Somebody school me in 12v power transfer please

[Paladin]

If you are powering 12v lights or whatever while in motion either via battery bank or alternator etc and then want to change over to shore power what is the best or conventional means to do it?
I'd imagine a converter but can you use a transfer switch to changeover or is it best to change over manually? I understand few complications etc so fewer gremlins but would a transfer switch such a the Iota 50 which is designed for 120v even switch with only 12 volts dc? I'm not even sure that automatic transfer of 12v is important anyway.

Sorry of this seems elementary but my mind is goo from all of the various planning I'm trying to do.


Another question, I'm thinking of picking up a Marinco power inlet for shore power. I know this is likely a can-o-worms but to cover all bases should I put in a 30a or 50? 120 or 240? I keep hearing that true 50a is a rarity and not really that useful. I'm avoiding 240 but don't want to limit options. Sooo what do you guys who have done this suggest? Sean???
 

[Sean]

DC does not work that way.

You connect all your DC loads (lights, fans, LP detector, whatever) to fuses or breakers in a DC panel, then connect the panel directly to the batteries through an appropriately sized "main" fuse, typically a class-T.  I recommend also putting in a high-current disconnect switch between the fuse and the batteries.

You then merely connect your charging sources -- alternator (via bridge solenoid), inverter/charger, stand-alone charger, or solar panels (via solar controller) to the battery bank.  No "switching" is needed among charging sources.

If you go with a converter (and there is plenty of converter vs. inverter/charger discussion and information in the archives), the converter has an internal transfer switch that switches your DC loads off the battery and onto the converter's regulated DC power supply, usually at the same time connecting an internal charger to the batteries.

With regard to the Marinco power inlet, I strongly advise you to go with the 50-amp, 120/240-volt model.  You can always adapt that down to 30-amp, 120-volt when that's the only thing available, but you can't ever get more power into the coach if you go the other way around.  Besides which, above five 120-volt circuits, or two "thermostatically controlled" appliances, you are legally required to install the larger service -- we just had a long discussion about this topic, as well, here on the board.  (Or was that the other board?).  Remember, 30-amp service delivers 3,600 watts, and 50-amp service delivers 12,000 watts -- more than three times as much power.

-Sean
http://ourodyssey.blogspot.com

[JackConrad]

Here is "our way".  when the engine is running the 12 volt alternator keeps the house batteries charged (putting back what the 12 volt appliances are using).  When we are plugged in or have the generator running, the charger function of our inverter keeps the batteries charged.  No switches needed.

We have a 50 amp Iota transfer switch for our AC (switching between the shoreline and generator). This switch requires 120 volt AC to activate the switch.

We used a 50 amp Marinco shoreline connector and our bus is wired for 50 amp/240 volt (all 50 amp RV connections are 240 volt). We can use an adapter to plug into 30 amp if necessary, but if you wire your bus for 30 amp, you cannot make use of 50 amps when available.

There are other ways to "skin the cat" so to speak and hopefully others will chime in with "their way".  Jack

[gus]

My approach is to always go for the simplest system possible.

My 12v and 120v systems are totally separate, no inverters or any other connections.

I charge my house and/or start batteries from the engine alternator. If I need to charge them while parked I use plain ole battery chargers.

We are mostly travelers so this works for us. If you are dry camped most of the time this might not be the best but it is one way to do it.

My theory is that inverters are not so good with 12v systems, better with 24v.

[Kwajdiver]

Mine is even simpler,   Lights and water pump, works off the 4 deep cycle marine batteries.  I charge them with a A/C batter charger.  It works when I'm  running the gen or connected to shore power.  They have never run low.


Bill

[PP]

My way is identical to Jack's Orange Blossom Spcl. Since removing my basement freezer, the new bats are working flawlessly The charge side of the inverter comes on when needed just like the doc ordered. When on the road the 12v Alternator keeps the house bats charged and the 24v Alternator keeps the coach batteries charged. When the genny is on it's just like being on shore power.
BTW-more parks are offering 50Amp service all the time. Even the Forest Service is upgrading their host sites to 50's.
Good luck with your project, Will

[gus]

Bill,

My setup is approximately the same as yours except I can charge from the bus engine alt if I choose.

All my batteries, start and house have individual shutoff switches, so I can charge or use one or all.

[Tim Strommen]

Easy way to switch 12V loads from the battery to shore power (when available) is to use a 120V-coil SPDT relay.  The comon pin goes to the power bus you want, the Normally-Closed relay input goes to the on-board battery bus supply, and the Normally-Open relay input goes to the converter.

Using this method, when you plug in shore power the relay will active once power gets to the coil, and power will be sourced from the 12V converter.


A few notes:

You may want to add another SPST 12V-coil relay just to ensure that the converter is actually outputting power before you switch the DC source (power the 12V coil from the output of the converter, and have the SPST circuit cut the power to the 120V-coil relay's coil).

You want to add a bunch of capacitance on the load-side of the relay (the comon "output" lead") to prevent the loads from shutting off during the switch event (now that I think of it, you'll also want to add capacitance to the two relay inputs to absorb the inductive "kick" that WILL happen by switching active loads with distance involved on the wires...).


My new way (still designing the PM-circuit boards):  dual battery rails, with a third power rail for shorepower (this has two settings, auto/manual-connect).  I have two strings of Optima D31A AGM batteries for the chassis, one entire power-rail can die or blow a fuse/breaker and I have another to fall back on on a circuit-by-circuit basis (to get well off the road, or keep driving if the nearest service station is 150Mi away).  When I plug into shorepower, I can either manually select loads I want to transfer to the pole, or I can have it automatically switch when power becomes available from that source.


-Tim


P.S. here's an idea I haven't heard of here on the board... install a 120V-coil SPDT relay that is connected to the prime-mover's starter wire (from the ignition).  The NC side is hooked to the starter solenoid/relay, while the NO side is connected to a sounder and bright light on the dashboard marked "Shore Power".  This way, if you try to start your engine while still plugged into the pole, you get an annoying alarm going off on your dashboard, instead of being able to start up and drag the pole with you...   -T