This installation tip was provided by Douglas Grubbs, applications engineer at Morningstar Corporation
Large off-grid solar and battery backup PV systems often require greater charging than a single controller can provide. To meet high power charging requirements, several solar controllers can be connected in parallel to a battery bank with each controller connected to its own separate PV subarray.
For MPPT controllers, MPPT tracking of each subarray is preferred, as there is less impact with voltage mismatches. In addition, failure with one controller will not affect the solar production from the rest of the array. Another consideration is that BOS requirements for larger arrays can be more challenging. Therefore, multiple solar controllers with separate subarrays should be considered not only feasible, but optimal.
When using parallel solar controllers, the battery must be sized to handle the combined charging current as recommended by the battery manufacturer. It’s also important to note that each controller will have its own separate solar array, with each array configured in accordance with the solar controller specifications.
Morningstar’s TriStar and TriStar MPPT controllers’ voltage regulation is accurate enough to synchronize and coordinate the charging regulation for parallel operation without the need for intercommunication. The controllers will transition to and from the different charging states at approximately the same time if all of the following conditions exist:
- All of the controllers are set with the same charging profile.
- A common battery voltage sense is connected to each controller. A
- battery remote temperature sensor (RTS) is used with every controller.
During absorption (or boost) charging, the parallel controllers will start limiting the voltage at approximately the same time, since they will all be sensing the same voltage and have the same charging profile. The controllers may end absorption charging at slightly different times, but this will typically be of little or no consequence.
The monitoring of the system in Figure 2 shows that the charging can alternate between the controllers. However, the total combined charging will fully charge and maintain float.