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Old 05-09-2021, 08:41 AM   #1
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Functions of the "Emergency Start" button

I love the "emergency start" button in my motorhome. I've had to rely on it once or twice when the chassis battery became discharged accidentally.

I suppose it could be called a "BIM bypass switch". In normal operation, the BIM (battery isolation manager) allows the chassis alternator to charge the house batteries. Yet, the BIM prevents the house battery from being discharged by chassis functions. In certain situations, like when the chassis battery is dead and we need to start the engine, the BIM does need to be bypassed. That is exactly what the "emergency start" button does.

In my motorhome, the Emergency Start button is a momentary switch. When you take your finger off, it stops bypassing. However, there are times when I might want that connection to persist for minutes or hours and I don't want to hold my finger on the button. Maybe my 4000W generator is running and I want to use it to charge my chassis battery.

There may be other times when I want to sever the connection from the BIM to the house batteries. Maybe I have solar panels on the roof to charge the house batteries and I want to give the chassis alternator a rest.

I think I would prefer to have a BIM control switch with three positions.
1. BIM is bypassed. House batteries and chassis battery are tied together electrically until position of this switch is changed. Current can travel in either direction.
2. Normal BIM operation. Current is allowed to travel to the house batteries, but not in the opposite direction.
3. Full isolation. Current is not allowed to travel in either direction.

How would I go about accomplishing this?
Would the BIM need to be changed?

I'm trying to consider any unintended consequences of this configuration. An inexperienced operator might leave the switch in the bypass position overnight which could kill the chassis battery.

Or, somebody might leave the switch in the isolation position for a 300 mile trip. We could arrive at our destination with dead house batteries.

I think I can live with either of those because I can use the generator to recover.
Are there any other issues that I should anticipate?
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Old 05-09-2021, 08:53 AM   #2
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You could pretty much achieve this by replacing the emergency start solenoid with a continous duty one like the one used for the salesman switch. You would have to run a new wire from it to the switch on your dash.

Momentary rocker switch up.. Batteries linked together.. Switch down
. Batteries isolated..
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Old 05-09-2021, 08:58 AM   #3
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There is a second emergency start function of that switch that isn't often mentioned. If your house battery is down, you can also use that switch to emergency start the generator from the chassis battery.

I think that if I wanted to do what you have in mind, rather than modifying the emergency start system, I would install a second, continuous duty solenoid in parallel with the emergency start with a separate switch to actuate it. I would also make that new switch one that has a light when closed to make sure that I didn't forget to open it again when finished.
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Old 05-09-2021, 09:04 AM   #4
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The solenoid for my emergency start is already a continuous duty solenoid. Sometimes I trigger it with a small jumper wire to keep it on. ~Craig
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Old 05-09-2021, 09:18 AM   #5
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You can address most of your issues by replacing your BIM with a BIRD controller.

It gives you Bi-directional charging with some logic control to not overcharge either battery bank.

Any time it senses charging on either bank, it will close the contiounus duty solenoid already in your system.

You would probably not need to add a tigle switch to combine the batteries for any reason.

In your 2017 Thor, it should already have a Bi-directional control, mine does.

There have been a few cases where Thor crossed up the wiring to it and it doesn't function properly, you may want to check into that.
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Old 05-09-2021, 09:27 AM   #6
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Quote:
Originally Posted by Concord View Post
You could pretty much achieve this by replacing the emergency start solenoid with a continous duty one like the one used for the salesman switch. You would have to run a new wire from it to the switch on your dash.

Momentary rocker switch up.. Batteries linked together.. Switch down
. Batteries isolated..
The Battery Disconnect solenoid ( salesman ) is a latching style relay. There are 4 wires between it and the control switch. Once latched, it draws no power. Its not rated to carry the load of starting an engine.

The BIM is a contionues duty device. It can be because it only gets actavated when a charging source is avalable.
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Old 05-09-2021, 09:31 AM   #7
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Quote:
Originally Posted by WileyOne View Post
I love the "emergency start" button in my motorhome. I've had to rely on it once or twice when the chassis battery became discharged accidentally.

I suppose it could be called a "BIM bypass switch". In normal operation, the BIM (battery isolation manager) allows the chassis alternator to charge the house batteries. Yet, the BIM prevents the house battery from being discharged by chassis functions. In certain situations, like when the chassis battery is dead and we need to start the engine, the BIM does need to be bypassed. That is exactly what the "emergency start" button does.

In my motorhome, the Emergency Start button is a momentary switch. When you take your finger off, it stops bypassing. However, there are times when I might want that connection to persist for minutes or hours and I don't want to hold my finger on the button. Maybe my 4000W generator is running and I want to use it to charge my chassis battery.

There may be other times when I want to sever the connection from the BIM to the house batteries. Maybe I have solar panels on the roof to charge the house batteries and I want to give the chassis alternator a rest.

I think I would prefer to have a BIM control switch with three positions.
1. BIM is bypassed. House batteries and chassis battery are tied together electrically until position of this switch is changed. Current can travel in either direction.
2. Normal BIM operation. Current is allowed to travel to the house batteries, but not in the opposite direction.
3. Full isolation. Current is not allowed to travel in either direction.

