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Old 05-12-2015, 02:58 PM   #1
Mini-Skoolie
 
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Charging from the Alternator

Hey All,

Last electricity related question, I swear!

Pretty much, as the total says, we are hoping to charge our house batteries from the alternator off of our 5.9L cummins in our '95 Carpenter. We know it might not contribute that much to the charge but it at least might slow the discharge rate after charging using our inverter charger.

So that we can avoid buying expensive battery isolators, we are looking at setting up something like this. Could I get your opinions on if you think this looks reasonable or are we crazy?

Thanks!
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Old 05-12-2015, 03:18 PM   #2
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To be more specific, I was wondering if the 10AWG wire from the crank batteries to the house batteries will be sufficient? Should we go with something heavier?

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Old 05-12-2015, 03:26 PM   #3
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Okay terms are confused...

While stationary engine off, you will be charging batteries with a convertor it will take 120V and step down to 12V, kind of like a power supply on your computer at home

an invertor takes 12V and steps it up to 120V

so what I did is this:

I put a HD 200 amp continuous solenoid between my alternator and battery bank, with a 175amp fuse

When I start my bus I can flip a switch that turns on the solenoid and use my alt to charge my house batteries, starting battery and power my invertor (12V TO 120V) for over the road use (microwave/rotisserie/fans etc)

when parked I use my solar to charge my batteries and use camp ground plug in

NOW, all my lights and stuff is 120V in my bus so I have no need to run 12V appliances, IF I did I wound install a convertor/charger like the Iota 45 amp model
This allows the 120V incoming load to convert to 45 amps of 12V to power stuff and the 4 stage charger keeps batteries happy
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Old 05-12-2015, 03:28 PM   #4
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I use the $50 solenoid to isolate my start batteries so they do not drain, I use a manual flip switch to do this, an oil pressure switch will do the same thing but automatically when engine starts and pressure builds up
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Old 05-12-2015, 06:39 PM   #5
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Everything in my bus is 12v. I use the Iota 55a, one of the best things I ever bought. Of course I have 120v too for a small fridge and water kettle.
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Old 05-12-2015, 07:39 PM   #6
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You might be interested in some of the ideas and things to look out for on another thread over there.
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Old 05-12-2015, 08:49 PM   #7
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Bansil - Did you use a special type of switch that was capable of holding a load like that? By any chance, could you link a suggested one?
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Old 05-12-2015, 09:01 PM   #8
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Also, I have no idea how you guys decipherd what I meant in the OP without this crucial link I forgot to include!

http://www.doityourselfrv.com/bring-...oid-stealth-rv

In the diagram in the link below, it only shows a 'live' connection going from the starting batteries to the house set via the solenoid. Will the house batteries charge without a 'neutral' line? If not, where in the diagram should that go?


Also, as a more general question, will the solenoid act to prevent draining of the batteries back into starting batteries?
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Old 05-13-2015, 04:58 AM   #9
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That diagram is almost what I said, issues I see are this:

The fused wire from fuse box to turn on when running, I put a basic 20/30 amp switch here, this let's me control it AND I can turn it on with bus off and let solar top off start batteries

And that 30 amp circuit breaker in my mind is way to small if you plan to use invertor on the road, I put in a 175amp fuse, so I can also use alt. To power it

Which also brings up the 10ga wiring, way to small especially since you just replaced 30amp breaker with a very big fuse, keep your cables large and keep the inverter as close as possible to batteries so you don't loose voltage and create excessive heat

Plenty of good free wire size calculators online
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Old 05-13-2015, 09:43 AM   #10
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I may not be answering your original question but I am attempting, if you will pardon the pun, to add some more "light" to this discussion.

Outside of when you are running down the road I would not plan on using the OEM alternator for charging your batteries.

While most school buses come from the factory with a large alternator they are not designed to provide juice at full load for long periods of time like the alternator on a genset is designed to provide.

Also, trying to charge things up while parked will be very hard on your engine and very inefficient. A small Honda genset will provide more juice at a fraction of the cost for fuel than running your bus engine to charge things up.

I would agree that 10 gauge wire is inadequate for any sort of heavy load.

You need to make sure there is no way in which the house batteries are directly connected to the start batteries except for when you are driving down the road. You do not want to find yourself at the back of beyond with a set of start batteries that have been drained flat. While an expensive isolator switch is one way in which the seperation can be accomplished there are other options out there. The expensive options do everything automatically. The simple ones require you to physically throw the switch every time. The problem with the simple solutions is if you are like me you can forget to switch and discover either your house batteries didn't get charged while going down the road or your start batteries got drained down. BTDT more than once.

You also need to make sure the batteries you use are correct for the application. Your start batteries will need a lot of cold cranking amps while your house batteries need a lot of reserve amps in a design that doesn't mind being cycled from full to empty on a regular basis.

As was noted, you want to keep wire runs to your batteries and to your invertor/convertor as short as possible. The longer the runs the heavier the gauge of the wire will need to be. If the size of the wire isn't sufficient you run the distinct risk of letting the smoke out of the wire. Letting the smoke out of the wire can be catastrophic.

Good luck.
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