Buyers remorse or do I have a good start to a working system?

[gadget]

I recent bought some parts to put together a 24v system. Here is what I have so far;

6 - Sunelec SV-T-200-HV
      200w 27.5 Vmp

1 - Xantrex C60

2 - 4D batteris

6 - Schottky diodes

2- 480 watt water heater elements

Original Plan was to hook up the panels directly to the batteries with the C60 as a dump control
to water heater loads to regulate charging. All panels running parallel with Schottky blocking diodes hooked to 2 4D batteries
in series for a 24v system. The C60 would go to two parallel 480watt water heater elements for diversion. Would have 1 or 2 24v to 110v modified sine wave inverters. There would also be a DC to DC down convert to 12v for trailer lighting.

My original intentions was to maximize unused solar power to heat water and have the batteries available for power AC during emergencies and long term power outage. I normal use grid power day to day so the batteries would not get used much. The main reason for the system is for emergency power. I also have a generator and lots of fuel but I don't want to get that debate started.

My main reason for posting is because I am getting ready to buy a couple of inverters and am having second thoughts on 24v  for a couple of reasons.

First, I am concerned that my panels will not be able to maintain a good and fast enough charge to the batteries due to the lack of over head on the voltage.

Second, it would be nice to be able to buy more common and available 12v inverters and not have to run a dc to dc converter for the trailer.

So, I guess I am looking for input on my panel voltage and if it is enough to charge the batteries with one full day of sun at 80% discharge. I am thinking about upgrading to a MPPT charger but then I would loose the diversion to hot water. I would run my 2 batteries in parallel for a 12v system and buy a 12v inverter. I don't know but I am guessing an outback FM80 would work but I may have to much panel for it and need to run 2 MPPT charges. I would really like to keep the water heating if possible. What do you think??

-Gadget

Thanks
-Gadget

[Watt]

Hello and welcome.

Wow, that's a lot of panel for 2 4d batteries.  Completely drained and assuming 200ah per battery, that is over 4kwh of power from the batteries.  The panels will most likely charge those two batteries in 5 to 6 hours if dead ( also depends on available sunlight ).  Instead of using the inverters to heat the water, I'd suggest using 24vdc water heating element(s) with the c60 and a good load managing  relay setup.  If you decide to go with mppt, the sky is the limit.  Chris Olson is successfully heating water with the waste not mode within the Midnite Classic.  He is on this site and others and I do know he has a thread on Anotherpower.com regarding the waste-not mode. 

Anyway, you do have a great start.  Batteries, batteries and more batteries, you can support more battery with that kind of solar.  Also, if managed well, you can make use of that solar to run daily loads ( of-course wattage dependent ) in say the 200 to 250 watt hour range.  I for sure wouldn't feel any remorse buying what you have so far.     

[gadget]

The heater elements would be supplied 24v from the C60 and it is PWM depending on the need of the batteries. The elements I have are ratted at 4800watt @240v.

I figured I had matched my batteries up pretty well but maybe not? Batteries are cheap and I can add more later. The panels where cheap at $1/w, I didn't want to skimp.

I just need to find out if they will charge my batteries at 27.5Vmp

[Watt]

The heater elements would be supplied 24v from the C60 and it is PWM depending on the need of the batteries. The elements I have are ratted at 4800watt @240v.

I figured I had matched my batteries up pretty well but maybe not? Batteries are cheap and I can add more later. The panels where cheap at $1/w, I didn't want to skimp.

I just need to find out if they will charge my batteries at 27.5Vmp

Yes they will charge them fine.  As the voltage moves above 27.5 ( not accounting for line losses ) panel maximum power will not be obtained but you will still get some amperage to the batteries.  By that time, you will be close to absorb anyway.  If you have trouble, which I doubt, you can always go the mppt route and series string 2 x 3 panels.  or even 3 x 2 panels.  In all reality with those two batteries, you may get 2.2kwh out of them before they would need a charge. 

Using the 240v elements with 24v?  You won't get near 4800 watts from those elements.

[gadget]

That is great news to hear. I may just go ahead and pickup a 24v inverter and give it a go. I was also thinking about not using the diodes for night time isolation and just using a 24hour time to disconnect the panels from the batteries at night. This would save .7v

I was hoping to get close to 480w from the 4800w after I do the math. Does that sound about right? Would a 960w diversion be enough?

[Watt]

I'd use the diodes, schottky if you can get them before you set everything up.  8watts is a pretty good price to pay for when you are not around and you come home to find your batteries are dead or worse, your panels are not as good as they were when you left. 

