choosing second charge controller

eajonesyk2

Sorry in advance if any of my terminology is off and please excuse my grammar too. I am in the planning stages of a permanent pv system for my off grid cabin. I have unfortunately already purchased many of the components that I thought I would use and have a decision to make regarding a new charge controller purchase. I currently have 4 dmsolar 145 watt panels and 2 uni-solar (thin film) 64 watt panels. I have one blu sky solar boost 2000e mppt charge controller and one Sunforce cheapy 7 amp charge controller. For best sun exposure the bulk of the pv array will have to be pole mounted approximately 75 to 100 feet from cabin. I want to stay with 12 volt battery bank so I can use my xantrex 1000 watt prosine inverter.

I am only using one of the uni-solar panels at this time. That panel is being run through the 7 amp charge controller and into a 12 volt marine/deep cycle battery. I only recently hooked up this panel and it is working fine for now. Only power draw is for a CFL light bulb and charging phones, etc. It will be tested this winter when the sun travels lower in the sky. I have instructed the neighbor who is currently staying at the cabin not to use the battery if the resting voltage dips below 12. volts. He has been living there without any electricty for over a year now so even one panel and a battery is a treat for him. The county kicked him off his property which was also without power.

So for my charge controller dilemma. My future power needs will be just under a 1000 watts a day during the heat of the summer and much less in the spring, fall and winter. The blu sky controller will only handle two or three of the panels so I will need a second charge controller for the other two, three or four panels. Money is a big issue so I am looking for the most economical (but reliable) solutions to choose from. Theft is also another big problem I will have to consider. I can make the panels harder to get at but the charge controllers will be an easy target if they are at ground level. Batteries are going to be an attractive target too so any ideas on making those hard to get at would be helpful.

Here are a few charge controller ideas I have tossed around.

Option one: Mount second uni-solar panel next to first on cabin roof and use blu sky controller (mounted in attic) for these two panels to charge a small 12 volt battery bank. Purchase a morningstar tristar 60 amp PWM controller ($200) to run all four 145 watt panels in parallel. Locate battery bank and inverter at base of this array and run ac to house. This should keep my wire cost lower and the 60 amp controller would leave room to add panels later if budget permits.

Option two: Leave current unisolar panel and 7 amp charge controller as is. Run two dmsolar 145 watt panels and one uni-solar 64 watt panel through blu sky controller and purchase a sunsaver 15 amp mppt controller ($223) for the other two dmsolar panels (those two panels would have to be connected in series, correct?). I would still have to locate battery bank and inverter at base of array to keep wire cost low. (Blu sky controller won't take panels wired in series)

Option three: Try to sell blu sky controller (what's it worth?) and buy a more expensive mppt controller ($500ish) that would allow me to hook up all six panels in series (60 volts nominal) allowing the charge controller, battery bank and inverter to be mounted at house because of reduced wire size required to the controller.

I realize there are many configurations of controllers that could be utilized so any suggestions will be appreciated. Main goals to keep in mind is that I want to keep remaining cost under $1000 which will include pole mounting cost, hardware, wire, etc. I want to stay with 12 volt battery bank. I won't be able to afford do overs so reliability will be important. Stuff grows legs down there so the harder it is to take and the less attractive it looks the better. If data is not sufficient let me know and I will supply any numbers or other info needed. Thanks so much. Eric

Cariboocoot

Re: choosing second charge controller

Welcome to the forum.

Well you do have a situation, don't you? Let's evaluate the components separately and then look at the compatibility.

DM 145 Watt panels. Vmp is 18 as I recall and Imp about 8. Perfectly usable in a parallel configuration with a PWM controller on a 12 Volt system. You've got four, so you'd need to be able to accommodate 4 * 8 = 32 Amps. This means a 40 Amp charge controller. The wire distance is a problem, as the Voltage drop over 100 feet would be significant; you'd need 4/0 wire to handle it. But they could also be configured as 2 x 2 and run through an MPPT type controller (like Morningstar's 45 Amp unit). This means you'd run 2X the Voltage and 1/2 the current and use 4 AWG. You could also run all four in series for 72 Vmp @ 8 Amps and use 8 AWG for the 100 foot run.

