wiring question

Re: wiring question

The MPPT portion of a charge controller can be thought of being like a Variac (variable AC transformer). The MPPT CC can take high voltage/low current and transform it into low voltage/high current.

Of course there are limitations. The optimum for the charge controller is to have the Vmp just a few volts above the the maximum battery charging voltage (internal switching losses are lower).

However, the size of the wire from the panels to the charge controller have to be larger (more copper, more money). So, many times people will choose to run the panels near Vmp=100 volts (140 VDC is typical max--have to allow for cold days with no load). There will be a couple percent more loss in the controller--but less losses in the wiring (see controller manual--the usually have a graph of losses versus voltage/current).

You can, if you wish, run Vmp=100 volts and charge your 12 volt bank very nicely (I would not worry one bit). And, if you want to go to a 24 or 48 VDC bank--the panels are all set up for it.

Also, notice that the charge controller output limit is based on current... For example the XW CC is 60 amps--so you have limits like:

P=I*V= 60 amp * 15 volts = 900 watts of solar panels
P= 60 amp * 30 volts = 1,800 watts of solar panels
P= 60 amp * 48 volts = 3,600 watts of solar panels

So--you will have practical energy limits on the solar side vs the battery bank voltage.

Note, you can put more solar panels on as most of the time the panels operate under less than maximum output (hot, dusty panels on roof)... The MPPT CC will not pass more power than its rating. In the winter, raise and tilt the panels near vertical (shed snow with room for snow to clear) combined with the cold and white snow field in front--and you can get some surprising output levels from your panels.

-Bill

Re: wiring question

Alter--not a problem. Solar panel MPPT charge controllers are pretty complex pieces of equipment. And tie that together with NEC (National Electric Code) requirements--and I am not even the best person to ask (I used to work with NEC with the equipment I designed--that was enough).

Anyway, you have it correct... The solar panels when added in series add voltage... Solar panels when added together in parallel--the current adds instead.

And, you will probably end up with series/parallel connections. Say 4 panels in series, in two parallel strings... So the voltage is 4xVmp, and the current is 2xImp.

Many vendors now have websites where you can put configurations together and see that all of the basic requirements are met.

The XW charge controller string calculator is here.

Lets say you are planning using kyocera KD210-GX-LP. The XW 60 amp charge controller on a 12 volt battery bank. And in Wisconsin, you plan on your days running from 0F to 90F.

You will get a charge of 1-3 modules (top) and 1-3 strings (left side).

So a 1x1 setup would give you 190 Watts PTC ("real" power after losses)

Or, a 2x2 will give you the same 760 watts (but Vpanel = 43-77 volts)

And notice that block on the lower left. Max Voc (maximum open circuit voltage)...

With no current being used (batteries fully charged) on a 0F day with sunlight on the panels--this is the maximum voltage the panels can reach. Voc needs to be less than Vocmax of the controller (150 VDC in this case) and needs to be less than 140 VDC for the controller to operate.

And Min Vmp (minimum Voltage for maximum power)--On a hot day, no wind, panels mounted to a roof, drawing full current--this is the minimum voltage that will be generated by the panels (and needs to be at or more than Vbatt charging).

Now, take the same thing, and change the battery to 48 volts.

Now, there is only one "supported" string configuration--has to be 3 panels. Fewer panels will not charge the battery bank. More panels may damage the charge controller (Vpanel = 65-115 volts).

So, that leaves you with strings 3x1, 3x2, etc. (3 per string, 1 or 2 or... parallel strings).

So--"supported" Kyocera 210 watt "24 volt panels" on a 48 volt battery bank leave you with 1x3=3panels or 2x3=6panels, or 3x3=9panels, etc...

You could not support 4 panels (your current "12 volt" configuration) on a "48 volt" battery bank... You would either have to drop one panel, or buy two more...

So, if you wanted to be "flexible"--choose the 1 string x 3 panels in series -- and all you would have to do is change to a 48 volt bank (and probably reprogram the controller for 48 volt bank--I have not read the XW CC manual, so you will have to check yourself on programming instructions).

If you want to wire for 12 volt battery--then 2x2 (series/parallel) or 4x1 (parallel) are the only options with your current panels.

Does that help?

-Bill