The instructions below are the same that I have previously forwarded to people in PDF format. (Until now, I hadn't figured out how to post this info on AirForums.)
The OEM converter is still installed in our Bambi; and by using this switch to individually charge each battery (overnight) once a month, the Optima Blue Top, deep-cycle marine/RV batteries that I purchased (one in 2006 and the other in 2007) are still going strong (as of September 2013).
FYI, our Bambi is connected to shore power 24/7, and the Perko switch is turned to OFF to prevent overcharging the batteries and boiling them dry. While on shore power, the converter alone powers all 12-volt DC lights and equipment.
1. Battery switch must be turned to 1, 2 or ALL when towing, since battery power is needed for the refrigerator and the electric brakes (if the trailer separates from the TV while underway). Also, this allows the TV to recharge the selected battery(ies) while driving.
2. Turning the battery switch to OFF before refueling the TV, automatically cuts propane flow to appliances in newer Airstreams that use electronic ignition instead of gas pilot lights. After refueling is completed, propane appliances should automatically restart when the battery switch is returned 1, 2 or ALL. However, we usually double-check the refrigerator to make sure it is ON.
The switch in the photo below is the same as the one at this link:
Perko 8501DP Marine Battery Selector Switch : Amazon.com : Automotive
There are other manufacturers that make switches that are similar in design, and there may be other sources that cost less. However, this is the one that I purchased.
Two of the four battery isolator switch mounting bolts can be seen near switch positions “2” and “OFF”. The bolt heads are inside the battery box, and nuts, flat washers and lock washers are installed on the outside, with Loctite thread lock sealant to hold the plastic switch securely in place without torqueing the mounting hardware so tight that it deforms the plastic switch body. The ragged white trim between the red switch body and the battery box is the remnant of a plastic lid that was used as insulation to keep the connector lugs on the back of the switch from shorting to the battery box. However, by design, a sufficient gap prevents shorting; and the additional “insulator” is unnecessary.
The two green cables running to the bottom of this switch are actually battery cables that look like this:
Accel 1843 Black 2-Gauge 3' Long Silicone Battery Cable Eyelet : Amazon.com : Automotive
Note: The above link is for illustration purposes. The actual cables were purchased at WalMart for a couple of dollars, each.
Also, there are actually three battery cables; a third one comes from underneath the battery box and cannot be seen in the photo.
Two of the three cables that run to the switch are like the battery cable shown in the link above. Both of these cables have one end attached to the back of the battery switch (see details in instructions that come with the switch), and the other ends are connected as follows:
One of these cables goes to the positive marine (wing nut) terminal on one of the batteries in the battery box. This line carries +12 volts from one of the batteries to the battery switch.
The second of these two cables is connected to the positive battery-post connector that comes from the trailer (i.e., the trailer “hot” line). The battery post adapter shown below is used to mate these two cable ends together. These uninsulated connectors are enclosed in the gray rectangular box (shown on top of one of the batteries) to prevent shorting to ground the +12 volts on these joined cable ends. The gray box is a non-conductive (plastic), 110-volt AC box used in regular house wiring, which was purchased from Home Depot for a couple of dollars.
The lug (above) goes in the red, +12 volt, battery-post connector on the trailer “hot” line, and the battery cable coming from the switch is connected to the bottom of the lug using the nut shown. Thus, the output from the switch uses this adaptor to connect to the trailer “hot” line.
The third line coming from the switch is a regular battery cable that connects to the positive battery post on one of the two batteries shown in the photos that follow. This is an extra battery cable that was purchased at WalMart for a couple of dollars.
All of the extra battery cables that I purchased had black insulation, because these were significantly cheaper. As you can see, I spray painted the ends red and added labels to identify polarity.
The new battery cables were inserted in sections of old (green) garden hose to protect the battery cable insulation from road hazards. There is no significance to the color; this is just what I had on hand.
Photos of batteries and cables shown from opposite sides of the trailer follow.
View below is from passenger/curb side of trailer (i.e., starboard side):
In this photo, one can see a short jumper that connects the two negative terminals together, and the ground connector from trailer (black, insulated, negative/ground post connector), which I believe are unchanged from the original factory installation. Original wiring had two batteries connected in parallel.
By using the battery isolator switch, one can now use battery #1, battery #2, or ALL, which connects both of them in parallel. The advantage is that if one battery fails, one can use the switch to connect to only the good battery, and isolate the bad battery from the circuit. In the original, unswitched parallel wiring, if one battery failed (usually an internal short), it would drain both batteries leaving the trailer without power.
By using only one battery at a time, it is easy to tell which one is weak or dead, and quit using it. Or, under high loads, “ALL” can be selected to access the power of both batteries at the same time. In addition, both can be completely disconnected from the trailer by selecting “OFF”, which prevents them from being overcharged when the trailer is continuously connected to shore power, or from being discharged by parasitic loads during storage.
Our Bambi is connected to shore power 24/7, but the battery switch is turned to “OFF”. Then, once a month, each battery is selected separately and charged overnight.
Note: The extra small-diameter cable connected to the positive (wing nut) terminal on the top battery is the wire that powers the electric tongue jack.
View below is from driver/street side of trailer (i.e., port side):
Note that the original, black, ground cable from the trailer is connected to the negative post on the top battery. And, the original, red, +12 volt cable from the trailer terminates inside the gray 110 volt wiring box where it connects to the cable carrying the +12 volt output from the battery switch. The red cable used to be connected to the positive post on the lower battery, when the batteries were connected in parallel.
Again, you can see the green garden hose used as armor sleeves to keep the +12 volt cables from shorting to the sharp cutouts in the battery box, and to protect the exposed cables (outside the box) from road hazards and grime.
While this switch was installed to keep the batteries from going dead in storage, using it once a month to charge the batteries (instead of leaving them connected all the time to the converter or an external battery charger) also prevents them from boiling dry.
I only turn the battery switch to “1”, “2” or “ALL” when driving (needed for the refrigerator, interior lights, etc. and MOST IMPORTANTLY, the ELECTRIC BRAKES; if and when the trailer separates from the tow vehicle, and the breakaway switch is pulled), or when dry camping (without shore power).
In storage and at RV parks when shore power is available, the switch is turned to “OFF”, unless the batteries are purposely being charged. I have NOT replaced the converter, as a 3-stage model is now unnecessary to protect the batteries from overcharging.
The batteries on our Airstream have been turned OFF for as long as 3-4 months while in storage, and they maintained a full charge. (Optima and other AGM batteries should hold a charge for a year or more.)
By the way, the terminal post adapter and insulating box allow all OEM 12-volt cables and connectors to be retained, should one decide to revert to the original Airstream factory wiring and parallel setup.