From the Morningstar website (kudos to them as they have the simplest of the plain english explanations I have seen)
This explanation is for a Matched setup as opposed to Master/Slave.
I personally prefer Master/Slave but that is waaay more expensive and more for domestic/industrial than a reccy user would need
For anyone who doesn't wish to read much further the most common problem encountered is syncing the Float Voltages and Equalisation of different Regs.
For the more techo inclined... enjoy
Key Factors of Parallel Setup:
Since Morningstar uses voltage based State of Charge (SOC) for voltage regulation, no direct communication between the controllers is necessary to ensure they charge correctly in tandem. 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
- Use the same DIP Switch settings for matching the charging control.
- If creating a custom setup in MSView, save the custom setup configuration by using “Write to File” in the Setup Wizard to program all of the controllers with the same settings.
- There is a connection from the battery bank to each controller’s Voltage Sense Terminal pair. This ensures each controller is measuring the same battery bank voltage. Note: paralleling between controllers’ Voltage Sense terminals, in order to share a single connection to the battery bank is an acceptable practice.
- A battery Remote Temperature Sensor (RTS) is used with every controller. This ensures all controllers will be measuring the same temperature for regulation charging, voltage temperature compensation. If the batteries are located indoors in a climate controlled area it may be acceptable to omit an RTS. Note: An RTS is included in the TriStar MPPT box, but not with the TriStar. If using TriStar controllers, RTS’s must be purchased separately for each controller. A single RTS CANNOT be paralleled or shared between multiple controllers.
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 RTS and voltage sense connections help to achieve this charging synchronicity.
The controllers may end absorption charging at slightly different times, but this will typically be of little or no consequence to the health of the batteries. Usually the lagging controller will continue to hold the voltage in regulation and complete the cycle shortly after the other controller(s) have already transitioned to float. In larger systems, a lagging controller may not apply enough charging current to offset the loads to maintain absorption voltage. Often the voltage will eventually decrease to the float voltage, so the float voltage stage is still in effect.
If a lagging controller is causing the voltage to stay higher than float, but not high enough to complete absorption charging, this does not necessarily indicate that there is a problem. At the end of an absorption charging state, it should not take much current to maintain absorption regulation voltage, so it may indicate that the batteries are not fully charged and additional charging may be needed for the given day.
Voltage intolerances can also cause one or more of the controllers to start and finish the absorption charging stage earlier than other controllers in the system. This should have no noticeable effect on battery health. However, if there is a concern about this, custom programming provides the ability to make small adjustments to the regulation voltages and timing to get the controllers to more closely match each other.
Since Morningstar’s voltage regulation is very accurate and there is a voltage sense connection to each controller (there are no voltage drops that might cause each controller to read different voltages) this setup as proven to be effective. Product polling over a communications bus has not been found to be necessary making it simpler to install. The controllers act autonomously without any concern with setting up a communications link.
If the batteries will be equalized, it is important that all parallel controllers equalize at the same time. The TriStar MPPT manual states that “if two or more TriStar MPPT controllers are charging in parallel, each controller may attempt to equalize on a different day. Systems with multiple controllers should only equalize manually to ensure synchronization between controllers.” If automatic equalize (DIP switch 7) is ON, the equalization schedule between the controllers may not be synched. This could cause multiple days of equalization unintentionally and may cause a high voltage disconnect (HVD) alarm for the controllers not equalizing on the given day. In some cases a single controller may provide enough charging power to complete the equalization by itself in which case one controller can be setup with the automatic equalize feature. If multiple controllers are needed to supply more power to complete the equalization stage it is recommended that DIP 7 be set to OFF for manual equalization. Typically, flooded batteries need regular maintenance and the manual equalization can be part of the scheduled maintenance for the system.
