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Old 07-02-2013, 03:14 PM   #1
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Boost Switch

Which batteries are primary and which are secondary when the boost switch is used? Do the house assist the chassis or the other way around? I assumed the house would assist the chassis starter circuit so you could start the vehicle under most any circumstance. But it may depend on the make and model. Anyone know how this works?

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Old 07-02-2013, 03:16 PM   #2
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I was under the impression that they assist each other. The switch connects them together.


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Old 07-02-2013, 03:23 PM   #3
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Hi zzjea,
Both Cliff and you are correct. The switch connects the house and chassis batteries. One usually uses the switch to assist in getting the coach engine to start. While the switch could be used to help the house batteries, the switch is a momentary switch. The circuit is not made to stay engaged for long periods of time.
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Old 07-02-2013, 04:06 PM   #4
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I have always assumed that the main purpose of the boost switch is to use the house batteries to get the engine started, but as others have said, it just connects both sets of batteries together so it would work either way!
Joe & Annette

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Old 07-02-2013, 04:15 PM   #5
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Had to use mine for the first time last weekend. Found out the front battery was a lot weak. Left the lights on for approx 5 minutes while in the store, with the trailer hooked up as well. When I tried to start all I got was clicking. Pushed the switch and it started right up. Got a new chassis battery Monday.

I'm sure they feed each other.
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Old 07-02-2013, 09:55 PM   #6
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the switch puts them in parallel so they have to work together but my understanding is that they are there to enable you to start the engine when the chassis battery is low for whatever reason. If you have low chassis battery and you use the coach batteries to start the motor home you can drive it to get new batteries. If you have dead coach batteries and tie the chassis battery in and keep your lights going while boondocking then you dont have any way to get new batteries or charged or basically anything. Even if you have a generator you have now way to start it. So my vote is that the primary use of the switch is to let you start the engine if the chassis battery is down.
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Old 07-03-2013, 09:47 AM   #7
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Actually, you are all correct. However, and this may be different by model. The relay that connects the battery systems together has to have power to close. It is basically an electromagnet. Usually, the relay power comes from the coach batteries. (I said usually) so if the coach batteries are dead or disconnected it will not work. So, that would imply that the manufacturer expected it to help start with a low chassis battery mostly. But with a good chassis battery and some power in the coach batteries (doesn't take much) it could help start the Genset to charge everything.
It is a cool system.
Just my 2C.
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Old 07-03-2013, 11:04 AM   #8
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tompen is correct, it could depend on which system the trigger solenoid is wired to. Of course electricity will flow either way, but without power to the solenoid it won't connect.
I was talking to someone who's system had a rocker switch that was double throw center off, could connect House to Chassis and vice versa to boost which ever side was weak. I don't see why a lead couldn't be fed from both systems to the trigger, with appropriate diodes to keep them isolated. I've used the boost switch to help get the generator started when house batteries were too weak, never had to use it to start the main engine ---- yet.

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Old 07-03-2013, 12:26 PM   #9
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I put a separate switch on mine (left the one on the dash) powered by the chassis battery. I did it while putting a switch in line so I can turn off the BIRD. I don't want it (BIRD) to work while in storage. I don't charge deep cycle and conventional batteries with the same charger while stored for a few months. A plus was the little blue light on the switch that now comes on and tells me when the relay has combined the batteries.
But then I can't ever leave well enough alone.
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Old 07-03-2013, 12:36 PM   #10
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On all of the coaches that I have owned, the aux start solonoid is the same one that is used to tie the battery banks together when driving. The solonoid is closed whenever the ignition switch is in the run position (but not the start position).

The aux switch uses coach (house) battery power to close the solonoid. That way, coach battery power will remain available when cranking the engine.

I realize that some coaches are not wired this way, but most (at least class A's) are.
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Old 07-03-2013, 01:05 PM   #11
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With a BIRD system, The ignition is involved, but it is charging voltage from either side reaching a certain voltage for a certain amount of time that closes the relay. It is the same relay. If you want to combine the batteries for starting you would have to push the button.

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