I think at this point, you are best to use the inverter but, you could save some wattage by using 24v elements.  Just a thought. 

[dave ames]

I was hoping to get close to 480w from the 4800w after I do the math. Does that sound about right? Would a 960w diversion be enough?

240v*240v/4800w = 12 ohm

24v/12 ohm = 2 amps = 48 watts each 

We need to rethink this part of the setup.

[gadget]

240v*240v/4800w = 12 ohm

24v/12 ohm = 2 amps = 48 watts each 

We need to rethink this part of the setup.

Oh Wow! Ok, gonna have to find another solution there. Is there a cheap DC water load available? Needs to be 24v DC so I can use the PWM from the controller.

I'd use the diodes, schottky if you can get them before you set everything up.  8watts is a pretty good price to pay for when you are not around and you come home to find your batteries are dead or worse, your panels are not as good as they were when you left.

I have the diodes and will give them a try. I will use one per panel before I tie them together ahead of the batteries. Would prefer to use them instead of the timer relay.

[Watt]

Just to show you they are available.

 http://www.altestore.com/store/Charge-Controllers/Dump-Loads-Dump-Load-Controllers/Diversion-LoadsDump-Loads/Water-Heating-Element-for-12V-or-24V-DC/p2326/

Also, I assumed you had two elements 4800watts total.  Not to say that would matter but at the dump load voltage, I got 63watt/hr.   

[hydrosun]

Those panels are not designed to charge a 24 volt battery bank directly. The maximum power point will drop as soon as the panels warm up. You need at least a mpp of 32 to charge a 24 volt battery and even then it won't do well to equalize when hotout. So the best is a mpp of 36 volts.a 24 volt system you will have to run pairs in series and use a maximum power point tracking controller. Many of those have auxilliary outputs to control Fets or relays to turn on loads like water heating elements.
If you don't need all the power now you could just hook up pairs in seriesdirectly to the 24 volt battery and add  the mppt controller later when you need more power.
Chris

[birdhouse]

gadget-
i think sticking with 24v is a good idea!  i run a trailer off my 24v banks also, and just purchased a more efficient 120 to 12v converter (30A) from china for $26. 

then there's the panel voltage of 27.5Vmp.  this is pretty low for 24v charging.  my bank can reach 31+ volts during equalize in the cold winter.  i run some 30.5Vmp panels, and they do pretty good with 24v system, though i feel there right on the cusp.  your 27.5Vmps will push much harder than standard panels with a PWM controller during MOST of the charging cycle, but i feel they'll fall short on the very upper end of the cycle.  especially with a diode installed (.7v drop). 

i have no idea of your budget, but if you could return the ts-60 and buy a ts-60mppt, you'd have a pretty awesome system!  morningstar makes a relay driver that could dump for you.  or you could keep the ts-60, buy a ts-60mppt, and have the set points just a little different.  the mppt controller would solve the issue of the 27.5Vmp panels, and the pwm/dump controller would do the water heating. 

just some thoughts.  either way, with a bit more battery, your looking at a pretty nice system in my book!

adam

[Watt]

Those panels are not designed to charge a 24 volt battery bank directly. The maximum power point will drop as soon as the panels warm up. You need at least a mpp of 32 to charge a 24 volt battery and even then it won't do well to equalize when hotout. So the best is a mpp of 36 volts.a 24 volt system you will have to run pairs in series and use a maximum power point tracking controller. Many of those have auxilliary outputs to control Fets or relays to turn on loads like water heating elements.
If you don't need all the power now you could just hook up pairs in seriesdirectly to the 24 volt battery and add  the mppt controller later when you need more power.
Chris

True for sure about the equalizing voltage needed during hot weather.  But, I can't agree with two panels in series connected directly to a 24v nominal battery group.  That will be a potential of 21+ amps frying those batteries for at least several hours in a day.  Not to mention the potential wattage loss by what the batteries will try to clamp the pv voltage to while charging.  With 1200 watts of solar, even with voltage loss in the heat of the day, there will be a potential of 40+ amps before losses are in order.  If he lost 50% of the current available at vmp during hot weather,  the potential current available to equalize the batteries will still be around 18 -20 amps.  Still too much if the wind is blowing and he lives in a cool climate.  What part of the country do you live in?

