Mixed Voltage Panels

[Rigelation]

Hi folks! My father and I built an off-grid system for his home back around 2015. We use 8 6V golf cart batteries and have 14 200-ish Watt, 24V panels, and an Outback inverter + charger. Recently, we decided that we could use more charging power, especially in the less sunny months. We bought three "48V", 400W panels from some solar warehouse in the Bay Area. I say 48V in quotes because only much later did we realize that they are actually rated for 47.5V output, rather than the higher output that I think is more standard (something more like 70V or more, intended for charging a 48V system). Is there a good circuit that we can build or buy to, say, put all 3 of the new panels in series for ~144V, then reduce the voltage to match the other panels without too much power loss? Apologies if there is a previous post that got to this, I did a bit of searching and couldn't find anything.

[Mike90250]

Midnight Solar has a 200V and a 250V model of their Classic MPPT controller. A MPPT controller will downconvert the high panel voltage to the battery charging voltage.

A less expensive way, is to use a "normal" 150VDC charge controller , and wire the 3 panels in parallel, for 47.5V @ 25A You will also need a combiner box for 3 solar strings
to safely combine the panels


400w x 3 @ 47.5V = 25A from the 3 panels in parallel. After down conversion to battery charging voltage of 28V, you would be adding about 42 charging amps to your system.

[Rigelation]

Thanks for the reply, Mike! Thanks to your answer, I am now starting to see that MPPT charge controllers (well PWM I guess too?) can be strung together, but some care needs to be given so that they don't try to equalize the batteries at different times, and maybe some other things. I had imagined that you couldn't combine charge controllers, because once one was applying a voltage to charge the batteries, how would another know what the state of the batteries was? But maybe it's something about current that the batteries will accept at a given voltage? Anyway, I look forward to learning more about this now that I know what to look into!

[Mike90250]

Each charge controller ( PWM & MPPT ) need their own solar array. But several charge controllers, adjusted to the proper voltages, can charge a battery bank at the same time

[chrisski]

With proper voltages you could even mix PWM and MPPT controllers.

For my RV, I use three different VIctron SCCs for three different arrays feeding the same battery bank. The VIctrons are set to the same absorption an float voltages. They even have a virtual network and work together. Documentation is a bit lacking on what this virtual network actually does but I did find out that if the charge profiles are not the same for the three SCCs, you get a warning, if I set one to equalize all three will equalize and the same when I turn equalize off, and build, absorption, and float is entered at the same time.