24V Panels for 12V System and Battery Bank

Re: 24V Panels for 12V System and Battery Bank

Welcome to the forum Aleman... I will take a try at your questions

aleman83 wrote: »

I've been looking around for 2 new solar panels for my house. I was looking for something in the over 200W range, like two 235W or something similar.

Larger solar panels tend to be cheaper on a $$$/Watt basis... However, shipping and insurance can kill you for small orders. So, confirm the cost delivered to your door step before placing the order. If shipping ends up being excessive, you can get ~135 watt panels that are less expensive to ship (smaller physical size).

I already have a big 12V Battery, and also some other 12V equipment like inverters, fridge and lighting which I would really like to keep.

I always like to go back and ask more details about the loads... In general, around 1,200 watts is a "natural" maximum for a 12 volt battery system... Basically, wiring for ~100 amps is not too expensive. Wiring for 200+ amps is expensive (lots of copper), plus supporting fuses/breakers/etc. are not cheap either.

And what size 12 volt battery do you have (amp*hour rating)? We also like to check the size of the battery vs the available charging current.

Many times, trying to "save/reuse" equipment while trying to "up-size" the PV/Battery system is not a good ideal. Better to go with a 24 or 48 volt system (less wiring/support equipment costs/plus solar charge controllers are rated for output current--a 24 volt battery bank will support 2x the solar array wattage and a 48 volt battery bank will support 4x the array wattage vs a 12 volt bank==The old Power=Voltage*Current... 2x or 4x the voltage, 2-4 times the power).

Now I've realized that pretty much all the big panels are 24V or even higher. What would be the most reasonable solution for this? Of course it makes a lot of sense for them to run a higher voltage as it helps a lot in wire size, but I don't want to get into setting up a whole new system.

Other than using a MPPT type charge controller (which is not cheap), you do not have any practical options. If you use a PWM controller--For example a ~35 volt panel charging a 12 volt battery bank, you will lose ~1/2 the power of the solar panel/array (solar panels are constant current devices... If the panel outputs 8 amps, it will be ~8 amps at 12 volts/17.5 volts/35 volts --- So the Power=V*I hits again).

MPPT charge controllers are typically "buck mode" switching power supplies which "down convert" from solar array's high voltage/low current to low voltage/high current needed by the battery bank (a little bit like a DC version of an AC Variac Transformer).

You can find Vmp~35 volt solar arrays that would work efficiently with PWM controllers and a 24 volt battery bank... It will save some costs (as long as the wiring distance from the array to the charge controller/battery bank shed is "short".

I've already heard people talk about mppt controllers like the Tristar MPPT 60A being able to work with higher input voltages and being able to charge 12V batteries. Would there be another solution for my problem, or would a MPPT controller the only way to go? I already tried to understand the specs from the Tristar, but could not completely follow. Do I understand correct that I can have voltages up to 150V to charge a 12V battery but only with a total wattage from the panels of 800W?

MPPT charge controllers are actually pretty neat pieces of equipment. They not only "match" the Vmp*Imp to Vbatt*Ibatt in your system (down converting), the MPPT charge controller can control their output current. For a "60 amp" MPPT controller, it will safely limit its output to 60 amps--No matter what the available array input power is (within input specs of the controller).

For the maximum array, I use this equation to give you a "rough" cost effective maximum array for a battery bank:

• 60 amp controller * 14.5 volt battery charging * 1/0.77 panel+controller derating = 1,130 Watt "max cost effective" solar array

Basically, the MPPT controller will operate near its 60 amp maximum output on hot days with "typically dusty panels". The controller will hit "maximum output current" on cool/cold days around noon time... So, you will lose some power, but not a large amount (at least not worth buying two 45 or 60 amp charge controllers).

My last question would then be: My friend has run his solar panels over 3 charge controllers to the same battery because he has added on to his system over the time. He is very picky with his system, and it has worked well for him that way.

Yep, it does happen... People need to add more solar panels. Or, another issue, as you have seen Solar Panel Mfgs are going out of business left and right--and even for vendors that stay in business, they change models every few years--So you cannot get the older panels (at least not for a good price). So, you would need to have separate MPPT charge controllers if the Vmp/Imp between old and new panels cannot be matched (correctly--there is some wiggle room for some configurations).

So would it theoretically or rather practically be possible for me to go with the Tristar 45A for my two 235W 24V Panels in series (to cut down wire size) charging my 12V battery, and if I need to add on to my system in the future, buy another Tristar 45A and two more 235W 24V Panels charging the same battery with the two setups?

