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Old 07-26-2005, 01:34 AM   #1
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Electrical Systems

Building my first bus, and I need some help. I've searched for all these things but cant seem to find answers...

I will not have a generator
Power from land hookup and Alternator

Will the car batteries charging detract from the charging of the house batteries?
Will I need an Isolator if I have a switch to switch from charging house (idling at night) to charging car (driving) to both (driving and using appliances)
Will a converter charge a battery or do I have to buy a charger?
What are advantages/disadvantages between smart and the simple kind of chargers?
How does an inverter/charger work?

Below is a technical diagram that comes from a PhD in electrical diagrams

Is this the easiest cheapest and simplest way to do it?
Any help would be greatly appreciated
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Old 07-26-2005, 05:39 AM   #2
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Hi noahyay,

I am planning on the same plan, but I'm leaning towards a switch instead of an isolator (cheaper and simpler). The only thing I'll have to worry about is not to forget to switch over to house batteries when I'm parked.

I, myself, am going to install a smart battery charger, mainly for the deep cycle batteries (a regular charger can supply too much juice for deep cells, hence damaging them).

A good source on batteries and charging systems is Phred's Poop Sheet.

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Old 07-26-2005, 08:50 AM   #3
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To answer some questions. An inverter charger is just that, it is either changing 12V to 110V to run your stuff or it is changing 110V to 12V to charge your batteries. You can turn each function on of course. They usually have a built in transfer switch also so that you don't have to manually switch between battery and shore power. I recommend getting this kind of setup.

The only reason you would need a isolator is if you:

A) You don't want your charger to charge your engine battery. (With the engine running you will still be charging both batteries.)

B) You don't want to drain your engine battery by using your inverter. The problem with this is:

1) The inverter will stop working anyway well before it drains your battery so much you can't start the bus.

2) If you run any of the fans/lights that were built into your bus from the factory that will be draining from just the engine battery, not the bank.
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Old 07-26-2005, 12:07 PM   #4
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thanks a lot

it was a big help
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Old 07-26-2005, 01:12 PM   #5
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Some converters (most, I think) also charge batteries. I have a Progressive Dynamics 60 AMP Electronic Power Converter. . I also bought the Charge Wizard module that goes with it.

Right now, I'm just using it to power a 12VDC muffin fan for my composting toilet exhaust, because I don't have any house batteries; therefore, I can't say how well it works at charging house batteries.

If you are going to charge your house batteries from the alternator, I think you are going the right way in using a switch rather than an isolator, especially if there is any distance between the house batteries and the alternator. Isolators use diodes, and diodes cause a significant voltage drop in the current that passes through them. Coupling that voltage drop with the voltage drop that you get as a result of transmission distance, and it might well be that your house batteries are never charged at proper rates.

The Charge Wizard is supposed to also have a de-sulfation mode that helps to maintain battery health. Again, I have no house batteries yet, so I haven't tried it.
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Old 07-26-2005, 01:23 PM   #6
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Please note that the drawing above is based on a normal RV electrical system setup. The only thing I would disagree with is that the 120VAC input to the Converter/Charger is also routed through a circuit breaker (typically 20 AMP). Looking at the drawing, you will see that in a normal RV:
1) Engine Alternator circuit - Isolator (electronic device) or constant current relay (about $15 at RV store) is between alternator and batteries. electronic isolator uses diodes that only allow batteries (starting and house) to be charged by alternator. Battries are not discharged through it. Constant current relay is controlled by a dashboard switch (on/off). Power to the control switch is normally from ignition switch which requires the ignition switch to be ON in order to cross connect starting and house batteries. When ignition switch is off, the relay is deenergized therefore, starting and house batteries are not cross connected (no voltage to dash select switch). Early Winnebago used a DUAL-NORM-MOM switch where Dual (on) charged both batteries, NORM (off) start battery and MOM (ON but was spring loaded back to NORM) allowed using the house battery to be used as a starting battery. If a normal ON/OFF switch is used, the MOM position would be available if switch is on and ignition is on. Because of the very high current draw for starting, these are heavy duty relays. A normal car starting relay cannot be used in place of these constant current relays because the relay will overheat if energized constantly. Ergo, the name constant current relay. Isolator (electronic) style may not charge batteries fully because of 0.7VDC drop across isolator.