Unisolar 64 Watt panels. Are these the ones with the 16.5 Vmp? Some of their panels in this size have a very high Vmp (60+) with very low current. Not a very practical panel either way. The 16.5 Vmp is low for charging a 12 Volt system; at higher temps there may not be enough Voltage from the panels to achieve charging. I would not invest too much time/effort/money on these.

The two charge controllers: neither would be on a list of my favourites. The Blue Sky is about $270 brand new. It has an input limit of 30 Volts so it's no good for dealing with the long wire run. I think the Sunforce is just a shunt controller; not much good for anything.

So you really need to change the charge controller to a decent MPPT type if you're going to handle that long run with any practical wire size.

What can you get out of those four 145 Watt panels? Pretty good: 580 Watt array will probably yield 1.2 kW hours per day AC (based on 4 hours sun). So it looks like those panels alone with a decent MPPT controller could handle your 1kW hour power need.

How are we doing so far?

eajonesyk2

Re: choosing second charge controller

Hi Cariboocoot, doing good so far. Yes, the uni-solar panels are the 16.5 volt ones. 200 ft of 4/0 wire will never be in my budget:). My wallet won't like to buy 200 ft of 4awg either. Since I might not need the uni-solar panels to reach 1kwh of daily power production then I could craigslist or ebay those panels along with the blu sky controller to get the dough needed for a mppt controller that would handle the 72 volts of all four 145 watt panels in series. 8 awg wire would be much friendlier to my budget.

What controllers would you recommend that would handle 72 volts? How efficient would the controllers be taking 72 volts down to 12 volts at the battery?

Cariboocoot

Re: choosing second charge controller

Two things to look at: the input max Voltage and the output current. Unfortunately for your situation those two things eliminate many charge controllers.
The input max will be 4X the Voc of the panels, which if I recall is 22 -ish so you would have 88+ Voc. The Rogue can't handle that high Voltage nor the potential 32 Amps output, so you are left with the choices of the 'upmarket' controllers

Morningstar TriStar 45 Amp MPPT: http://www.solar-electric.com/motr45ampmps.html $400
MidNite Classic Lite 150: http://www.solar-electric.com/misoclli150m.html $500
Outback FM60: http://www.solar-electric.com/oupofl60mpso.html $517

In my opinion these would be your best choices. The Morningstar and MidNite both have optional meters which can be added later. The Outback's display is built-in.

You are correct that conversion efficiency will fall off with higher array Voltage to battery Voltage. But at worse you are only looking at 2-3% difference. As opposed to losing a very large % of power over that long wire run if you try to bring the array Voltage down.

eajonesyk2

Re: choosing second charge controller

Thanks Cariboocoot. I think the tristar 45 might work well. I would like to mount the controller in the attic area of the cabin to discourage theft. The tristar specs indicate it can handle 45 celcius with out derating. How does the derating affect the controller if the temps got hotter than that? Also I have four Deka gel golf cart type batteries which I would also like to put in the attic. Any advice on putting batteries in attic spaces. This area use to be the loft until I closed it up so there is a small 12 inch by 12 inch window up there which I keep open year round. I have no other roof venting up there, other than cracks and gaps from sloppy construction. The gaps and cracks were big enough for the squirrels and bats to get in until I stapled hardware cloth over them. It does get warm up there (not sure about the temp) in July and August but it isn't to bad the rest of the year.

If I am able to mount the tristar in the attic the optional remote display would be very handy. Thanks for the suggestions.

Cariboocoot

Re: choosing second charge controller

I don't suppose you could dig a hole in the ground instead? As a rule electronic components do not like heat. Very bad for them, heat. The controller would not be able to handle full current (much of current handling has to do with being able to dissipate heat - the hotter the environment, the harder it becomes). Batteries likewise do not like to be hot. The charge Voltages have to be adjusted downwards - a remote battery temperature sensor is a must - and they will have less actual capacity.

If you could construct some venting that would enable the space to draft air through it would help. Either 'chimney effect' or forced air. If the temp is likely to exceed 40C you will have trouble.

