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Old 09-16-2018, 09:51 AM   #1
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Battery boost systems - how do they work?

Last week, we were looking at a Renegade Verona LE that was on a dealer lot. They were going to pull it out of its parking spot but apparently, the chassis batteries were so dead that it wouldn’t even light the dash with the ignition key turned on.

They fired up the generator (I don’t know if that was necessary) to try to use the “battery boost” to charge the chassis battery but the dash lights still wouldn’t come on after about 20 minutes.

This got me thinking about how these systems work. When the “battery boost” is on, is it designed to just trickle charge the chassis battery or are they wired such that you could actually start the engine off of the coach batteries?

They were saying that the chassis battery (or batteries) could be bad but I couldn’t imagine the dash lights not even coming on if the battery boost was on.

Are these systems pretty standard across manufacturers or do the way they work vary?

The service manager came out and messed with it for a little while then left. They never did get it started so we left after about 30 minutes. The problem shook my confidence in those systems because I thought they were designed to get you out of exactly this type of situation.
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Old 09-16-2018, 10:23 AM   #2
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If the coach batteries were (also) down/dead, how did they get the generator to start? The genset usually starts off the coach batteries.
Methinks they either didn't have the coach batteries enabled when they tried to boost the chassis battery, but that would be unlikely, if they knew what they were doing.
I'd say there were other issues with the coach. Hard to say for sure.
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Old 09-16-2018, 10:29 AM   #3
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Usually the battery boost is a momentary switch so it has to be held down for current to flow. Did they just hold it down for a few seconds or hold it down for the 20 min they tried to charge the batteries?
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Old 09-16-2018, 11:13 AM   #4
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That one may be different by the "Boost" or "Aux start" switch on my coach momentarily connects the house batteries to the chassis batteries for starting the engine when the chassis batteries are low.
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Old 09-16-2018, 11:44 AM   #5
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That battery boost switch activation circuit may be wired to the start battery.

In that case, with a totally dead start battery, it wouldn't work. The solenoid needs 5 to 8 volts to pull in.

They should have thrown a portable jumper pack on the start battery, just to give it a little voltage.

Another option is to jump across the 2 big terminals on the boost solenoid. That would send some voltage to the totally dead start battery and get things working.

Even if it was built to charge the chassis battery with the generator running the house battery charger, there are some battery isolation systems that need some kind of voltage in both batteries to work.
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Old 09-16-2018, 11:47 AM   #6
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Thanks for the replies! In response to Winterbagoal, the coach batteries must have been up because there were lights on as we were walking through it and there was no shore power.

Mr_D, I wasn't in a position where I could see exactly what the dealer staff was doing so I couldn't tell if they were holding anything down or they flipped a switch somewhere. Maybe they didn't do anything at all with the battery boost. I just assumed the MH had it and that they were using it but after some of the experiences I've had with dealers and their service departments, perhaps I shouldn't make that assumption.
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Old 09-16-2018, 12:45 PM   #7
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The Aux Start, aka Boost, mechanism is very simple. There is a solenoid that connects the house batteries in parallel with the chassis (engine) batteries, enabling whatever power the house batteries have available to be employed for starting, operating the dash displays, etc. The switch for Aux/Boost is momentary, i.e. it holds the solenoid engaged only as long as the switch is held down.


These systems are essentially identical in all RVs. The differences are in where/how thee wiring runs, solenoid location, etc., but the operating method is the same.
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Old 09-17-2018, 09:18 PM   #8
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Winterbagoal my generator has its own battery and nothing else runs off from it.
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Old 09-27-2018, 06:24 PM   #9
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My boost system is two way, on a three way switch. Both momentary positions from center off connect the house and chassis battery in parallel. The difference is which battery is used to drive the solenoid, so whether the house or chassis is run down I can boost from the other.

As other have said, they may have been trying to boost from a battery that was too dead to close the solenoid. Or like the OP said, maybe they just didn't know what they were doing.

Either way doing your research here is the best idea. One would love to trust in the knowledge and advice of the expert sale persons, but they rarely know as much as they should. I prefer to come into any transaction with my own knowledge, especially a larger purchase like a RV.

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Old 09-27-2018, 06:34 PM   #10
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Originally Posted by Truckerdave View Post
Winterbagoal my generator has its own battery and nothing else runs off from it.


So you have 3 totally separate sets of batteries? 1 for starting,1 for house lights,and 1 for just starting genny.
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Old 09-29-2018, 02:44 PM   #11
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Coaches with gensets used to have a separate generator starter battery, at least on the higher end diesel models. Few do that any more, probably not since around 1998.
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Old 09-29-2018, 05:08 PM   #12
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Originally Posted by Gary RVRoamer View Post
Coaches with gensets used to have a separate generator starter battery, at least on the higher end diesel models. Few do that any more, probably not since around 1998.
Exactly! A member with a 97 Magna in Country Coach forum said his has a separate battery for genset. Then a member with a 2007 Allure wanted a dedicated battery for his genset as he had a hard time to start his genset from house battery.

I am going another direction - I reduced the number of batteries and only have one battery pack for the entire thing. it works every time
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Old 09-30-2018, 04:58 PM   #13
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How come no one has asked about the easiest thing to overlook? Does the unit have a chassis battery isolation switch (salesman switch)?
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Old 09-30-2018, 05:41 PM   #14
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How come no one has asked about the easiest thing to overlook? Does the unit have a chassis battery isolation switch (salesman switch)?
Because it is not a given that the salesman's switch relay has anything to do with the boost relay. I know it does not on my unit. Separate functions with different connections.

FWIW - there is no one way everything is wired. Every time someone says "they are all that way" someone comes up with a "mine is not". If there is more than one way to do anything then they will all be around.
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