Start RV from House Battery

[Diver007]

Anyone know if you can jumper the House batteries to a dead chassis battery to start the engine ?

One-year-old house battery has Twice come up dead out of 50 times. Can't seem to find out what can draw down the battery with the key off and transfer switch off. Any ideas are welcome.

[booster]

Originally Posted by Diver007

Anyone know if you can jumper the House batteries to a dead chassis battery to start the engine ?

One-year-old house battery has Twice come up dead out of 50 times. Can't seem to find out what can draw down the battery with the key off and transfer switch off. Any ideas are welcome.

We can do it our 07 Raodtrek, but when I upgraded the electrical I made sure I used large enough components to do it safely.


You would need to get rid of the automatic battery separator that ties the front and rear together or jumper around it when needed, most likely need to upsize the cable to the rear along with the circuit breakers in that cable.


If you don't have the lithium or an engine generator you would probably have a #4 cable and 80 amp breakers in to the coach which isn't large enough for engine starting. We have very large cabling and battery capacity so no problem there, and use a manual charge relay to combine the front are rear batteries so very easy for use to do with a switch.



Don't just jumper around the existing separator without confirming the wiring can handle it. It is OK to charge the chassis battery from the coach on shore or solar power though, and the separator should do that for you automatically.

[Diver007]

Originally Posted by booster

We can do it our 07 Raodtrek, but when I upgraded the electrical I made sure I used large enough components to do it safely.


You would need to get rid of the automatic battery separator that ties the front and rear together or jumper around it when needed, most likely need to upsize the cable to the rear along with the circuit breakers in that cable.


If you don't have the lithium or an engine generator you would probably have a #4 cable and 80 amp breakers in to the coach which isn't large enough for engine starting. We have very large cabling and battery capacity so no problem there, and use a manual charge relay to combine the front are rear batteries so very easy for use to do with a switch.



Don't just jumper around the existing separator without confirming the wiring can handle it. It is OK to charge the chassis battery from the coach on shore or solar power though, and the separator should do that for you automatically.

Thanks Booster. In an emergency can you just use a single jumper cable to jump around the isolator?

[booster]

Originally Posted by Diver007

Thanks Booster. In an emergency can you just use a single jumper cable to jump around the isolator?

Yes, but only if your wire size to the coach and breakers in that cable are big enough to carry starting level currents which are often over 100+ amps. If your setup was like ours, you would be limited to 80 amps and may or may turn the motor, but probably would overheat connections, and would trip the 80 amp breakers.

[SeabeckStan]

I can't tell you how many times I've come up with a dead battery in my RV. I could call AAA, but instead carry a Clore Automotive Jump-N-Carry 12 Volt Jump Starter. It can stay plugged in all the time, so whenever we're on shore power it can recharge. Easily starts the diesel in my Roadtrek Sprinter. Check with Amazon.

[mloganusda]

Some MH have a monetary switch that connects house batteries to motor battery. You hold it down for several minuets and then start motor.

[rogerstg]

Originally Posted by Diver007

Thanks Booster. In an emergency can you just use a single jumper cable to jump around the isolator?

Originally Posted by booster

Yes, but only if your wire size to the coach and breakers in that cable are big enough to carry starting level currents which are often over 100+ amps.

True. Or, you can jump it and wait for the weak battery to draw from the full house battery. It would be best if one has a genset to run, charging the house and vehicle batteries, so you don't end up with dead house batteries too. Disconnect the jumper before trying to start.

I'm going to check my set up, because I think that I might be able to jump the directly from my house batteries with 12' jumper cables.

[booster]

Originally Posted by rogerstg

True. Or, you can jump it and wait for the weak battery to draw from the full house battery. It would be best if one has a genset to run, charging the house and vehicle batteries, so you don't end up with dead house batteries too. Disconnect the jumper before trying to start.

I'm going to check my set up, because I think that I might be able to jump the directly from my house batteries with 12' jumper cables.

Yes, you can try that, but often there is either a high inrush of current that will trip small breakers if the battery bank is big enough. In other cases there may not be enough voltage difference to get the staring battery high enough charge to start. Biggest risk is that you can wind up with all the batteries dead, but you might also start the engine.


