ASK THE EXPERTS: Controller-Array Sizing

Zack's PV array.

I had a question regarding Zeke Yewdall’s comment about replacing 30-year-old batteries (“Ask the Experts” in HP183). He said that some charge controllers, such as MidNite Solar’s, are current-sensing, and therefore can be purposely sized with an array that is larger than theoretically recommended.

I have an off-grid 1.9 kW PV array (ten 190 W Sanyo modules), and approximately 2 kW of 320 W modules in storage. I would like to add these extra modules to the array to increase PV production on cloudy days and keep my 760 Ah battery at a higher state of charge. Do I need to replace my charge controller?

Zack Gore • Ontario, Canada

While you don’t necessarily need to replace your charge controller, you will need to add a second charge controller to accommodate your new modules and their string voltage. But first I would like to further explain the concept discussed in HP183. Oversizing a PV array can be done with some MPPT charge controllers that can regulate maximum output current. But what situations would warrant this?

For the majority of the winter, the weather may be poor and the sun low in the sky, with the location rarely receiving more than 50% full sun. In the summer, the days are much sunnier, and we don’t need as large of an array. Doubling the array size, but not increasing the charge controller’s size, could be effective in increasing the system’s winter production. In the winter, when it’s often less sunny, a smaller charge controller would generally not be a limiting factor. In the summer, the charge controller would limit the output amperage. This could eliminate the need for upsizing the charge controller, wiring, etc.

But you’ll need to confirm with the manufacturer that the MPPT charge controller can limit output amperage and make sure the array does not exceed the charge controller’s rated input current. For example, you can’t put 60 A into a 30 A charge controller and have it limit the output to just 30 A.  When array size is increased beyond a charge controller’s traditional sizing, the array voltage, not its current, needs to be increased. The array’s Voc maximum will need to remain less than the charge controller’s maximum input voltage, too.

Regardless of the controller sizing, the array must consist of similar PV modules to use the same controller—at least the same Vmp for each string of modules. Mixing newer modules with the older ones will not work. Using multiple controllers allows different PV modules in the system, especially if there is a mix of 36-cell, 60-cell, 72-cell, or 96-cell modules, or different PV technologies (crystalline and amorphous, for example).

Zeke Yewdall • Ward, Colorado

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