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blacksquirrel avatar image
blacksquirrel asked

SS MPPT 150 / 85 MC4 VE Works with 3 panels, not 4?

I'm building up a small system, and was doing some preliminary testing and getting some strange results. Important stats:

  • 16S1P lithium iron phosphate with BMS. Charge level is about 50%. This pack is known to be functional and is in good balance (<0.05 V maximum voltage difference). 200 Ah capacity.
  • SmartSolar MPPT 150 / 85 MC4 VE, rated for 48V packs (says so on the label affixed to the unit). Sorry I don't have a serial number at the moment but can get one if needed.
  • I have a set of identical solar panels, all 100W nominal 12 volt panels, Voc measures about 21-23 volts on all panels in full noon sun. All appear to be functioning.
  • I understand that the charge controller needs at least 5V higher solar voltage than the battery voltage.

I was doing some preliminary testing with just a few solar panels before ramping up any capacity, mostly just to make sure the MPPT was working, and I saw some strange results. I'm sure I'm just missing something obvious.

I wired 3 panels in series and measured a voltage in full sun of around 66 V. Plugged that into the MPPT and it worked perfectly. After only a few seconds it started putting 245 W into the battery pack, and I confirmed charge current with my clamp meter as well as the battery pack's BMS. So far so good, and not bad considering I'm not that far above the battery pack voltage.

The MPPT controller is rated for up to 150 V open circuit, so I think I should be good to add a 4th panel in series. Which should bring the panel voltage up into the 88-90 V range. Well within the 150 V rating. I added a 4th panel, and the MPPT reported the expected voltage (confirmed with a voltmeter just to be safe). But after adding a 4th panel, I get a charge of 1-5 W, even after waiting around 10 minutes in full sun. Clearly the charge controller isn't happy, but I'm nowhere near the capacity in either voltage or current (unless I misread something or looked at the wrong spec sheet, totally possible!).

I initially assumed a possible bad panel or connector, but I have a bunch of the same model panel, and tried swapping out a few along with checking all connections. All good, plus the MPPT is showing expected panel voltage. I used the built-in LiFePo4 settings in the MPPT configuration (settable via the DIP switch on the unit), which seem to work fine with the 3 panels.

Like I said, I'm sure it's something dumb, but what am I missing? Thanks!

MPPT Controllers
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6 Answers
snoobler avatar image
snoobler answered ·

It should actually take 5.

When you removed the 4th and went back to 3, did it work?

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blacksquirrel avatar image
blacksquirrel answered ·

Indeed it did, and I left it running for a few hours afterwards. No problems, ended up putting about 500 Wh into the battery pack.

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snoobler avatar image
snoobler answered ·

Did you try 5?

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blacksquirrel avatar image
blacksquirrel answered ·

I forgot to include it in the original post, but yes I did! 5 panels also shows 1-5 W of charge, even after 10 minutes. Also when I dropped back to 3 panels I included the (suspect?) panel #4 I had added, dropping one of the original 3. It worked great, 245 W of power.

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kevgermany avatar image
kevgermany answered ·

Might be the BMS shutting off charging as the MPPT tries to push the current higher.

Would be good to see a screen shot of the MPPT when charging with three panels and when not charging with 4 panels.

Also the battery settings in the MPPT.

More BMS/battery info would help also.

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blacksquirrel avatar image
blacksquirrel answered ·

Unfortunately I didn't grab screenshots, and I won't be able to test the panels again until the weekend at the earliest. (Argh I know, rude to ask a question then immediately fail to have the information needed for others to help, sorry!). I will say it threw no errors at any point.

I'm currently using a Daly 200A BMS, but I also tried swapping out a Heltec active balancer with no change in behavior. I've used the Daly BMS with this pack as an UPS and it routinely charged with way more current than this mini solar setup (22 amps when charging from wall power, versus 2-3 amps with solar). The BMS also throws no errors of any kind. (That's actually what kicked this off, I have a functioning homemade UPS and wanted to see if I could add supplementary solar to it.)

I'll pull the battery setting from the MPPT when I get home later.

At this point I'm guessing one of the panels has an intermittent poor connection. They were discount panels and I hand-crimped the MC4 connectors myself, so there is an elevated chance of a problem. I'm thinking that the poor connection is presenting as a resistance, which doesn't show up in the open-circuit voltage but the MPPT sees the second it starts drawing real current. When swapping panels around I may have just gotten lucky/unlucky with the connection quality. Just a guess at this point though. I think I need to test each panel individually and with current draw. I'm no stranger to hunting down problems like this.

I mostly just wanted a reality check that I'm using the MPPT within it's specifications and not doing anything obviously wrong.

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