Ed Note: The following is an email message sent in response to a reader inquiry to our Mailbox. We found the information a unique contribution to the technical literature concerning some of the fine points of RV electrical systems, and have therefore included it here as a free standing article.

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Hi,

After reading your letter in the RV'ers Online Mailbox, I just couldn't refuse to add my two-centavos worth:

For quite a number of years I repaired RV electricals, remanufactured around 10,000 domestic alternators (my poor hands!) and performed battery manufacturing and engineering tests. Because all three fields are inter related I received quite an education in the process. Here are a few of my views on the subjects that you listed.
 
Motorhome manufacturers purchase either a cab and chassis (Class C) or just the chassis (class A) from the automotive manufacturer. Until recently, very little real engineering went toward providing a comprehensive AC and DC electrical system. Style not function was the main criteria. Such items as converters, AC fuse panels, DC chassis electrical fuse panels, and critical relays, isolators, and other vital components were haphazardly placed with little or no documentation or even written specifications, diagrams, or schematics, to be furnished to the consumer. Indeed, most "RV manuals" are little more than a compilation of various component user manuals (heater, water heater, refrigerator, awning) that were provided to the manufacturer by the vendor. The manufactured coach itself sometimes has a scant dozen or so pages devoted to it. As I used to gripe "The darned plastic chandelier over the dining table has more print devoted to it than the entire electrical syst! em"
 
Of all the various remedies you can take to alleviate future problems, perhaps the best one is to access information about your rig from a professional, and because electrical problems seem to be high on the list of potential and continuing ailment it is perhaps there you should focus your first efforts:
 
Finding a qualified electrical technician is not going to be a snap. I have found that perhaps three percent of those individuals who claim to be adept with RV electricals are qualified to use that term. But I would have to say that in every city of perhaps a hundred thousand souls there usually is one shop that qualifies as one to do business with (and it may or may not have RV's as a primary customer base).
 
An independent battery shop, that in addition rebuilds -all- of it's own alternators is a good place to start. You may find that the personnel in such a place to be somewhat "fussy" and that is usually a good first sign (experts are usually quite picky and somewhat short tempered). Have them checkout your coach for the following criteria:
 
1 Clean, charge and test all batteries with a load tester such as a VAT-40 and hydrometer test all battery cells and replace bad batteries as needed.
2 Load test the alternator with the VAT-40 and make absolutely certain that the amperage output of the alternator meets factory specifications: Two rules of mine: The first is that most alternator models have several submodels with varying amperage outputs --- choose the highest rating available, it will save you hundreds of dollars in fuel and batteries. Make certain that the battery wiring in the coach can accept at least ninety five percent of the alternator's maximum output (I cannot begin to tell you how few motorhome systems pass this test and how much agony and money it costs the owner). A 100 amp alternator MUST have the ability to charge -either- the vehicle or the house batteries at 95 amps. Have repairs made to correct this problem.
3 Test the chassis battery electricals for "drains". A drain is an electrical load that "shouldn't be there" when the engine is stopped for purposes of parking or storage. A maximum of .040 (forty milli-amps) is acceptable but today's greedy microprocessor circuits may snack on more than that amount at rest. If the rig is going to be stored for longer than a month, disconnect the chassis battery.
4 Check the coach or house batteries for drain. They should not have -any- drain whatsoever. If more than a 10 milliamp drain is found, you can either install a master cut-off switch (on the negative side please), or if enough sunlight is available wire a small solar panel into the circuit.
5 The engine cranking amperes will be examined and if found to be excessive the problem should be repaired right there and then.

NOTE: Of course you haven't the faintest idea of what a Sun Vat-40 is, and milliamps and such may be incomprehensible to you. But you aren't the one that's going to do the tests. Print out and tear off the instructions above. A shop that knows its way around will immediately recognize the tests and although they may mutter a little about the specific values that I have listed won't challenge any of the tests. If they challenge the tests outright, find another shop. To you, the five steps are sort of a "litmus test" to find qualified technicians.

TIP: Each battery post or stud should have a single (one) wire going to it. Multiple connections should be made at a remote "Connector Stud Block" far enough away from the battery to avoid corrosion. Marine supply houses such as "West Marine" sell excellent battery terminal posts that are housed in plastic. If you have a large number of wires going to a single post, use a brass machine nut to allow stacking of more wires on top. West Marine also sells an aerosol corrosion spray by the brand name "Boeshield" (Made by the Boeing Aircraft Works). This stuff works excellently and seems to be a lot less messy than other corrosion sprays.

Well, I hope this helps out...

¡Saludos de México!

David

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