For what it's worth I have considered moving my equipment upstairs for space considerations. The knowledge that the temp in the area I'd like to place it easily exceeds 40C in Summer is why it's still at ground level. If I could figure out a convenient way to access it from above it would all be going below the main level. This is in a 60 year-old log cabin in the Cariboo, where our ambient temps swing from -40C to +40C through the year. Subterranean storage will keep temps moderate and consistent, which batteries especially like.

eajonesyk2

Re: choosing second charge controller

ok i will give some more thought to the placement of the controller and batts. We don't get quite the extremes you do but it does hit 100f. every once in awhile and we get a handful of below 0f days in the winter along with plenty of snow (5200 ele.).

I definitely think the tristar 45 will be the right fit for me. Solar panels have come down in price an unbelievable amount in the last year or two. Do you expect any of the electronics (i.e charge controllers) to follow this price march downward?

Cariboocoot

Re: choosing second charge controller

Here's some good news for you: at 5200 feet elevation the panels will produce better than expected. Normally you'd see about an average 77% of the nameplate rating for power output with an MPPT controllers. I'm at 3200 feet and mine run 80% to 84% depending on the time of year. You could see as much 86% due to the greater insolation at higher elevation (thinner atmosphere).

Some equipment has come down in price, some has remained stable. I don't sell the stuff (none of us moderators are associated with the forum host company in any way) but I think those who do would agree that prices in general are about at the bottom now. The only exception would be new technology which inevitably comes out at one price and then slides down at least a bit. I noticed recently that the new Outback Radian inverter has dropped about $300, but you won't be buying one of those. MidNite only recently announced its 'lite' version of the Classic; took the LCD display out and dropped the price.

Occasionally manufacturers introduce new versions of models which means the old ones may be had 'new' at a discount if you shop around a bit.

eajonesyk2

Re: choosing second charge controller

that is good news about the greater insolation, i guess that will make the four panels that much more likely to meet my 1kwh goal. That should help offset my complaints of having to use sunscreen all the time. I am going to wait until xmas to order a controller. We may get a bonus at work this year, if things stay busy. I am in in no hurry to install the system at the cabin. but i wanted to play around with everything here at home first. I am planning on making a trip down there in late december (snow or no snow) to check on how the sun is shining on different parts of the property.

I wish my other neighbor from down there could read this forum. He has been taught that deep cycle batteries should be run way down (like ni-cads) before charging them back up. He was running two banks of 12 volt marine batteries from wally world and would run one bank until the inverter shut down and then switch over to the other bank. He had around a dozen batteries in each bank but is now down to one bank. Amazingly enough some of the wally world batteries are from 2004 and still hanging on. He is currently using a xantrex c-35 controller which is probably way to light for the size of battery bank(s) he was running.

Cariboocoot

Re: choosing second charge controller

Well "marine-RV" batteries aren't true deep cycle; they're automotive batteries with thicker plates. For RE purposes they don't work well. Aside from that, lead-acid batteries have no memory effect so draining them down low is not necessary. Nor is it desirable: the deeper the discharge, the shorter the life. About 25% DOD seems to work best for battery performance vs. longevity.

With enough panels behind it, a C35 can put out 35 Amps which is normally what you'd pick for peak current potential on a 350 Amp hour battery bank. Those RV batteries are likely 90 to 100 Amp hours.

Lots of people get it wrong, mainly because of all the misinformation that's out there. We try to get it right here, but usually have to deal with generalized answers for the most part. Fine tuning a system to the particular location and use can be as much art as science.

eajonesyk2

Re: choosing second charge controller

Hi Cariboocoot, I had a couple of more questions if you have time.

Could I run six of the 145 watt panels in series (108 volts give or take) through the tristar mppt 45? The Voc on each panel is 22.3 volts. If so could I then get by with 10 awg wire. The 10 awg wire is about half the price of the 8 awg wire.

Cariboocoot wrote: »

Welcome to the forum.

Well you do have a situation, don't you? Let's evaluate the components separately and then look at the compatibility.

DM 145 Watt panels. Vmp is 18 as I recall and Imp about 8. Perfectly usable in a parallel configuration with a PWM controller on a 12 Volt system. You've got four, so you'd need to be able to accommodate 4 * 8 = 32 Amps. This means a 40 Amp charge controller. The wire distance is a problem, as the Voltage drop over 100 feet would be significant; you'd need 4/0 wire to handle it. But they could also be configured as 2 x 2 and run through an MPPT type controller (like Morningstar's 45 Amp unit). This means you'd run 2X the Voltage and 1/2 the current and use 4 AWG. You could also run all four in series for 72 Vmp @ 8 Amps and use 8 AWG for the 100 foot run.

