Our Odyssey

House DC Electrical System

It can be argued that the house DC system is the heart of any motorhome. Without it, little else will function -- it's impossible even to flush the toilet without the electricity to run the water pump. And the heart of any DC house system is the battery plant.

Since Odyssey is our full-time residence, and since we spend most of our time away from sources of AC power (we detest commercial campgrounds), we put the biggest battery plant we could fit into the coach. We also did not want to deal with the safety and maintenance issues of flooded batteries, so we specified Absorbed Glass Matt (AGM) items. Eight size 8D AGMs fit nicely into the space above the front wheel wells (which also helped out with some weight-and-balance issues). These happen to be Xantrex-branded batteries, which Xantrex no longer markets (so we got a good deal). These units hold about 225Ah apiece, giving us a 900Ah total capacity. We expect that should be good for two to three days of routine living between charges, and would maintain basic environmental conditions inside a parked coach for over a week if we had to leave it someplace. Here's a schematic diagram of our battery system -- the batteries themselves are shown with positional accuracy, and the other items are "near" where they are shown in the drawing.

As with the chassis system, there is a center tap from the battery plant to support 12-volt loads. We were able to find 24-volt items for most of our needs, such as main lighting and the water pumps, but a few specialty lights, the stove ignitor, and specialty items like the satellite dish and computer hardware come in 12-volt only. Here again a Vanner 50-amp battery equalizer is used to balance the battery levels. One consequence of the 12-volt tap and equalizer is that the batteries can not be arranged as four series strings of two batteries each. All the center points of the batteries must be connected for the battery system to remain balanced. Accordingly, the batteries are instead arranged as two groups of four parallel batteries each. You may note that each group is connected "on the diagonal," meaning the negative connection is made at the left-most battery in the group, and the positive connection is made at the right-most battery. This tends to even out the loads on the batteries, which otherwise would be impacted by the resistance of the paralleling cables and their connection points.

Unlike the chassis batteries, we don't ever expect to "turn off" the house batteries, except in an emergency or for maintenance. Among other reasons for this, just too many things "lose their minds" when the power is completely disconnected, including the Trace SW4024, the Link-10 battery monitor, and a host of electronics. Nevertheless, disconnect switches are included on both the 24-volt and 12-volt outputs of the battery plant, and these are large, high-current, manually operated levers. There are also large fuses just ahead of these disconnects (400amp and 200amp respectively). Here's a diagram, which includes the chassis system.

The single biggest load on the DC system is the Xantrex/Trace SW4024 Inverter/Charger, drawing up to 330amps. For this reason, the SW4024 is located in the same area over the front wheels. This unit also serves as the main charge source for the house batteries, supplying up to 150amps of 3-stage, temperature-compensated charging. It can also start the generator when battery charge level drops below a set point. The SW4024 is described in more detail in the AC power section of this site.

DC house loads are organized into circuits and fed from two groups of hydraulic marine-type circuit breakers that we've named "Panel 24" and "Panel 12." These panels are, in turn, fed from the main fuses discussed above. Here is a directory of house DC circuits. Note that some circuits have two "sizes" specified -- the first is the ampacity of wire used (in case we need to upgrade the circuit later) and the second is the size breaker installed, which may be lower due to availability of breakers (we had a bunch lying around) or due to the specification of the load device installed on the circuit. Panels 24 and 12 are co-located with the AC panelboards in the end of the bookcase cabinetry at the top of the stairs.

Three other items in the DC power system bear mention here. One is the Xantrex PathMaker battery bank combiner, which is discussed in more detail in the chassis section. A 1/0 feeder cable from the chassis batteries at the rear of the coach is run up to the house battery area in the front, where the PathMaker is located. Another is the Link-10 battery monitor, which acts as a "fuel gauge" for the house batteries and is connected by means of a shunt between the battery negative and the rest of the system. Last is the pair of #8 wires that runs from the battery bay up to the roof, for future solar panel installation.

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