I had no problems keeping and using wattage with 4 - 8d batteries at 24v nominal and 28.4 vmp from a 600watt solar array.  I kicked myself in the rear as he is doing worrying over what I was going to do.  Maybe I was a bit unclear on the mppt as birdhouse suggested, these are the bees knees.  You can even pick up some wattage by installing one. 

I might also add: Even though summer time equalization will be touchy, why not equalize with the 6 panels as parallel but instead of at 24v nominal battery string, equalize manually at 12v - the two batteries in parallel. An hour or so before solar noon on a clear day to an hour or two after on that clear day.  I'm not sure how often there will be a need to equalize but, a hydrometer should tell.   With mppt, there is no need for diode to protect the batteries, only a schottky to protect individual strings in the controller connected array.  And, as panels are strung together, the current in that string stays the same and so does the voltage drop.  Same " protection " wattage no mater the string voltage so long as the diode can sustain the current and voltage.  Just double the wattage of the diode in use or triple and so on or added strings up to the controller maximum amperage or wattage.

1 Midnite Classic 200 will allow all the panels to be strung together.  No diode, no potential battery voltage charge issues, waste not mode for heating water ( extra equipment required to load the elements but can be controlled from the classic, easily set to equalize and by no means the end of its capabilities, log system parameters.  Yes, I like my classics.

 

[hydrosun]

My comment was to warn that these panels could cronically under charge a 24 volt battery bank. The output drastically drops above the mpp and that drops as soon as the temp goes up. It  might work with a small battery bank in a cool climate. So ideally you would use several in series through a mppt controller. In a pinch you could use two in series and shunt the powwer when the battery is full. That was a way to use the equipment he now has to get started. It does throw away half of the potential power but doesn't destroy the battery with undercharging and sulfation.
Chris

[birdhouse]

alright, so as per watt, his 28.4 Vmp panels performed well with his 24v system, so maybe your 27.5Vmp panels will do well for you as well! 

a quick idea here...  a few switches (old disconnects for example) could be used to switch the panels from parallel to series/parallel for your equalization periods.  then you'd have everything sorted with what you've got (minus figuring out your water heating stuff)!

the standard ts-60 would handle it fine, and it would make equalizing as easy as throwing two switches, in the right order. 

adam

[birdhouse]

My comment was to warn that these panels could cronically under charge a 24 volt battery bank. The output drastically drops above the mpp and that drops as soon as the temp goes up. It  might work with a small battery bank in a cool climate. So ideally you would use several in series through a mppt controller. In a pinch you could use two in series and shunt the powwer when the battery is full. That was a way to use the equipment he now has to get started. It does throw away half of the potential power but doesn't destroy the battery with undercharging and sulfation.
Chris

i don't fully agree with this.  for example:  i've got some 36Vmp panels for a 24v bank, and when i had them on a PWM controller, i was always wasting a LOT of power due to the rather large mismatch in voltage.  as stated earlier, i think 27.5Vmp may be a little low, but not by much.  personally, i think if you don't have the cash for MPPT, then ~31Vmp panels will outperform typical 36Vmp panels for a 24v system any day of the week! 

even those 27.5Vmp panels probably have an open circuit of around 34v?    at 34v, yes they have fallen entirely off the curve as far as amps go, but with a smallish battery bank like gadget has, only a few amps should be needed to keep his bank at equalize, and it will never need 34v to get there. i would guess that 1200W of these low voltage panels WOULD hit equalize on two 4D batteries even in the summer. 

the one fact that's working FOR us in this situation, is summer temps lower panel voltage, yet also lower battery equalize voltage!  when it's warm, my bank only hits around 29V during equalize due to the controller's temp sensor. 

adam

[Watt]

I know this is not an apples for apples example but just to show how the voc comes into play is all this is doing.  This is all while in absorb.  The Vin is 120.3  120.4Voc first pic.  113.5Vin 120.4voc with slight load second pic

With batteries at absorb, and yes this is not a similar system to the one in the thread.  However, this does show power at voc nonetheless.



This is adding around 300watt/hrs load.  Panels, when managed right, make usable power beyond the magic Mpp.  Greed sometimes gets in the way of making what ya got work. 




Running at Voc, I am getting 1448kwh from the panels ~181watts per.  28.4Vmp and 30.1Voc - today is close to 90f, no wind to speak of and the panels are hot.  So, by running at Vmp, I picked up 300watt/hrs and yet at Voc for rated 230w panels I still make power.

[Watt]

Rereading this, I failed miserably explaining my power point beyond vmp.  I should have just pointed out the current difference between when Voc matched Vin. and when Vin was lower than Voc.  And that power was possible because the voltage difference between the battery voltage and Vin.  Anyway, I edited what is in red.  Sorry for the blunder.   

