Mix Panel for off grid battery system

[monogram]

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

Can I connect two 72cell (175W & 360W) panel in parallel and what is the total wattage will I get for this setup?
Does voltage different between those two panel (35.8V vs 38.4v) matter?

[bcroe]

Can I connect two 72cell (175W & 360W) panel in parallel and what is the total wattage will I get
for this setup? Does voltage different between those two panel (35.8V vs 38.4v) matter?

My rule is, panels of the same technology and NUMBER OF CELLS can be efficiently paralleled.
There is a somewhat broad operating curve, allowing this. Bruce Roe

[Wy_White_Wolf]

Can I connect two 72cell (175W & 360W) panel in parallel and what is the total wattage will I get for this setup?
Does voltage different between those two panel (35.8V vs 38.4v) matter?

35.8V to 38.4V is about a 7% difference. While higher than the suggested 5% maximum difference it is workable. Just plan on losing an extra 7% of the 38.4V panels wattage in your calculations.

WWW

[bcroe]

If a panel is operated 1% below its MPPT voltage, you don't lose a whole percent of power, because
as the voltage drops, the current increases. Same thing going the other way. So I suggest that for
small differences, the % power loss is LESS than the % voltage difference. But I'm not going to run the
experiment to prove/disprove it. Bruce Roe

[monogram]

Thank you all for your reply, please let me know what design below produce the most power (wattage)?
Please note, there is 2.6V different between the 2 panel. Both panel are different manufacture.

[littleharbor]

Series wiring the panels will cause the current of the larger panel to be limited to the current of the smaller panel. NOT GOOD
.
Parallel wiring the two will not produce voltage high enough to get the benefits of the MPPT controller.

The only way I see to get the best performance would be to parallel wire the panels and configure the batteries in a 12 volt bank.

[SupraLance]

Originally posted by SupraLance View Post
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)

60cell panels don't reach a high enough voltage to top off a 24v battery. They can only charge it partially, leaving it to die an early death from chronic undercharging. If you think about the fact that a 12v battery requires a 36cell panel to have an 18v potential to fully charge a "12v" battery, than it stands to reason that double the battery to 24v requires double the cells to 72cell. Unscrupulous sellers may refer to 60cell as nominal 24volts, but they are only nominal 20v. It will seem to work but your battery will die early.

Originally posted by SupraLance View Post
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?

Most of those questions are answered by looking at my math:
Suntech 250w + LG 360w in series, 30.7v@8.1a + 38v@9.8a, will pull the 38v panel down to the roughly 8a of the other panel, resulting in 38v@8a = 304w from your 360w panel. 304w + 250w = 550w [This is rough math and is rounded because it isn't precise anyway. What actually happens is far more complex, but this gives you a general idea of what to expect.]

You mention 4.9a, so you must be calculating the BP 175w panel in series with the LG 360w. 35.8v@4.9a + 38v@9.8a, will pull the 38v panel down to 4.9a but leave the volts alone, resulting in 38v x 4.9a=186w from your 360w panel. 175w + 186w = 361w

In parallel works the other way. 35.8v@4.9a + 38v@9.8a, will pull the 38v panel down to 35.8v but leave the amps alone, resulting in 35.8v x 9.8a=350w from your 360w panel. 175w + 350w = 525w

As for real vs fake mppt, lots of good info on youtube about that, but the cheapest confirmed real mppt I'm aware of is the EPever.

Parallel wiring the two will not produce voltage high enough to get the benefits of the MPPT controller.

Littleharbor is right, and this is a serious issue. A real MPPT requires higher input voltage than output voltage. Your better off with separate pwm controller on each 72cell panel, and only using mppt if you can series similar amperage 60cell or 72cell panels.