Battery charging vs Gas mileage

[Henry J Fate]

My driving habits are as fuel friendly as possible. Loading the vehicle (fuel water etc) is always done to a minimum unless of course I drop in an area that will save a quarter or more on a gallon of fuel then I fill her up. I check my tire pressures always. I installed easy access valves and extenders that ended the battle with the stock nonesense. I have drove large vehicles off an on throughout my history and do a pretty good job of it.

My best mpg was a little over 9 which is pretty good. I rarely dip below 8 and I use that number as a reference.

Another reason for changing the switch  from a momentary to a fixed 2 position would be to engage the chassis and house batteries for charging when the engine isn't running. That feature existed on one of my older units early 90s maybe late 80s. I still prefer that design as I feel capable of managing it properly which was probably one of the reasons the designers took it away and left chassis batteries to suffer and die early.

I also have a brand new old stock switch that matches the momentary but in a 2 position style to toggle my new set up. It should fit in the existing slot for the momentary.

I do alot of flat road stuff 300 - 400 mile trips so I should be able to confirm the lousy results fairly quickly after wiring it up.

I use a deep cycle battery for my chassis so having it connected when charging with the converter is good. All the batteries are deep cycle. I would think that on long trips that the house batteries would appreciate a break in the high voltage charging from the alternator. That could possibly extend the useful life of the batteries.

It seems like a good modification and I am trying to get it done fairly quickly. The solenoid is boxed in the front engine compartment so I should be able to get at it if needed.

I think Gary came up with .08 miles per gallon. Doesn't sound very promising but all the advantages may be worth it.

 

[Gary RV_Wizard]

It costs almost nothing to try, whereas the calculations (including mine) are theoretical. Have fun giving it a try.

 

[WILDEBILL308]

Just add 2.5 lbs air preshure to each tire and that should offset the loss on the alternator by lowering rolling resistance.
Bill

 

[Arch Hoagland]

I'm looking forward to hearing how much it improves your gas mileage. 

8 MPG on a 454 sounds pretty good to me as is. 

 

[Old_Crow]

I'm looking forward to hearing how much it improves your gas mileage. 

8 MPG on a 454 sounds pretty good to me as is.

That's what I was thinking.  Must be one of the later, fuel injected, motors.  On my 454 with the quadrajet I was happy to see 6.

 

[Mark_K5LXP]

You will never see any observable economy difference by disconnecting the house batteries from the alternator.  Batteries are not damaged in any way by being held at absorption voltage even for hours at a time, that's their usual charge profile.  It's to your advantage to keep the house side charge maintained so any loads aren't drawing the batteries down during travel.  I'm all on board with the capability to bridge house and chassis batteries for purposes of floating during storage and having extra house capacity when needed.  I made up a short jumper cable for mine to do that very thing.

changing the switch  from a momentary to a fixed 2 position

A caution there is I would look at the spec's for your boost solenoid.  If it's rated for continuous duty indeed all you would have to do is replace the momentary dash switch with a toggle.  In that scenario though the solenoid will be drawing coil current (a few watts) any time the batteries are bridged so there's a bit of power draw associated with that.  Other options would be a passive switch across the solenoid you actuate manually when needed, or replacing or bridging the solenoid with a latching type like the ones often used as the "salesman switch".

Mark B.
Albuquerque, NM

 

[Gary RV_Wizard]

If it's the same solenoid as is used to cross-connect house and chassis while driving, then it has to be capable of continuous duty because it is going to be engaged for hours at a time in normal use.  Typically those are latching relays as well, e.g. the Intelletec Big Boy or similar, with zero amp draw to keep the solenoid engaged.


That said, the very first step is to verify the things that Mark described. Failure to do so could lead to an early death of the solenoid.

 

[Henry J Fate]

Mark brings up some good things to think about. It would seem that the solenoid used for emergency starting would be the same method of combining the batteries for alternator charging. If not, it is possible they direct connected the engine alternator to both battery banks full time with blocking diodes of some type. I think it's more likely they used the same solenoid as the emergency start switch. I also think that it is a long standing design that had been used with a two position switch but at some point they changed it to a momentary to prevent chassis battery drain.

Guess I will need to open the box and verify that the solenoid is responsible for both functions or not. If so, I would conclude it to be fine full time.

I do think that the voltage from the alternator is a little high for batteries that are looking for a maintenance voltage. It would be during that maintenance time that it may be helpful to discontinue charging. I know voltage is decreased as needed when done by a battery charger. I would not expect a float charge over 14 volts on any battery charger for lead acid. Maybe it is fine that high but it Doesn't  appear to be the voltage of choice full time.

The gas mileage will be interesting. I do keep records when I highway travel so I have things to compare too. I do believe I can hear a louder whine from the engine when I have low house batteries. It is noticeable when I first start the engine up. I suspect the .08 mpg gain may be all I could expect but hopefully I can gather good info to prove it out. I will try. It is not something I will use all of the time as I do have the need to recharge in between stops fairly frequently.

 

[Lou Schneider]

My 1993 Damon motorhome had two battery paralleling solenoids, one right above the other.

The first was a short duty cycle Ford starting solenoid, activated by the momentary Boost switch.

The second was a similar looking continuous duty solenoid.  +V came from the chassis battery, ground was through a second oil pressure switch so it was only grounded and engaged the solenoid when the engine was running and the oil pressure was up.  Seemed like an elegant and simple solution to only paralleling the house and chassis batteries when the engine was actually running.

 

[John From Detroit]

The numbers vary from Rig to Rig but on my Motor home the Alternator can not keep up with a 500 watt load, that's about 50 amps at 12 volts.  and about 1/2 horsepower load on the engine (all figures rounded)

Not worth the effort do disable it.

Some RV's they can put 100-200-300 amps into the batteries... Mine can not.

 

[Henry J Fate]

Stop beating a dead horse John

Only joking.

I suspect my alternator will deliver more that 50 amps but I may be wrong. I will try to determine what the rating is when I start digging into things.

That 1993 Damon solenoid design is interesting and may complicate things if my 2001 Winnebago did something similar. I am fairly certain that all relative solenoids will be mounted in that box up front inside the hood. It seems a little excessive using the oil pressure as a switch but I assume that would certainly limit the coupling only when the engine is running and not respond to the ignition key. That could also be a safety of some sorts to limit current floating around in the case of an accident. Just a guess.

I may have some time to start looking next week. My plan is to have the new switching schematic figured out by the beginning of April. I have a month's stay in April that will allow me to move forward with the modification so it will be done in April. Lots of other things to tackle during that month too. Hopefully the weather in southwest Florida allows for a productive month of April.

 

[Larry N.]

It seems a little excessive using the oil pressure as a switch but I assume that would certainly limit the coupling only when the engine is running and not respond to the ignition key.

That approach has been used for decades in light aircraft to run the Hobbs meter (hour timer), so that it only records time the engine is actually running.