Mix Panel for off grid battery system

[monogram]

Hello everyone,

I have a saltwater fish tank that required 2 pumps to run 24/7/365 with some LED's that run for 8 hours a day (all devices are on 24V DC). With my current solar system, 250W panel (30.7V;8.1A; 60 cells) with 4 brand new GC2 battery at 215A, it barely enough power the pumps for 24 hrs if I get good sun. I"m planning to add another panel 360W, 38V, 9.8A, 72 cells (cost less than 60 cents per wattage), what is the best setup for it ?

I also have another solar setup for my fresh water tank with 175W, 35.8V, 4.9A, 72 cells to charge my 4 deep cycle battery at 55Ah each in 24V setup.

Here is the list of all the parts I had:

1. BP Solar panel: 175W, 72 cells, 35.8V, 4.9A
2. SunTech STP250-20WD panel: 250W, 60 cells, 30.7V, 8.1A
3. Goftcart battery: 6V GC2-215A x 4
4. Deep Cycle battery: 12V-55A x 4
5. Cheap Chinese MPPT charge controller.

Going to buy:

• LG panel LG360N2W-B3 (from friend): 360W, 72 cells, 38V, 9.8A

Load:

1. 24V @ 3A pumps ==> 72W/Hr ==> 1728W/Day (need to be run for 24/7/365)
2. 24V LED's light @ 6A ==> 144W/Hr (time ON can be adjust, to meet the battery supply)

Can I connect the 2 panel with different voltage/Wattage in parallel as shown on the drawing below? I
If not, what is the correct setup for it?

Thank you in advance for all your help.

Regards,
Key

FYI, I'm in Southern California.

[monogram]

Anyone?
Please help.

[PNPmacnab]

Depends how cheap that MPPT controller is. Some of them aren't real, only MPPT with wind. Nothing will be optimal using the 4.9A 60 cell. Parallel is your best bet as your controller is PWM and the currents will add together.

[Mike90250]

your 250 & 360 w panels are not well matched, putting them in parallel will reduce the 360w to 300w. No harm, but you will only get 30.7V @ 17A
A PWM controller would work, giving you a harvest of 425w , a top of line MPPT would give you 530w is the price differential worth the cost ??

[monogram]

Depends how cheap that MPPT controller is. Some of them aren't real, only MPPT with wind. Nothing will be optimal using the 4.9A 60 cell. Parallel is your best bet as your controller is PWM and the currents will add together.

Hi PNPmacnab, just want to be clear, the panel with 4.9A is 175W with 72 cell not 60 cell like you mentioned.


With my cheap MPPT, the Suntech panel (250W, 8.1A, 60 cell) can go up to 10.2A on the sunny day with cool temperature.

My question is can I connect the Suntech (30.7V, 60 cell) in parralel with new LG (38V, 72 cell)?

If not, what are other solution for the connection on 2 panels with different voltage?

[Sunking]

So please explain why you want to pay 5 to 10 times more for power?

[monogram]

So please explain why you want to pay 5 to 10 times more for power?

What do you mean by paying 5 to 10 times more for power?

• First of all, I want to run keep my fish tank with stable power by solar/battery just in case the energy company interrupt the power during summer time.
• Second, I will save the money on long term run with Solar/battery. May be after 5 years, the saving will break-even on the battery and the panel.
• I will get free power after that 5 years, right?

My question is can I connect the Suntech (30.7V, 60 cell) in parralel with new LG (38V, 72 cell)?

If not, what are other option for the connection on 2 panels with different voltage?

[Sunking]

What do you mean by paying 5 to 10 times more for power?

If you were to use battery power and utilize all the power your panels can generate, just in battery cost alone is going to cost you 5 to 10 times more than buying it from the POCO. Used as backup 100 times more.

Once you look at the cost of just the batteries, nothing else like panels, controller, wiring, hardware, or inverter, just the batteries alone is going to cost you 60-cents to $1 per Kwh. Not sure where you live but POCO's charge 6 to 20 cents per Kwh. So if your fishies use 1 Kwh per day with batteries will cost you $1 in batteries to save 10-cents on your electric bill.

