Our Odyssey

Energy Management System

One of the great things about the SW4024 is it can run almost everything we need on the coach on a daily basis. Indeed, many high-end motor coaches come with one of these where all the power for the coach runs through it, except for maybe an air conditioner.

Since boondocking in quiet (read: generator not running) solitude is our preferred mode, we arranged things so that everything we need is connected to the SW4024 and can run from the batteries. The things that are not connected to the SW4024 are generally things that help us take advantage of a big shore connection when it's available (to save some diesel), such as the electric heat for domestic hot water (normally supplied by our hydronic system) and electric space heaters (likewise).

Because the SW4024, as a 120-volt device, is necessarily connected to only one of two legs of the generator (or shore power, when available), and since it is big enough, by itself, to use up most of that leg, absolutely everything else is connected to the other leg, which I will call the "dry" side. Even so, it is hard to balance the load on the generator this way.

Bearing this in mind, there are still some times when it is desirable to connect some things, which are normally relegated to the dry side, to the inverter instead. Specifically, when shore power is available, but it is less than a full 50-amp service. This is because one of the features of the SW4024 is "supplemental load support," wherein the inverter will supplement the capacity of the incoming AC line with energy from the batteries to run the load. In this manner, for example, two air conditioners could be run on one 30-amp service. (The running load of an air conditioner is around 10 amps, while starting load can approach 20 amps. The inverter will easily deliver up to 60 amps to the output for such starting surges, even with an AC input of, say, only 15 amps).

When 50-amp service is available, or when the generator is running, it is decidedly disadvantageous to leave these loads on the inverter side. The dry side would be underloaded (bad for the generator, if not the shore power), but, worse, AC power being used to run loads through the inverter is unavailable for battery charging (the SW4024 can draw up to 35 amps, AC, to charge batteries). Lastly, it is certainly not desirable to leave loads like extra air conditioners, or heaters, etc. inadvertently connected to the inverter when running on batteries alone.

To resolve this dilemma, we implemented an "Energy Management System" to automatically move loads from the dry side to the inverter and back. This system starts at the main transfer switch, where an 80-amp contactor with a 240-volt coil allows shore power through to Panel N (the "dry" panel) only when a 240-volt (and undoubtedly, therefore, 50-amp) shore connection exists.

This same contactor passes a 120-volt control signal from the shore connection (therefore only hot when shore is connected) on to "Panel S" -- our "Energy Management" panel -- only when the contactor is "open" (no 240-volt present).

In normal operation, this 120-volt control signal connects the feed for "Panel S" to the inverter via a 45-amp circuit on "Panel I" by energizing a 45-amp double-throw relay. In the absence of this control signal, the relay is de-energized and the feed for "Panel S" is connected to "dry power" via a 45-amp circuit on "Panel N". A pull-to-unlock type toggle switch allows us to override this in either direction, forcing the relay to connect Panel S to either side as required.

A schematic of this arrangement is here. Note that Panels I, N, and S are actually three buss bars in one enclosure. This is necessary because the three panels share a common neutral bus, and a single neutral back to the main ATS. The main ATS diagram, also showing the EMS control signal, is here.

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