Looking for an explanation……….Last week we decided to go camping. Motorhome had not been started in a month. Also, the previous 2 nights got into the low 20s so decided to plug block heater in (Cummins ISB). Got all loaded up and after about 2 hrs. attempted to start the engine. Turned key and had 2 “growls” and then engine would not turn over. Waited about a minute or so and tried again, nothing. I then depressed the “bat boost” switch and tried to start again but had the same results, nothing. I checked the operation of the “bat boost” contactor and it was working as I could hear it energize when the switch was depressed. The house batteries were fully charged as I leave the motorhome plugged in. Put a battery charger on the chassis batteries for about an hour and tried the above again with the “bat boost” but still had nothing. I then decided to call my road service. After a few minutes a tech came on the line and I explained all of the above to him.He then wanted me to try one more thing. He said get in the motorhome and hold the “bat boost” switch down for 1 minute but do not try to start. After 1 minute he told me (still depressing the “bat boost” switch) to try to start the engine. Turned the key and the engine turned over and fired right up. He could give me no explanation as to why the “bat boost” switch had to be held down for a minute so he apparently had run into this situation previously.So, perhaps someone can enlighten me as to what transpired for that minute the “bat boost” switch was held on and the house/chassis batteries were “shorted” together to allow the engine to turn over.Thanks, Richard

Not enough juice can go thru the jumper wire to run the starter but during your one minute wait the the battery got "charged" up enough to run the starter.

Probably the chassis batteries needed a little charge to build up the voltage. I would have thought pushing the battery boost would immediately start the engine. Batteries are strange sometimes.

Probably the chassis batteries were so low that most of the DC current from the house batteries were going that direction rather than the starter. Also your battery charger might not have enough power to have much of an effect after an hour considering the size of the engine start batteries needed for a diesel engine.Holding the AUX START for a minute or so pushed a lot of amps into the chassis batteries to give it a bit more of a charge than what your charger could provide. Started dead cars many times by connecting the jumper cables and just let it sit for a couple of minutes.

K Charles wrote:Not enough juice can go thru the jumper wire to run the starter but during your one minute wait the the battery got "charged" up enough to run the starter.X2 Makes sense to me.

Had the exact same experience with our Dynasty. Chassis batteries were about 4 yrs. old and I had the motorhome in storage for about 2 months plugged in to 30 amp. I've never been away from the motorhome that long in it's whole life, she missed me!The coach would not start on first turn. I held the boost switch and tried to start, no go. I then held the switch for a minute as described above and it turned over but the chassis batteries would not keep a charge so I had them replaced (this was back in June). Mark

"Battery boost" question | Good Sam Community

J-Rooster
Explorer
Nov 29, 2014
Richard, thanks for the tip! I don't like to admit this but I had trouble with my battery boost last week it wouldn't work in my coach. I was ready to go get the CRV for the jump start and the DW came out and turned the battery off/on switch to the on position it started right up, another senior moment!
wa8yxm
Explorer III
Nov 29, 2014
The wires to the engine from the house are not strong enough for the starting current (Too much voltage drop) when you held the botton down power was transferred from the HOUSE batteries to the STARTING batteries, once they had enough in them then, with the help of the house, they got it done.

This is fairly common, this is why when I jump some stranded motorist we hook up with my car running, let it sit for a while, then crank up the stranded.
Ivylog
Explorer III
Nov 29, 2014
Running the generator to power the inverter/charger increases the voltage slightly which equals more watts into the dead battery. The size and length of the wires still limit the number of amps that the boost will pass. It's normal for it to take several minutes of boosting to get enough charge into the dead batteries before the engine will turn over.
Scrooge39
Explorer
Nov 28, 2014
Thanks for all the responses.
Picture palm of hand slapping forehead....I never thought of the inv/conv charging the chassis batteries when the "bat boost"
switch held down. Although I don't know how much charge was imparted for that minute but apparently with inv/conv charge
coupled with the fully charged house batteries, there was enough to turn over the engine to start it.

Since this episode, I have ordered a TRIK-L-START to install to prevent a reoccurance.
deandec
Explorer
Nov 28, 2014
The battery boost switch on my old rig just ties the two batteries together like one car battery jumping another car's battery. It works much better though if the generator is running and charging the house battery at the same time as the boost switch is being used.

In my younger days in sunny but cold Minnesota, it was common to let the boosting car run at high idle for a couple of minutes to allow some charge into the battery of the boosted car before engaging the starter from the dead battery.