I know this is not an apples for apples example but just to show how the voc comes into play is all this is doing.  This is all while in absorb.  The Vin is 120.3  120.4Voc first pic.  113.5Vin 120.4voc with slight load second pic panel Voc as tested lower temps is somewhere in the 32.? range.

With batteries at absorb, and yes this is not a similar system to the one in the thread.  However, this does show power up to voc nonetheless.



This is adding around 300watt/hrs load.  Panels, when managed right, make usable power beyond the magic Mpp.  Greed sometimes gets in the way of making what ya got work. 




Running at Voc, between Voc and battery voltage, I am getting 1448kwh from the panels ~181watts per.  28.4Vmp and 30.1Voc - today is close to 90f, no wind to speak of and the panels are hot.  So, by running at Vmp ( not panel Vmp but current required to keep batteries at absorb), I picked up 300watt/hrs and yet at Voc for rated 230w panels I still make power.

[hydrosun]

I'm not disagreeing with anyones observations. Most solar panels hooked to a battery through a pwm controller throws away some of the power to ensure that the voltage will get high enough to fully charge the battery in less than ideal situations. With the original situation with a large solar array and a relatively small battery bank the low output at high voltage probably would work, but if he changed to a larger bank in the future,  or someone else reading this would be ruing a battery bank with too low a charging voltage. In the case of a 27volt mpp panels there will be situations that the battery won't be fully charged and could gradually lose capacity through sulfation. My sugestions were were ways to keep the battery bank working longer. Other people have given switching ideas to do the same thing. and I keep repeating that a mppt controller is the best option to get the most power out of th epanels and ensure that the batteries last a long time by getting fully charge and equallized frequently.
Chris

[gadget]

Great info and lots to think about. Glad I posted my question here! Thanks everyone!

I took one panel and hooked it up to the batteries fully charged and I was getting 28.5 volts and 32v open circuit. I have not done any other testing yet.

I had another idea that came to me last night. I was thinking about hooking the panels up to a 12v battery bank ( two 4d's in parallel) and diverting the left over power to the hot water. This would allow me to run 12v inverters and plenty of overhead for the batteries, especially for equalizing. I spoke with Xantrex but they where of not much help but they did say it would work. I would need plenty of diversion load to the water heaters. Not sure how much charge capability I would loose to the batteries. Any thoughts??

Thanks again for all the help

[Watt]

Great info and lots to think about. Glad I posted my question here! Thanks everyone!

I took one panel and hooked it up to the batteries fully charged and I was getting 28.5 volts and 32v open circuit. I have not done any other testing yet.

I had another idea that came to me last night. I was thinking about hooking the panels up to a 12v battery bank ( two 4d's in parallel) and diverting the left over power to the hot water. This would allow me to run 12v inverters and plenty of overhead for the batteries, especially for equalizing. I spoke with Xantrex but they where of not much help but they did say it would work. I would need plenty of diversion load to the water heaters. Not sure how much charge capability I would loose to the batteries. Any thoughts??

Thanks again for all the help

With the panel hooked to the batteries, was the voltage rising? 

Your diversion load will be the heating elements but, You will need a bunch of them.  Maybe birdhouse or hydrosun can help with your controller specifics.  Good luck.

[gadget]

With the panel hooked to the batteries, was the voltage rising? 

I didn't leave it on for more than a few seconds.

Looks like I am in a gray area with this system and just need to hook it up and start doing some testing. I will
report back results.

[SparWeb]

Looks like I am in a gray area with this system and just need to hook it up and start doing some testing.

No amount of testing will prove that your panels match your battery.  When I got my solar panels, I thought they were "just barely enough" at Vmp 17.4  (17.4 x 2 = 34.8V charging to my 28v batteries).
If you don't have the V-I graph for your panels, go find one for a similar type of cell, and look at what happens when voltage is higher than the Vmp.  The current drops off steeply.
Think of batteries as "14 volt" and "28 volt" when you are picturing the system to charge them.
I would consider 2 x 27.4v = 54.8v...   but it reduces the array rating from 1200W to about 700W.  Bummer.  On the plus side, I wonder if it may work almost as well on cloudy days as sunny ones...  not sure about the last thought but with reduced light the V-open-circuit is still above battery voltage, so some current can still flow.  I'll defer to someone with more experience on the "tricks" to get more out of them.