Combined watts lower than strings

[Bluedog225]

Good afternoon,

Tl-dr- I’ve been trying to put together two strings of three panels in parallel. It’s not going well. This voltage is as expected around 100 V. (Maybe that’s low but it’s consistent. ).

But the amperage drops when I combine strings. The amperage does not drop when I run a single string. Total production on a single string is a 750 watt range. But around 500 with the combined strings.

These are all from the same batch of used panels that looked fairly new. But maybe they’re jacked. I can pull some others out of storage and see if they work differently.

The individual panels seem to test well. And the strings test at 100 volts consistently.

The amps on the individual panels are in the 7 to 8 range. When I combine the strings every time it drops the amps to 4 to 5 amps

I’ve got three strings of three seeing if I can isolate something wrong. But it’s consistent. Panels the produce 7 A on the tester produced 4 to 5 A when put in series and hooked up to the charge controller.

It’s quite hot out here if that makes a difference. Does anything obvious jump out at you guys? I have tested the strings individually and in different combinations. Same results.

Not sure what to do at this point. I don’t have fuses or combiner box so I’m limited to two parallel strings. At least for this trip. I’ve got two MPPT‘s and was going to run 32P on each MPPT.

Well, I guess I still am. Sorry if I’m rambling. It’s pretty hot.

Any ideas?

 

[MisterSandals]

put together two strings of three panels in parallel.

So that’s 3 panels in series for each string?

 

[Mattb4]

If each string produces but when you parallel them they do not increase in output, but decrease, it would suggest a bad parallel connection.

 

[Bluedog225]

Yes. But only seeing 9 amps at the mppt. I’d expect 14 or so based on the panel testing.

I tested amps with a panel tester. Can I short the panels and test a mow with the fluke clamp meter? Maybe my panel tester isn’t accurate.

This is 6 panels 3S2P. Production is up.

 

[Bluedog225]

Agreed. I’ve redone them a couple of times. Will try something besides a wago or wire nut. I’ve got some big honking ferrules.

 

[Mattb4]

Panel testing via a meter at short circuit does not tell you what you will get in real production. Your MPPT is loading at a voltage whereas a short circuit through the meter you have no idea what the voltage is and therefore can not calculate wattage.

 

[MisterSandals]

Battery charge current oddly an even number 15.00A

Is the charging limited to 15A?

 

[Bluedog225]

Battery charge current oddly an even number 15.00A

Is the charging limited to 15A?

Holy smokes. Yes it is set to 15 A gosh dang it.

Can I just set that to 35 A to match the MPPT locally on the Victron app?

 

[Bluedog225]

And thank you!

This is my second outing trying to get this worked out. I’ve been out here since 530 this morning.

Did I mention it’s a little warm?

 

[MisterSandals]

Can I just set that to 35 A to match the MPPT locally on the Victron app?

Set it to what your batteries can handle.

 

[gnappi]

Matt is spot on. I recently 86'd two panels that individually checked OK with a DMM but connecting them to even a small load like my attic fan they went into the toilet.

 

[Bluedog225]

Worked great. Approaching 1000 watts now. My panels are pretty much flat on the ground, which is not an ideal angle, and they’re used, and I’ve lost the overhead sun, and a hazy day, and the panels are used, and dirty.

1050 watts out of a rated 1650 seems like the right ballpark.

I have no idea how that setting got jacked on the 150/35 MPPT.

It’s been driving me crazy all day trying to figure out what mistake I was making.

Embarrassed to admit, I don’t know what my batteries can handle. I’ve never had enough charging capacity to worry about it. It’s four 48 V rack batteries at 100 amps each. 35 amps is charging at 9%.

 

[MisterSandals]

100Ah x 4 = 400Ah

400Ah x .2C = 80A

1000W / 58V charging voltage = 17A

You could max out your 150/35 and your batteries could handle it easily.

 

[Bluedog225]

Thanks. Do you happen to know if 2C is the max for these things?

I can look it up when I get home, but I thought you might know.

 

[MisterSandals]

Do you happen to know if 2C is the max for these things?

2C is 200A per 100Ah battery or 800A total.

I mentioned .2C which is a conservative LFP charge and discharge number.

 

[Bluedog225]

I have to say thanks again. I would never, repeat never, have thought to check the settings.

Been a rough day. Now that it’s working, I can set the rest up at night.