You will be replacing those batteries every few years. There is absolutely no chance to ever save a dime. It is a liability. That is why I asked why do you want to pay so much more for power?

[Mike90250]

I will get free power after that 5 years, right?

Not quite - now it's time to buy a new battery.

Getting a large computer UPS, with enough VA capacity to keep you tank going for however many hours you require, is less expensive than solar, charge controllers. Simply recharge from the household power, at $0.20 kwh - 1 kwh should run a tank for 3-7 days.
A UPS will still have a 5 year battery replacement cycle, but the rest of the gear is less than solar

[bcroe]

If not, what are other option for the connection on 2 panels with different voltage

Your panels are too far apart in operating voltage for efficient parallel operation. If they were
about the same current rating, you might connect them is series using a properly rated MPPT
controller. If I had to parallel them, I might put about 10 big 10A diodes in series with the 72
cell panel to bring down the available voltage, they would run very hot. Bruce Roe

[SupraLance]

1st, a single 60cell panel should never be used with a 24v system, it doesn't produce enough voltage, and that is why your issues. You need another matching 60cell to use this in series through an mppt controller, or you need to use it for a 12v system.

If you add a 72cell panel to a 60cell panel in parallel, it will pull the voltage of the 72cell panel down too low for a 24v system as well. Don't do that, you'd be better off leaving the 60cell panel out.
If you put the panels in series, the voltage will add and you will be good there (IF your controller is truly MPPT), but the amperage will be pulled down to match reducing the watts. So both panels would end up producing at about 8a, one @30v and one @38v, for a combined 68v & about 550w. BUT more likely your controller is actually PWM and can't do this, and that much of an amperage mismatch isn't considered safe. Would not recommend this either.

With an actual MPPT controller, the two 72cell panels (175w & 360w) could be ran in parallel, although this would force them to operate at the lower voltage of the 175w panel. This would only cost you about 10w, producing 525w (at STC). But then your left with the 60cell panel for the other system, and it really needs a 60cell buddy to be any use at 24v.

I think your best bet is to buy another 60cell 250w panel instead of the 360w 72cell panel, and then buy a real MPPT controller to use them. Or sell the 60cell panel and stick to 72cell panels, since they will work with 24v systems through less expensive PWM or fake MPPT.

[monogram]

So please explain why you want to pay 5 to 10 times more for power?

I don't think I pay 5 to 10 times more for the power, correct me if I'm wrong, here is how:

4 years ago:

• Bought a 175W and 250W panel for $150 & $200
• 2 x 12V 100A marine battery @ $80 each
• 5 x 12V 55A AGM battery (free from friend)
• 2 set of solar cable @ $30 each
• 2 set of cheap charge controller at $40 each

==> Total cost: $650

Load:

• 1600W per day ==> 584kW per year
• At $0.25/kW ==> $146 per year

==> The total cost after 4 years usage: $584

Note: All the load I have for my fishies are only 24V therefore I don't need any inverter or loosing the power due to power conversion.


Since I just bought another 4 GC2 battery @ $85 each therefore i'm looking to add another 360W panel (for $200) so it can charge on those 4 new battery. (the other 2 older battery will be use for USP backup or other task). For this new set, this will be for $1700W daily load.

What would you recommend on this new string, should I connect it to an existing string or just build up new string with another cable set and another CC?

[monogram]

1st, a single 60cell panel should never be used with a 24v system, it doesn't produce enough voltage, and that is why your issues. You need another matching 60cell to use this in series through an mppt controller, or you need to use it for a 12v system.

• Would you please explain to me why the 60cell panel can't used with 24V? I have been using this panel for the last 4 years to charge my 24V battery but I don't see anything wrong with it. On my cheap MPPT charge controller, I set my cutoff voltage at 27.2V and power off load at 24V. Some time the MPPT display 26.2V @ 10.2A (on a sunny day with cool temp at around 65-70F)

If you add a 72cell panel to a 60cell panel in parallel, it will pull the voltage of the 72cell panel down too low for a 24v system as well. Don't do that, you'd be better off leaving the 60cell panel out.

• OK, I won't. Thanks.