For a 45 amp MPPT controller:

• 45 amp controller * 14.5 volt battery charging * 1/0.77 panel+controller derating = 847 Watt "max cost effective" solar array

So--Assuming you can get matching panels later, you could build out to ~850 watt cost effective array with one 45 amp MPPT controller (note, I am carrying out decimal places so you can reproduce my math--there is nothing magical about 847 watts vs 850 watts in this discussion--typically if you are within 10%--That is virtually "the same" in solar math).

Question over questions. It all seems really simple at times, but then it gets me again . If it would be possible to use individual charging setups like my friend uses that would be really great as I wouldn't have to predetermine the total size of my future system as it might grow . And also it would give me some security in case something breaks on one system I could still keep the other part running while I fix the other one.

Back to loads--Peak Watts, average watts, average watts*hours of operation per day. How much sun do you get where it will be installed. What is your need (summer cabin, 3 season, 4 season, backup genset, grid power and solar emergency backup, etc.)....

It is very easy to get in the "every expanding" system mode--But that can get expensive and frustrating as you start hitting limits and figuring out you really did not save a lot of money overall between your "starter" system and your "ideal/final" setup.

Solar PV systems do not "grow" well. I would suggest discussing/measuring your loads (Amp*hours or Watt*hours per day) and doing a couple "paper designs" before you start laying out cash. You might find better solutions once you have done the comparisons/tradeoffs.

-Bill

Re: 24V Panels for 12V System and Battery Bank

You are very welcome Aleman--We are still missing some information. How many "hours of noon-time equivalent" sun do you have where you live?

Many islands have significant marine layer effects which can kill average production--and/or cause multiple days of less than optimum production (why a backup genset is still needed for most people).

For example, say your usage is an average of 2,550 Watt*Hours per day (note, you should only plan on using ~66 to 75% of your "average" production per day--some days will be better, and some days will be worse). Assuming ~4.5 hours of "average sun" for at least 9 months of the year:

• 2,550 Watt*Hours per day load * 1/0.52 system efficiency * 1/4.5 hours of sun per day = 1,089 Watt Solar array minimum

Since you are looking at a 1,800 watt or so array, you will be able to easily meet the 2.55 kWH per day estimated loads (based on battery bank size):

• 1,800 watts * 0.52 system eff * 4.5 hours of sun = 5,022 WH per day

Notice, that we are sizing the array both on Battery bank capacity and amount of power per day... If you have a smaller bank (1 day of power) and don't use much electricty, you can go with a smaller array. If you have a bank with 3 days of backup (and 50% maximum discharge), the bank is very large and you need a good size array for proper charging (letting a battery bank sit partially discharged for days/weeks/months causes them to sulfate and fail in months/year or so--instead of the 15+ years you would hope for from a good quality forklift battery.

Towards that end, I would suggest a hydrometer, accurate volt meter or DC Current Clamp Meter (Sears has an "OK' one at a good price), and a Battery Monitor (Victron is another good brand). Batteries are easy to kill with improper loading/charging. A Battery Monitor makes it much easier to keep an eye on your battery bank state of charge (especially if you have a spouse, kids, and/or house guests).

Also a Kill-a-Watt meter (or equivalent) for measuring your AC loads (there are 230 VAC versions if that is your line voltage)

Also, if you do things like run loads during the day (washing, vacuuming, etc.)--That will allow you to run directly from the array and skip the ~80% battery efficiency (another 25% more "free" power).

Please note, I am being fairly conservative here in my recommendations. This allows you some growth in power usage and for system/battery aging and dusty panels, etc... Plus I do not know your location/needs/locally available devices--So I would rather have you happy with a "conservative" design vs something that is limping along a few years from now.

Regarding array voltage... If you have some distance to cover--Yes, keep the voltage of the array high (typically, around 100 volts Vmp maximum for a 150 VDC max input controller--The vendor panels have calculators that you can use to verify operating voltages. Also, because you are on a Caribbean island, you most likely don't have sub zero weather ever--so Voc-cold will not rise very much).

So, for your wiring... Assume 240 watt panel with Vmp~30 volts (keep numbers simple). That would be 3xVmp=~90 volts for Vmp-array (perfectly OK).

