Panels for a 24 volt battery system

[Tired sparky]

Hello, I’m currently building a small 24 volt Solar system and have 4 x 160 watt 20 volt panels wired 2S2P which is fine. I might be needing some more panels at some point so have been looking for used ones online. Most of the used panels near me for collection are I think the domestic grid tie type with 30 volt max output? I’ve read these are unsuitable for charging a 24 volt battery bank? I use an MPPT charge controller btw.
There aren’t any used 72 cell 40 whatever volt panels around so is my only option to get some more 20 volt max type and series them again? Or can I use the readily available 30 volt ones with my MPPT?

Thanks,

Steve

[Mike90250]

When connecting random panels in SERIES, you want to match the amps of each panel, and your end amps will be the same at any point in the circuit. The panel voltages will add up.

When connecting in PARALLEL, you match the voltage of each panel and your voltage stays the same, but your amps all add up (on a suitably sunny day)

For charging a 24V battery with a MPPT charger, you want at least 45V Vmp from the panels going into the charge controller.
( keep your Voc well below the max voltage limit of the controller.)

[Tired sparky]

Ok thanks. Looks like all these Vpm 30 volt panels are unsuitable for my 24 volt batteries. Will series some more of the 44 volt Voc, 36 volt Vmp panels.

[littleharbor]

60 cell panels are the most versatile format for typical MPPT controllers. Mike mentioned 45 volts as a minimum input voltage for a 24 volt system.

[Tired sparky]

That’s 45 volts Voc surely? 72 cell panels would be great but I can’t get them here. 60 cell panels only reach 36 or 38 volts Voc so are no good either. My only option is to series two ‘12’ volt panels which give 44 or 45 volts Voc?

[littleharbor]

I'm referring to series wiring those 60 cell panels . Not sure you're getting the higher voltage MPPT thing. Also Voc is only important when considering the max voltage your controller will take. Vmp. is the working voltage you need to achieve to allow MPPT controllers to do their thing.

[Tired sparky]

Ok I’m starting to get it, done some more reading here. Seemes like mppt cc prefer a higher voltage up to the controllers max input allowed?

Since i I already have 4 panels in a 2S2P configuration sending 38 Vmp to the controller am I now limited to that voltage or can I add new panels to the system with 60 Vmp? Can I mix several arrays of different voltages?

[Tired sparky]

Following on I suppose I also have the option of series wiring all four panels and sending 76 Vmp to the controller? I have an Epever 4010 AN which is ok up to 100 volts input. Would this be the best way to charge my 24 volt batteries? Btw, I’m in the UK so the panels don’t get too hot too often!!

[Mike90250]

Following on I suppose I also have the option of series wiring all four panels and sending 76 Vmp to the controller? I have an Epever 4010 AN which is ok up to 100 volts input. Would this be the best way to charge my 24 volt batteries? Btw, I’m in the UK so the panels don’t get too hot too often!!

It's a balancing act. The further the PV voltage is from the battery voltage, you get a little bit more loss from the down-conversion. Weigh that against the expense of heavier copper in the wire as you add more panels in parallel
Cold is the killer of charge controllers. When the PV panels are cold, the Voc goes higher than nameplate spec, so you need to use the PV sizing chart that most MPPT mfg websites have. You input your panel model # or spec, and your local record COLD temperature.

[Tired sparky]

It is indeed a proper tricky balancing act!!! So there is a sweet spot where the charge controller has enough volts to do its MPPT dance but not too many where it starts to struggle with down regulating the volts to the battery bank size?

I dont have cable size issues so much with my set up. It’s a small system to start with and the PV array is less than 10 metres (30’) from the controller and battery’s which are together in a shed. Plus I was an electrician and have cable coming out my ears!!! . I have run mine in 6mm square cable ( 9 or 10 gauge).

As for weather temperatures, -5 to 30 degrees C (23 to 86 degrees F).

I think i will just have to run the system in its current 2S2P configuration and see what happens? If it’s ok I can easily add more panels of the same size.

I’m guessing I can’t mix two arrays into the same controller, one imputing 37 volts Vmp and the other 60 Vmp?

[Mike90250]

........

I’m guessing I can’t mix two arrays into the same controller, one imputing 37 volts Vmp and the other 60 Vmp?

Correct, the actual voltage will be pulled to the lowest panel in the parallel array. So your 60V panels will reduce nearly in half, as they get pulled down to 37v. And, that may cause a problem with the 37V panels being backfeed so much.

In parallel, match volts
in series, match amps

[Tired sparky]

Thanks, think I’m all set with info to get me going now!

Steve