That voltage will vary depending on the charge mode of the controller as most of them have three stage charging.

[ddruker]

You've got it exactly right -

24V solar panel ---> MPPT Controller ---> 12V Airstream systems

The output of the MPPT controller is nominal 12V power that both runs any 12V loads you happen to have turned on, and also charges your trailer's battery / batteries at the same time.

I still think it would be a good idea to speak with an expert to help you decide which MPPT controller to buy, what size wiring to install, etc.

[purman]

Ok so here is another question. If I hook it up to my 12 v panel. It will then works is way to the converter and charge the batteries without a direct line to the batteries. Like my TV cord does This can not be as efficient as running directly from the controller to the batteries. So I will still run both wires.

We have a somewhat local solar place I will give a call to later today.

[0557]

Would an MPPT controller be beneficial if you have a 24volt inverter? Like a Xantrex 4024

[Wabbiteer]

I will still run both wires

YES. Heavy up the copper gauge on the PV feed lines and the controller outputs to the batteries as it will give 1, 3, 5% bonus power in perpetuity and its not like you're carrying the additional weight in a backpack. Also, add Doomsday inline fuses so it can truly be a forgettable installation.

1st thing is don't get caught up in a perpetual upgrade loop, the marketers are focused on impulse buyers and their products perceived value. Don't latch onto that first thing you picked up, I know all you want right now is a little longer battery life & all that rot but renewable energy is addictive and saying "no inverter for me" might not be an accurate prediction

Buy enough 'charge controller capacity' to cover future expansion of your system.

With a glut of PV panels on the market right now I'm seeing 60¢/watt for thin film and $1/watt for mono/poly crystalline panels so you might want to retail off the panel you already bought (craigslist?) and custom specify panels that match the roof real estate AND power requirements.

With solar panels as cheap as they are now simply adding another panel could be a great less expensive option - but you still need a charge controller to keep from cooking the batteries.

MPPT controllers are nice but not truly needed even with slight panel mismatch now that the PV panels are as cheap as they are.

Beware electric interference from some MPPT/PWM controllers - do the research before the switching RF noise invades your electronics.

Hope you return from your visit with the somewhat local solar place without spending money - build the system on paper through twenty revisions before committing!

[garry]

Originally Posted by purman

OK, my brain hurts from reading and reading about solar installation. I am going with a MPPT controller, but an not putting in an inverter. I do however want to hook it up to my 12v system, so i can run lights and fans straight of the solar panel, during the day instead of the battery..

You have this backwards you charge the battery during the day from the solar panel so you can run the 12V things at night from the battery.

If you want to charge the battery and run all the systems at the same time you need to add up the total system requirement at maximum load in total amps (or power) and size the system from there.

I suspect you will have around 8 amps from the controller with the solar panel you now have.

[purman]

Originally Posted by garry

You have this backwards you charge the battery during the day from the solar panel so you can run the 12V things at night from the battery.

If you want to charge the battery and run all the systems at the same time you need to add up the total system requirement at maximum load in total amps (or power) and size the system from there.

I suspect you will have around 8 amps from the controller with the solar panel you now have.

I'm going to do both. The batteries are not going to get drained that much so they won't have to charge all day.. When i'm gone during the day it's nice to set the thermostat on the fantastic fan, so it won't run all the time.. Plus I can charge my 12 volt stuff through my 12 volt sockets..

But I mostly want it to charge the batteries and it won't get much use during the day... But nice to have it if I do want to use it..