• 240 Watt panel / 30 Volts Vmp = 8.0 Amp Imp

A three panel array in series would be 90 volts Vmp with 8 amp Imp and 50 foot run and 1% to 3% recommended voltage drop. Using Voltage drop calculator:

• 90 volts * 1% = 0.9 volt drop => 50 feet, 10 awg wire -> 0.96 volt drop
• 90 volts * 3% = 2.7 volt drop => 50 feet, 14 awg wire -> 2.4 volt drop

If you did the same thing with 30 volt Vmp (three panels in parallel) and 24 amp Imp, the 3% drop would be:

• 30 volts and 0.9 volt drop => 50 feet, 4 AWG -> 0.6 volt drop

Much more expensive copper wire for the run.

There is a bit of a down side with many MPPT charge controllers... They tend to run most efficiently with Vmp roughly 2x battery charging voltage (12 volt battery would be around 30 volts Vmp-array). So, you may lose a couple percent in overall efficiency with a controller having Vmp-array of ~90 volts and a ~14.5 volt charging battery (read the manuals, many include efficiency charts for different Vmp-array vs battery bank voltages).

I probably would not recommend running at 30 volts Vmp-array and 4+ awg cable--Also, placing three or more solar panels in parallel, you should add fuses/breakers (one breaker/fuse per string). This helps limit short circuit current in one shorted panel when driven by 2 or more parallel connected strings (look for the series protection fuse rating in your solar panel specifications).

Also check the price of shipping... Breaking your order into 2 or more lots may kill you on shipping/packing/insurance charges.

Otherwise, yes, placing two or more charge controllers in parallel ("home run" the wire from each controller back to your battery bus connections... Do not "daisy chain" from battery to Controller A to Controller B--The controllers tend to be electrically noisy and can confuse each other if sharing the same wire--i.e., changing current and voltage drops makes for "unstable" battery voltage readings between controllers).

The MorningStar TS MPPT controllers are very nice... However, if you add panels to your array--You might look at the Midnite or Outback solar charge controllers, they are rated at 80 amps or more.

Some more night time reading:

Deep Cycle Battery FAQ
www.batteryfaq.org
All About Charge Controllers
Read this page about power tracking controllers
All About Inverters
Choosing an inverter for water pumping

Do you have lightning strikes in your area?

-Bill

Re: 24V Panels for 12V System and Battery Bank

Yes, the controller will run hotter--but if you have good ventilation and not very hot, in general (no desert), would not worry about too much. Again, different brands/designs will have difference performance/efficiencies.

Some vendors put a fan on their controller--So, if the fan runs a lot, you may need to replace it very 3-5 years or so--Still a lot less than the copper. Especially if you end up with two array's worth (that can easily be several thousand dollars worth of copper wiring).

You can look at PV Watts and see if there are any sites that are near where you live (there are a lot of Honduran locations--but I don't know if their weather is similar or not) and if they have similar weather patterns (I don't know if the land nearby blocks weather or not for you). Anyway, here is Belize Intl airport:

Month Solar Radiation (kWh/m2/day)
1 4.40
2 5.01
3 5.62
4 5.69
5 5.28
6 4.93
7 5.20
8 5.18
9 5.12
10 4.83
11 4.57
12 4.41
Year 5.02

Around 4.5 hours of "full noon time equivalent sun" does not look too far off...

You can contact NAWS directly (start with a phone call if you can):

Northern Arizona Wind & Sun
4091 E Huntington Drive
Flagstaff, AZ 86004

Sales & Service: 800-383-0195 | 928-526-8017
Fax: 928-527-0729

Our office hours are from 8AM to 4PM, MST Monday through Friday. We are closed for major holidays. If you are in the Flagstaff area, please stop by our store and visit.

As Vtmaps said--Only the two admins (Windsun and Rick) work at NAWS. The rest of us here are volunteers.

-Bill

Re: 24V Panels for 12V System and Battery Bank

Battery capacity in Amp*Hours at 20 hour rate... (i.e., 140 AH battery discharged at 20 hour rate = 7 amps for 20 hours).

5% of battery 20 hour capacity:

• 0.05 rate of charge * 140 AH (20 hour capacity) = 7 amp rate of charge minimum

Numbers happen to work out that C/20 is 1/20 = 0.05 = 5%

Just picking rules of thumb that happen to match other rules of thumb/measurement conditions. If we picked C/10 as our "battery capacity standard", then we may have chosen 4% / 0.04 as the minimum rate of charge.

I think Surrette/Rolls uses C/10 capacity numbers for their rules of thumbs (as I recall).

-Bill