Re: [A/S] Electrical Independence

[kurt@xxxxxx]

First--apologies for replying to this somewhat old topic.  I'm a new 
member (and an electrical engineer who works in DC power systems for 
telephone companies.)


> So I would build a crossover switch between my alternator and 
puller's 
> battery.  When I throw the switch the alternator's output is 
redirected 
> through #4 wire to a plug installed very closeby. on the front end 
of the 
> puller.


This 'crossover switch' should be a continuous-duty solenoid rated at 
or above the maximum current you expect to see.  You might consider a 
Cole-Hersee 24143 (200A continuous, about $43) or maybe a 24059 (85A 
continuous, smaller & about $28.)  Hook one power stud to the trailer 
battery feed (that #4 welding cable) and one to the alternator 
terminal (throught a circuit breaker directly to the stud.)  Hook the 
control line to an ignition-hot lead so the two batteries are only 
connected when there's power.  Ideal would be a lead that's only hot 
when the engine is actually running but those can be hard to find on 
older vehicles.

For an improvement on this, rig the control line to an SPDT switch so 
you have three choices:  Normal (as above), off (for when the 
trailer's disconnected and you don't want to send power back there), 
and forced on (uses your trailer battery to jump-start the truck.)



> I park the vehicle next to the trailer's special battery charging 
plug.  (2 
> of the new high tech 12 volt batts.) and boost the idle speed to 
1500 
> rpm?  Assuming I have a big 125 amp hour alternator, should I not 
be able 
> to get a decent charge in an hour or so.

Your charging current will ba a function of engine RPM and the truck 
battery's state of charge.  You want a maximum of C/5 (20% of the amp-
hour capacity of the battery bank) to prevent gassing.  On a pair of 
6V golf cart batteries (far and away the best deal in a deep-cycle) 
rated at 225 AH, this would be 45 Amps of charging current.  You 
could use a 60A breaker which (running at its maximum 80% continuous 
load) would also allow you to use the smaller solenoid.  Measure the 
current with a clamp-on ammeter (borrow one form the battery dealer 
if you can) to determine what RPM is needed to get this amount of 
current flowing to your trailer bank.  Monitor the battery voltage 
when you actually do this out camping to determine when to shut off 
the truck.

A small solar panel is still useful for topping off the battery bank, 
since running the truck for the 5-15 hours needed at this lower 
current would be rather silly.