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Scrooge39's avatar
Scrooge39
Explorer
Nov 28, 2014

"Battery boost" question

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
Class A
  • jorbill2or's avatar
    jorbill2or
    Explorer II
    Nov 28, 2014
    If for some reason you weren't holding the boost switch down while trying to start originally ( I infered that from the way you told what happened) that may have been the problem ! the switch is a momentary switch and must be held down to connect the two battery systems.m release it and the batteries disconnect.
    It sounds like your chassis batteries were totally discharged and may be shot.
  • dougrainer's avatar
    dougrainer
    Nomad
    Nov 28, 2014
    1. HOLDING the Battery Boost will NOT allow extra charge to the chassis batteries unless you have SHORE or GENSET power connected.
    2. This will NOT happen if you are self contained with no external 120 power
    3. By HOLDING the Boost button, you allow your Inverter/Charger or CONVERTER which ever you have to push charge current to the chassis batteries.
    4. A FASTER way to do this is to disconnect the Shore or Genset for 1 minute. Reconnect Shore or Genset and then either Hold or pin the boost switch in the ON position. This will allow the Inverter/charger which will be in BULK(high) charge mode to charge the chassis faster.
    5. What is strange about this post is the fact he claims to have used a stand alone battery charger for 1 hour and still no go. So, the OP either has a BAD charger or he needs to make sure he gets good clean connections the next time he uses a stand alone charger. Doug
  • gemert's avatar
    gemert
    Explorer
    Nov 28, 2014
    I'm pretty new to this but thats the way I've been told it's supposed to work. The Boost switch in mine is spring loaded so you have to hold it down to charge the chassis batteries first and then while starting. I've jumped cars with dead batteries before where you have to hook up and just let the dead batteries charge for a few minutes before they will actually start the engine. I'm guessing that it has something to do with the jumper cables not being able to support the current draw when starting. Just a thought.
  • Mr_Mark1's avatar
    Mr_Mark1
    Explorer
    Nov 28, 2014
    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
  • Alfred622's avatar
    Alfred622
    Explorer
    Nov 28, 2014
    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.
  • Dale_Traveling's avatar
    Dale_Traveling
    Explorer II
    Nov 28, 2014
    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.
  • Mandalay_Parr's avatar
    Mandalay_Parr
    Explorer
    Nov 28, 2014
    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.
  • K_Charles's avatar
    K_Charles
    Explorer
    Nov 28, 2014
    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.

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