How would I go about accomplishing this?
Would the BIM need to be changed?

I'm trying to consider any unintended consequences of this configuration. An inexperienced operator might leave the switch in the bypass position overnight which could kill the chassis battery.

Or, somebody might leave the switch in the isolation position for a 300 mile trip. We could arrive at our destination with dead house batteries.

I think I can live with either of those because I can use the generator to recover.
Are there any other issues that I should anticipate?
Don't you already have a BIRD device in your motorhome? If so it does everything you need automatically. If you're plugged into shore power or running the generator, it will charge up both your chassis and house batteries, and not overcharge them. If you're boondocking and idle your engine, after a few minutes it will automatically begin charging the house batteries.
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Old 05-09-2021, 09:44 AM   #8
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Quote:
Originally Posted by twinboat View Post
You can address most of your issues by replacing your BIM with a BIRD controller.

It gives you Bi-directional charging with some logic control to not overcharge either battery bank.

Any time it senses charging on either bank, it will close the continuous duty solenoid already in your system.

In your 2017 Thor, it should already have a Bi-directional control, mine does.
.
I suppose I should start by going under the hood to find out what BIM is actually in there. Where would I expect to find it?

I'm not ready to plunge into anything yet. Right now, just thinking it through.
Before I buy anything, I'll want to make sure I'm ready for Lithium batteries in the future.
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Old 05-09-2021, 12:05 PM   #9
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Bluesea has a few options of bi directional automatic charge relays, starting at about $80 for simple ACR to about $170 for magnet latching ARC with or without extra manual connect disconnect. The latter two can replace the boost solenoid as well but need a selector switch an extra control wires.
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Old 05-09-2021, 12:24 PM   #10
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I suppose I should start by going under the hood to find out what BIM is actually in there. Where would I expect to find it?

I'm not ready to plunge into anything yet. Right now, just thinking it through.
Before I buy anything, I'll want to make sure I'm ready for Lithium batteries in the future.
If going with lithium, you may want to investigate the lithium BIM.

My BIM looks like this, but white, instead of green.

You can download the install instructions of either one and check the wiring.Click image for larger version

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Old 05-09-2021, 02:47 PM   #11
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Originally Posted by SKP Kirk View Post
I think that if I wanted to do what you have in mind, rather than modifying the emergency start system, I would install a second, continuous duty solenoid in parallel with the emergency start with a separate switch to actuate it. I would also make that new switch one that has a light when closed to make sure that I didn't forget to open it again when finished.
That a great idea. The add-on switch could be hidden under the dash where nobody can find it but me. When it is closed, the Emerg Start button would have no effect.

But, still nobody has suggested a good way to get total isolation, meaning no charging current allowed to go from chassis to house.
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Old 05-11-2021, 01:04 PM   #12
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Originally Posted by WileyOne View Post
But, still nobody has suggested a good way to get total isolation, meaning no charging current allowed to go from chassis to house.
Your current setup only charges the house batteries when traveling on the highway so why change it? There is a relay that connects the two once the voltage comes up from starting the engine but opens when you turn the engine off. It could be disabled but why?
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Old 05-11-2021, 01:23 PM   #13
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Originally Posted by WileyOne View Post
That a great idea. The add-on switch could be hidden under the dash where nobody can find it but me. When it is closed, the Emerg Start button would have no effect.

But, still nobody has suggested a good way to get total isolation, meaning no charging current allowed to go from chassis to house.
I would suggest to see if you can obtain a wiring diagram for your Motorhome, on mine, there is a continuous duty relay under the hood in front of the driver side (not sure where yours would be). That relay connects the chassis battery power to the coach battery power when either the key is turned on (which allow the engine alternator to charge the coach batteries), or if the emergency start button is pushed. My plans are to do something similar where I don't want the chassis and coached tied together (when the key is turned on) and the way to do that is to find the wire that has 12v power only when the key is on that turns on the relay (mine is right there at the emergency start switch) and disconnect it. Once you do that, the coach and chassis batteries will be isolated, unless you have other circuits that bypass this connection (as I will have once I install a DC2DC charger) or you press the momentary emergency start switch. I will likely also switch out the momentary emergency start switch with a standard spst with a safety cover switch so I can manually connect the two circuits whenever I desire to do so. ~CA
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Old 05-11-2021, 01:31 PM   #14
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Your current setup only charges the house batteries when traveling on the highway so why change it? There is a relay that connects the two once the voltage comes up from starting the engine but opens when you turn the engine off. It could be disabled but why?
Hi SKP Kirk, While I don't know the OP's reasoning, the reason I will be doing something similar is that I am installing LifePo4 batteries in the coach and the engine alternator charge profile isn't optimal for LifePo4 batteries which the dc2dc charger I plan to install is optimal, and the opposite is also true where my new inverter\charger will be set for LifePo4 batteries and that profile is a bit more voltage than optimal for the lead acid chassis battery. That is my reasons for not desiring the solenoid relay to be on (connecting the chassis and coach batteries) anymore without me directly turning it on, say for an emergency start situation. ~Craig
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