12 volt mobile equipment conversion to solar off-grid cabin...

[J.P.M.]

Gotcha...thanks!

If that was directed to me, you're welcome.

[Ky Speedracer]

I've seen several references on here and other places where people suggest keeping your solar panel voltage as close to the battery voltage as you can. I'm using a 12 volt battery bank in my setup. Most newer panels all seem to have a 20 volt nominal rating. I understanding that the MPPT CCs are designed to take advantage of additional power and manage or optimize panel output for the battery bank. Is the purpose of "keeping your solar panel voltage as close to the battery voltage as you can" to keep the CC from having to work to hard??

The array that I'm considering will be 930 watts (three 310 watt panels).

• In "parallel" the voltage is obviously lower - The current is 28.5 amps. Charging @ 14.4 volts is 65 amps. The VMP is 32.67 volts. The VOC is 40 volts (46 volts at -0 F).
• In "series" the amperage is obviously lower - The current is 9.5 amps. Charging @ 14.4 volts is 65 amps. The VMP is 98 volts. The VOC is 119 volts (138 volts at -0 F).

The MN Classic sizing tool gives me the "OK" to use a CL150 CC (or a CL200) for either parallel or series.

I guess my questions here are -

1. Why, and/or when is the panel voltage a concern when using an MPPT?
2. Is it better for me to wire the panels in parallel to keep the voltage down so the CC can work more efficiently?

I understand that lower amperage will allow me to run smaller wire from the array, but that really does not have an effect on my setup. Does any of the above really matter in a small array setup?

Maybe I'm way over thinking this...

[bcroe]

If you connect a 20V panel to a non MPPT control, the max current available to the
batt will be the same as the panel output. Put 2 of these in series, the max current
available will be THE SAME, the second panel is wasted. Put them in parallel and
double the current.

If you connect several panels in series, an MPPT controller will REDUCE the input
voltage to what the battery needs, and INCREASE the output current by the inverse
ratio, giving you nearly the same POWER out as in.

The MPPT will almost always deliver more power, because it provides a near
perfect panel-battery match, instead of an approximate one by choosing a panel.
But it needs a little extra input voltage to do its job. Bruce Roe

[neweclipse]

Seems odd that CC are built for voltages 150, 200, 250, 600 and 1000 and yet a few pepole spread the idea to keep nearer to battery voltage...a 12 volt system is yet again a troubled system.

[littleharbor]

Super high voltage CC's are going to be 48 volt (and possibly higher) system voltages. The Benefit of these high voltage comes with being able to run the array output a long distance without having to use large expensive wire. Yes there are efficiency losses, stepping 100+ volts down to 12 volts but they're not so large as to be a deal killer. The controllers are designed to do this. The only problem I see with using 12 volt systems is that they are severely limited in array size and inverting up to high voltage AC power causes issues in inverter size and fusing/wiring. Basically 12 volt is fine for small systems ie; led lights, phone charging, RV type water pump intermittent use. As soon as you add a refer, other appliances, larger water pumping, etc. you will need to step up the system voltage.

[neweclipse]

12volt can work...it's like using a sledge hammer to drive a tack though...but it will get the job done, none the less.

[Ky Speedracer]

12volt can work...it's like using a sledge hammer to drive a tack though...but it will get the job done, none the less.

Thanks for the feedback guys! This whole exercise has been incredibly educational to say the least!
I'm fortunate to have been given the equipment (12 volt) to experiment with a small off-grid system.
The equipment that I still will need to purchase (the CC and the panels) will be purchased with future expansion in mind. All the battery interconnecting cabling is 4/0 and came with the system. The only cabling I need to purchase is from the batteries to the 12v 3000w inverter (less than 4') and from the CC to the batteries.
All the equipment will be over-sized for a 24v or 48v system, but can still be used if I decide to expand in the future.

[neweclipse]

Indeed fortunate to have received all that at no cost.
Very wise to approach this with FUTURE capabilities being kept in mind. Better than getting painted into a corner and no easy way out, since you can reconfigure those free cables and the few new bought parts, if that idea to expand comes about.