2) IN normal RV, 12VDC Lights and such (reefer, furnace, fans, etc) are feed from the house batteries via a fuse (or breaker) panel. They are not feed from the starting battery. Prevents depletion of the starting battery this way.

3) Older converters used a heavy duty internal relay to switch from converter to battery (battery disconnected from circuit). Newer converters can best be viewed like an alternator because the converter output connects across the house battery (like an alternator) and performs both house battery charging and 12VDC load support (like an alternator). Older style (with heavy relay) had seperate charger lead but was limited to about 6 amps of charge (real small output).

4) In above drawing, inverter (120VAC from battery) would only draw from house batteries. The electronic isolator would not allow power to be drawn from start battery. Proper design of a constant current circuit would deenergize relay when ignition key is off thereby preventing inverter from pulling current from start battery. If the inverter is operated with engine running, it would draw current from the altenator (requires dash switch to be ON for constant current relay style setup)

Combinations and possibilties are endless. Primary goal should be, use house battery circuits for house support. Only use start battery for engine and such. Use converters, solar cells, shore power, and/or generators to support house functions (load/charging, etc.)
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Old 07-27-2005, 12:25 PM   #7
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woah, i almost understand that

so what you are saying is the only thing i should change is that the inverter be powered directly from the alternator while the engine is running?

and mabey that the converter not have a circut breaker? why not?
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Old 07-28-2005, 10:05 AM   #8
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The drawing that is there now is not the same one that I spoke to before. Actually, the one that is there now has way to many switches/relays in it. I have a good drawing but not sure where to post it. The BB does not support file transfer to it.
I can say, you only need a isolator or constant voltage relay at the alternator. Thats only one relay not 2.
There is no need for all the switches at the input to the 12 volt breaker panel. It's all one common buss connection there. House battery is always connected with todays converters. Check out the Progressive Dynamics site for a history of converters and how they are connected.
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Old 07-28-2005, 02:24 PM   #9
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winchester i sent you a pm.

can you leave the alternator always charging the deep cycles or is that bad for them?

i was thinking useing switches like the ones here

does anybody have suggestions on inverters chargers and batteries?
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Old 07-28-2005, 07:23 PM   #10
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Lets backup a few steps.
Both alternator and converter only supply the amount of current they sense is needed based on voltage level.
You do not want to run an alternator at max amps for extended periods. They will overheat and burn out. Typically only run continuiosly at 25% capacity

Automotive alternators are constant voltage charging devices that were designed to support starting batteries (short bursts of high current). The amperage rate drops of sharply as battery voltage increases. You will have high current for a short period of time and then it will taper off very fast. Multi-stage regulators are available but costly.

You do not want to recharge a discharged deep cycle battery at greater than 25% of it's rated capacity. You will damage it. They are not harmed by an alternator because of the short high current cycle inherent to automotive alternator circuit design.

Todays converters (120VAC to 12VDC) are connected across the deep cycle battery the same way an alternator is. When there are no RV loads, it is basically just a charger. As RV loads come on, it puts out more current (battery recharge load + RV loads) up to max capability (ie 55 amps). Some use 3 stage smart charger circutry (charge wizard) to keep current high longer before changing over to voltage sensing mode. Allows faster charge rate.
A 250amp switch is really overkill for most RV installations. Might be needed (high current draw) on a diesel setup if you tried to use the house battery to start a diesel. Normal house side loads are rarely large enough to need that large of a switch. You typically will never have that many loads on at the same time. Besides, that large of a draw would need a very large battery bank because it would be depleted real fast. Boondocking idea is to use small amounts of energy over a long period of time.

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