Holding the boost toggle switch engaged does the same thing as the high idle routine.
dougrainer
Nomad
Nov 28, 2014
Ivylog wrote:
Rarely do I disagree with Doug so here goes. Using the boost switch on a DP is like trying to jump a car with a cheap set of jumper cables. Will not start the car at first but after several minutes of putting some amps into the dead battery the combination has enough amps to start. Unfortunately the boost system on a DP is like a set of cheap cables as it can only supply less than 200amps which mainly goes into the dead batteries from the house batteries. Trying to start a minute later has done a quick charge on the dead batteries so now there is enough combined amps to turn the engine over. Doubt the OPs charger was more than 20 amps so even after an hour on two batteries that are 900+ amps each has not done much. I 'm actually surprised that it started after just one minute as the cables going to the bird are much smaller than the starter cables.
I have 00 welder jump cables that plug into the front of my truck(do not have to raise the hood) which can handle the 2000 amps from my two diesel truck batteries and rarely do I have to wait to start anything unlike the cables on a DPs boost circuit .

Sorry, I have had more than my share of DEAD/LOW chassis batteries on our service lot over the years. Just pushing and holding the battery boost for even 5 minutes is NOT going to transfer needed power to extremely low or dead chassis batteries. NOT GOING TO HAPPEN. You MUST have any type of external charging going thru the system. Either the Coach charging system or an external battery charger. YES, the battery boost SOLENOID and the cables usually going thru it are usually not large enough to get the needed current to the starter. I HAVE used jumper cables between the coach and chassis when they are close enough to start an engine when the battery boost would not, even tho the boost solenoid was operating. WE also have a battery boost cart that has 2 31 series chassis batteries and we keep it plugged in with a battery maintainer for a full charge and sometimes the chassis batteries are so dead, that even those 2 extra batteries will not start the engine until we get a small boost charge on the chassis batteries. It appears from my experience that completely DEAD 31 series batteries will actually pull any current into them before it can be pushed to the starter circuit. Once we get a short(30 minutes or less) small charge on the dead batteries the Start boost will then operate and the coach batteries will give enough power to start the engine. Doug

PS, There is a BIG difference between "theory" and what actually happens in the REAL world.
mowermech
Explorer
Nov 28, 2014
When batteries are connected in parallel, if one (or more) of them is discharged, the fully charged one(s) WILL transfer energy to the low voltage one(s).
Yes, this might take a minute or two.
When the "boost" or "emergency start" or whatever switch is pushed, it DOES connect all the batteries in the coach in parallel.
MrWizard
Moderator
Nov 28, 2014
most of the answers are in the ball park
a few are in left feild

consider this
in my situation the house batteries and engine batteries are in the same pull out tray in the rear
BUT the boost relay is on the engine compartment wall about 6ft distance
two cables pos & neg for each bank
thats a minimum of 12ft wire run.. for the positive..NOT counting the run to the starter
add the starter wire and the negative cables and its 24ft to boost the chassis batteries another XX run to the starter

the chassis batteries are pulling amps from the house batteries
they can't supply both the starter and the recharge power at the same time the total is hundreds of amps

a minute or two of recharge time into the chassis batteries lessens the total load (at start up) and allows the weak chassis battery to help the house batteries spin the starter
it voltagee drop and heavy amps needed to spin that high torque engine
Sully2
Explorer
Nov 28, 2014
Put a battery booster in your coach that constantly gives the engine start battery a small charge IF you keep your coach batterys on a charger of some sort. The small charge a booster gives the start battery is big help
Ivylog
Explorer III
Nov 28, 2014
Rarely do I disagree with Doug so here goes. Using the boost switch on a DP is like trying to jump a car with a cheap set of jumper cables. Will not start the car at first but after several minutes of putting some amps into the dead battery the combination has enough amps to start. Unfortunately the boost system on a DP is like a set of cheap cables as it can only supply less than 200amps which mainly goes into the dead batteries from the house batteries. Trying to start a minute later has done a quick charge on the dead batteries so now there is enough combined amps to turn the engine over. Doubt the OPs charger was more than 20 amps so even after an hour on two batteries that are 900+ amps each has not done much. I 'm actually surprised that it started after just one minute as the cables going to the bird are much smaller than the starter cables.
I have 00 welder jump cables that plug into the front of my truck(do not have to raise the hood) which can handle the 2000 amps from my two diesel truck batteries and rarely do I have to wait to start anything unlike the cables on a DPs boost circuit .