Unisolar 64 Watt panels. Are these the ones with the 16.5 Vmp? Some of their panels in this size have a very high Vmp (60+) with very low current. Not a very practical panel either way. The 16.5 Vmp is low for charging a 12 Volt system; at higher temps there may not be enough Voltage from the panels to achieve charging. I would not invest too much time/effort/money on these.

The two charge controllers: neither would be on a list of my favourites. The Blue Sky is about $270 brand new. It has an input limit of 30 Volts so it's no good for dealing with the long wire run. I think the Sunforce is just a shunt controller; not much good for anything.

So you really need to change the charge controller to a decent MPPT type if you're going to handle that long run with any practical wire size.

What can you get out of those four 145 Watt panels? Pretty good: 580 Watt array will probably yield 1.2 kW hours per day AC (based on 4 hours sun). So it looks like those panels alone with a decent MPPT controller could handle your 1kW hour power need.

How are we doing so far?

Cariboocoot

Re: choosing second charge controller

eajonesyk2 wrote: »

Hi Cariboocoot, I had a couple of more questions if you have time.

Could I run six of the 145 watt panels in series (108 volts give or take) through the tristar mppt 45? The Voc on each panel is 22.3 volts. If so could I then get by with 10 awg wire. The 10 awg wire is about half the price of the 8 awg wire.

Six of those panels would exceed the TriStar 45's max output current: 6 * 145 = 870 Watts @ 77% efficiency = 670 / 12 = 55 Amps out. So for that many panels you'd need the 60 Amp version: http://www.solar-electric.com/motr60ampmps.html
But the Voc could also be a problem (22.3 * 6 = 133.8) if you get any cold temps; the V in max of the controller being 150 this could be exceeded.

Keep in mind the main problem with Voltage drop occurs only at maximum power output. This will be at the lowest battery Voltage when they need the most current. Losing a couple Volts from the array at that stage will not have a serious effect on charging.

eajonesyk2

Re: choosing second charge controller

What about the midnite classic lite 150 or 200? Would that safely run the six panels in series during the cold spells. We do get plenty of single digit and low teen temps with sun.

I could save about $210 (based on 250ft roll purchase) on wire if I could run the 10 awg. The 145 watt panels are going for $320 a pair. Regarding the couple of volts loss you mentioned, does that mean I could run the 10 awg with the 4 panel 72 volt set up and not lose much in charging power over the 8 awg wire? That would be my preferred option for obvious reasons.

Cariboocoot wrote: »

Six of those panels would exceed the TriStar 45's max output current: 6 * 145 = 870 Watts @ 77% efficiency = 670 / 12 = 55 Amps out. So for that many panels you'd need the 60 Amp version: http://www.solar-electric.com/motr60ampmps.html
But the Voc could also be a problem (22.3 * 6 = 133.8) if you get any cold temps; the V in max of the controller being 150 this could be exceeded.

Keep in mind the main problem with Voltage drop occurs only at maximum power output. This will be at the lowest battery Voltage when they need the most current. Losing a couple Volts from the array at that stage will not have a serious effect on charging.

Cariboocoot

Re: choosing second charge controller

The MidNite Classic controllers have the added bonus of 'hyper-Voc' which allows them to take up to the V rating + battery system Voltage. So the max in on the 150 with a 12 Volt system is actually 162. Your six panels could run up to (133 * 1.3) 173 Volts, depending on how cold it gets there. The Classic 200 would handle this without question. Keep in mind they don't operate at these Voltages, they just don't fry or quit. (The number rating is the peak operating Voltage.)

You've got a 100' wire run. The array with four panels would be 72 Volts and 8 Amps. On 10 AWG you'd have a maximum of 3.5% Voltage drop, so there would still be plenty to charge with. As the batteries come up the current demand will go down and the Voltage will rise. When the current drops to 7 Amps the V-drop is about 3%, and at 6 Amps it's around 2.5% which is under the arbitrary '3% maximum allowable' loss.

If you go to six panels in series your Vmp will be far away from your battery bank Voltage and the conversion loss might exceed the power loss from V-drop.

