Emergency Start Button - Connects both Battery banks

Flyfasteddie wrote:
At least on my '91 Ford chassis, the push button "emergency start" relay would engage anytime you turned the key - in the 'on' and 'start' position.

The main reason is I'm usually boondocking; so the last thing I want to do is leave early in the morning with a heavily drained house battery and as soon as I start to crank the engine I'm connecting the lower voltage house battery to the chassis battery which is now sending a bunch of amps to the house battery instead of going to the starter

I think you're mistaken, when the key hits the start position the only thing getting power is the starter and ignition circuit. Your headlights, wipers and everything else go dead until the key returns to the run position.

If your motorhome was not changed by a subsequent owner it should have a 100 amp alternator that is designed to take a big load like that.

Winnebago uses a spdt mom-off-on switch for that- the 2 outer terminals are power feeds, one (momentary) coming from the coach battery, the other (latched on) from an ignition switched source. This way the single solenoid is used for both emergency and battery charging, but the ignition must be on for charging.

Appreciate all your thoughtful replies and good reasons. I will not be changing the momentary switch. The benefits of the momentary switch are many and I understand them better now. Thank You All!!!

fourthclassC wrote:
Want to have the option to draw from the engine battery if needed, if I kill the coach battery bank. What do you all think about this? Thank you

BAD idea. YOu only have to run both batteries down ONCE to know how bad.

A much better plan:
Run your generator when the coach batteries start getting weak.
OR
Run the chassis engine. It should automatically connect both sets together and charge both while it is running.

Quicksilver's Delco intermittant relay can draw from any of two banks. This is a start only operation but I do not have to play goosey-loosey guessing which bank to power the relay coil from. Just a single momentary pushbutton.

I put a switch on mine as well. Not so much to have the coach battery boost the engine battery (although the switch would work just as well as a push button switch if the need arises - but don't use it to get more TV time if your house battery is dead!) but to be able to keep the batteries isolated rather than have them connected any time the ignition is on. At least on my '91 Ford chassis, the push button "emergency start" relay would engage anytime you turned the key - in the 'on' and 'start' position.

The main reason is I'm usually boondocking; so the last thing I want to do is leave early in the morning with a heavily drained house battery and as soon as I start to crank the engine I'm connecting the lower voltage house battery to the chassis battery which is now sending a bunch of amps to the house battery instead of going to the starter.

Then after the engine starts I'm forcing the alternator to deliver 40+ amps to the house battery in addition to the amount going to the chassis battery to top it up after the start.

I would much rather keep the batteries isolated, let the chassis battery crank the starter, and let my solar charge the house battery while I'm on the road.

But if I want to give some solar to the chassis battery, or give some alternator amps to the house battery, I can easily do that with the flip of the switch. I used a lighted toggle switch so that it is obvious when it's engaged.

I installed a regular switch in my HR as you describe. It is handy to keep my chassis battery fully charged when we have an electrical hook-up. The coach stereo and a few other things run off the chassis battery in my coach. However I have not used the switch to help the house batteries. I would use my generator to do that.

On edit: make sure that your solenoid is rated for continuous duty.

Not a good idea mainly for the reasons Doug states. It's momentary for that reasn.

I think it's a bad idea, especially because of your intended use. You could pretty easily find yourself with no battery power to do anything, be it start the main engine or start the generator. The starting battery is not a deep cycle battery and isn't particularly well suited for significant discharges; it's instead more optimized to supply brief, high currents such as the starter needs.

On many motorhomes, the same relay is used for the emergency start as for the battery isolator system, as was mentioned, and often this is controlled for the isolator system by the engine run power line from the key switch. If that's the case, you can combine them temporarily by putting the key in the run position without starting the engine. I sometimes do this when exercising the generator for the opposite reason--to try to keep up the starting battery charge during longer-term storage.

Doug is right
that is a fairly hefty relay, it has considerable power drain that you don't want
When the engine is not running
You really don't want to be using the engine battery to support the house batteries
If you don't have enough house battery, then that is the situation that needs to be addressed