If you use the 12' cables, you usually can use both cables on the positive side of the connections and none on the negative as the grounds are already tied together in the van. This will give less voltage drop in the cables which tend to not be very big wire so lots of voltage loss.

[Lennie]

This isn't the same thing; I carry a small lithium jump starter with me. I used it for my RV once, I had an appointment to get a new house battery installed and it turned out the vehicle battery was dead.

I have used it numerous times to start other vehicles and friends' vehicles. One friend liked it so much he purchased one.

[GallenH]

Originally Posted by Lennie

This isn't the same thing; I carry a small lithium jump starter with me. I used it for my RV once, I had an appointment to get a new house battery installed and it turned out the vehicle battery was dead.

I have used it numerous times to start other vehicles and friends' vehicles. One friend liked it so much he purchased one.

I carry the same with me. It's a small package. Smaller than carrying a coil of jumper cables. That said, I've never used it. I've had friends who have used theirs on vans (not RVs) and they've said it worked well.

[Nspekter]

I haven’t needed to use it yet but our ERA has a dash switch for exactly that purpose.

[MarkS]

The jump pack is a good idea, but you have to make sure it stays charged. It's easy to forget about if you don't use it.
Another option is to carry a small battery charger. Start up the genny, plug in the charger, and let your battery charge for a while. If you have shore power, even better.

[Lennie]

I put things like filter changes and charge jump starter on my calendar otherwise I would forget and have to do it the night before I left which got to be a hassle.

[ProB]

I like the little lithium battery boosters. However, I have experienced that they--like many lithium ion batteries such as in cameras--their output is greatly diminished when they are cold. So don't rely on them to reliably start your rig when they have been sitting in a very cold vehicle.

However, I don't know if warming them up first helps. Maybe others can chime in.

[ProB]

In regard to the OP, one stunt that I'm currently doing is to keep the engine battery in really good shape with constant charging. My rig has the ability to charge my house batteries from solar, engine alternator, and shore power, so my house batteries are generally in a state of good charge.

I take advantage of this to constantly charge my engine battery via an inexpensive 115V AC Intelligent battery trickle charger. I thus (hopefully) am never in the situation where my engine battery has gone flat.

To the OP, this may be a better strategy than going through machinations to start your engine via your house battery.

Further, by the time the engine battery is so low that it needs boosting, it's been damaged already (so some degree). So it really is a good idea to net let things get to that state.

My stunt is currently working. Due to the pandemic limiting my travel, I have barely started my rig in like a year. But my constantly charged engine battery starts up the engine effortlessly every time because it's always at 100%.

Edit: to add that my trickle charger is "intelligent". This is important as very basic models if left on all the time to charge a battery may actually kill it via overcharging.

[Sleeping Dog]

Originally Posted by MarkS

The jump pack is a good idea, but you have to make sure it stays charged. It's easy to forget about if you don't use it.
Another option is to carry a small battery charger. Start up the genny, plug in the charger, and let your battery charge for a while. If you have shore power, even better.

Something to consider is what caused the battery to go flat. If it is because someone went into work and left the lights on, looks at wife, then then jump starting or charging will work. But modern batteries die slowly and then fail all at once. Coming across country last winter, my car battery fired the engine up quickly at ~20F, after sitting all night, but when I came out from a potty stop a couple of hours later, nothing. Got a jump and drove about 15 miles to the nearest auto parts store. After confirming they had the battery, I turned the car off and tried restarting, nothing.

This happened another time around 15 years before, when I car was at the airport for a week while I was traveling. Battery was flat, got a jump and drove home and put it on the charger overnight and it was dead in the AM.

Originally Posted by ProB

I like the little lithium battery boosters. However, I have experienced that they--like many lithium ion batteries such as in cameras--their output is greatly diminished when they are cold. So don't rely on them to reliably start your rig when they have been sitting in a very cold vehicle.

However, I don't know if warming them up first helps. Maybe others can chime in.

From my experience with lithium starter batteries in my motorcycles, the cold does effect power and when cold they may not start the vehicle. What I've learned is that you may need to 'warm' the battery before placing a heavy load on it. 'Warming' can be accomplished by simply turning on a low draw function like the lights for 30 or so seconds before cranking.