Then if you need to add more panels later because of an increase in battery bank size you would have no room for expansion as the V-drop under full current will get worse.

So now it's time to think about what future expansion might be, and plan for it carefully.

eajonesyk2

Re: choosing second charge controller

Ok, So If I bought the classic 200 and ran 10 awg wire from the array, I could simply add two panels in series but would suffer conversion losses that may be greater than the v-drop? Or I could add two more panels in series and up the voltage of the battery bank to lets say 48 volts and avoid much of the conversion losses and the v-drop?

Cariboocoot wrote: »

The MidNite Classic controllers have the added bonus of 'hyper-Voc' which allows them to take up to the V rating + battery system Voltage. So the max in on the 150 with a 12 Volt system is actually 162. Your six panels could run up to (133 * 1.3) 173 Volts, depending on how cold it gets there. The Classic 200 would handle this without question. Keep in mind they don't operate at these Voltages, they just don't fry or quit. (The number rating is the peak operating Voltage.)

You've got a 100' wire run. The array with four panels would be 72 Volts and 8 Amps. On 10 AWG you'd have a maximum of 3.5% Voltage drop, so there would still be plenty to charge with. As the batteries come up the current demand will go down and the Voltage will rise. When the current drops to 7 Amps the V-drop is about 3%, and at 6 Amps it's around 2.5% which is under the arbitrary '3% maximum allowable' loss.

If you go to six panels in series your Vmp will be far away from your battery bank Voltage and the conversion loss might exceed the power loss from V-drop.

Then if you need to add more panels later because of an increase in battery bank size you would have no room for expansion as the V-drop under full current will get worse.

So now it's time to think about what future expansion might be, and plan for it carefully.

Cariboocoot

Re: choosing second charge controller

eajonesyk2 wrote: »

Ok, So If I bought the classic 200 and ran 10 awg wire from the array, I could simply add two panels in series but would suffer conversion losses that may be greater than the v-drop? Or I could add two more panels in series and up the voltage of the battery bank to lets say 48 volts and avoid much of the conversion losses and the v-drop?

Yes; the difference in conversion efficiency at such high Voltage on a 12 Volt system could be greater than the 3.5% max V-drop from running the 72 Volt array.

However, increasing your system Voltage to 48 would reduce the conversion loss considerably. So if you want to "suffer along" for a while with a 12 Volt system losing about 6% max power over-all until you up the system Voltage then the Classic 200 would be the way to go.

eajonesyk2

Re: choosing second charge controller

Cariboocoot can you explain how you calculate conversion losses?

And to put the 6% conversion loss in amps would I be correct with this scenario:

Six 145 watt panels connected in series (about 108 volts) and run through 100 ft of 10 awg to Midnite Classic 200 and into 12 volt battery bank would yield about 51 or so amps going to the battery? If this is close to accurate what gain in amps might I see if the battery bank was converted to 48 volts?

Once again I am sorry for stretching out this post so long but you have provided very good info and I want to learn everything I can before spending another dime.

Cariboocoot wrote: »

Yes; the difference in conversion efficiency at such high Voltage on a 12 Volt system could be greater than the 3.5% max V-drop from running the 72 Volt array.

However, increasing your system Voltage to 48 would reduce the conversion loss considerably. So if you want to "suffer along" for a while with a 12 Volt system losing about 6% max power over-all until you up the system Voltage then the Classic 200 would be the way to go.

Cariboocoot

Re: choosing second charge controller

I don't calculate conversion losses, but the companies that make the charge controllers do. Basically the higher the array Voltage is in comparison to the system Voltage the less efficient the conversion. So an array with 72 Volts is just fine on a 48 Volt system and will run about 97% efficient but the same array on a 12 Volt system with the same charge controller might run at 93% efficiency (power loss).

This is not the same as the power loss from Voltage drop in the wiring, but the effect is cumulative. So if you lose 3% in the wiring and another 3% in the conversion you get a 6% total power loss.

It may not sound like much, but it adds up. An array + charge controller usually runs 77% of the 'nameplate' rating. If you have this extra 6% loss from wiring and extra-high Voltage the efficiency drops to 71%. That is in effect losing more than 1/4 of your array.