If you put the panels in series, the voltage will add and you will be good there (IF your controller is truly MPPT), but the amperage will be pulled down to match reducing the watts. So both panels would end up producing at about 8a, one @30v and one @38v, for a combined 68v & about 550w. BUT more likely your controller is actually PWM and can't do this, and that much of an amperage mismatch isn't considered safe. Would not recommend this either.

• What is the risk for amperage mismatch?
• I thought when putting 2 panel in series, the voltage will increase while current is the same which I will get 4.9A output for 68V which equal to 333W, am I right?
• How do I know if the CC is real MPPT, fake MPPT or PWM?

With an actual MPPT controller, the two 72cell panels (175w & 360w) could be ran in parallel, although this would force them to operate at the lower voltage of the 175w panel. This would only cost you about 10w, producing 525w (at STC). But then your left with the 60cell panel for the other system, and it really needs a 60cell buddy to be any use at 24v.

• Please explain on how do I get 525W when connect two 72cell panels (172W & 360W) in parallel?
• Does voltage different between 2 panel (35.8V vs 38.4v) matter?

I think your best bet is to buy another 60cell 250w panel instead of the 360w 72cell panel, and then buy a real MPPT controller to use them. Or sell the 60cell panel and stick to 72cell panels, since they will work with 24v systems through less expensive PWM or fake MPPT.

• I could not find any 60cell 250W panel from my local seller. The only thing I can find now is 360W 72cell panel.

Hi SupraLance, Thank you for your explanation.

What is your recommendation on the new panel? should I build up a new string along with another cheap MPPT for it?

Sorry to ask to many questions as I'm new to pro-solar forum.

[Sunking]

You have absolutely no need for solar, wasting your money, and risking killing your fish. No professional would use solar because it cost too much, unreliable, and prone to failure. In other words you are planning to kill your fish.

You can do what you want, but does not require solar. What pros would use is a special type of UPS, and you can easily make one yourself for a lot less money than solar. Not only is it less expensive to build and operate, but far more effective and reliable.

You first determine how much time you want to be able to run without commercial power. Say 24 to 48 hours. Example let's say 24 hours and in that 24 hours your pumps and heaters use 2 Kwh. That will require a 4 Kwh battery. At that size you are looking at a 12 volt battery system. The required battery Amp Hours = 4000 wh / 12 volts = 333 AH. So you go shopping for 2 x 350 AH 6-volt batteries.

Next step is size a 12 volt battery charger large enough to charge the batteries and run the equipment at the same time. It is real simple C/10, it will take a 12 volt 30 amp charger. Last step is add a 12 volt Inverter around 250 watts which is more than enough. 2 Kwh / 24 hours = 83 watts.

When connected and running, everything is running off commercial power. When and if power ever fails, the batteries take over. When commercial power comes back on, the batteries get recharged and float until the next outage. Your batteries will last twice as long.

To do the same your way with solar to give you the same usable 2 Kwh per day will require:

Panel Wattage = 750 to 1000 watts. $1000 to $1500
60 to 80 Amp MPPT Charger, $400 to $600
10 Kwh 12 volt 800 AH battery, $1300 to $1500
250 watt Inverter $100

Total = $2800 to $3700

My way works out to:

12 volt 30 amp charger, $200
12 volt 350 AH battery, $650 to $800
250 watt Inverter, $100

Total $950 to $1100

Not only is my way 1/3 the cost and more reliable, the battery will last you twice as long before replacement. Listen to SK, he knows what he is doing. So do it your way, you can kiss your money and fishies good-bye

[Wy_White_Wolf]

What do you mean by paying 5 to 10 times more for power?

• First of all, I want to run keep my fish tank with stable power by solar/battery just in case the energy company interrupt the power during summer time.
• Second, I will save the money on long term run with Solar/battery. May be after 5 years, the saving will break-even on the battery and the panel.
• I will get free power after that 5 years, right?

My question is can I connect the Suntech (30.7V, 60 cell) in parralel with new LG (38V, 72 cell)?

If not, what are other option for the connection on 2 panels with different voltage?

1. You can but would be an idiot to do so. You will be throwing away 25% of the power of the new panel wired parallel and about 50% wired in series.

2. Only sensible way is to use 2 charge controllers. One for each panel. With the voltages both would need to MPPT.

WWW