Battery charging vs Gas mileage

[Henry J Fate]

I have been throwing around the idea of disabling house battery charging from the engine alternator when heading to a destination that offers free electric with the rent. There have been times when I have  more than 50% depleted house batteries and head out to my next stop with free electric. It would seem wise to pause charging until I get to my next stop rather than to have the engine do all the work before I get there.

What affects on gas economy would one expect? Has anyone tried this?

 

[Old_Crow]

The alternator is going to be working anyway while the engine is running.  I think the increase in load caused by charging the coach batteries probably wouldn't even register any noticeable mileage increase.

Now, if you were trying to get every available horsepower out of the engine to decrease ET's at the strip, then...maybe, but for practical street applications not worth it.

Besides, if you broke down 1/2 way to the free electric spot, you'd probably be thankful for each amp hour the alternator stuffed into the batteries to that point.

Similar to the reasons I always travel with a full water tank.  May not need it, but if you do...

 

[NY_Dutch]

I wouldn't expect enough effect on MPG to be noticeable or even measurable with any thing less than lab grade equipment. Driving habits alone can have a much bigger effect.

 

[Gary RV_Wizard]

Agree fully with the others.  No significant benefit.

You are correct that generating power consumes horsepower, but the tiny loss to house battery charging will get lost in a myriad of other factors. You could do as well or better by simply focusing on accelerating more slowly and coasting to a stop rather than braking.

 

[RedandSilver]

I also agree with the others.

I usually drive 55-60 mph and that seems to be the sweet spot for best MPG.
Slowing down (cruising) will have a much greater impact on MPG then charging a set of batteries.
I tow a trailer sometimes and sometimes not.  I see very little difference either way.
So my point is that I would assume that leaving or taking a 5,000 lbs trailer having very little difference,
means disconnecting a battery bank would be useless IMO.

Now I have a diesel - so a gas unit MIGHT have a bigger difference then a diesel,
but if so it's probably not much.

But go ahead and disconnect them and then report back with the results.
Although keep in mind that it would have to be the same roads and weather conditions to get anywhere near
realistic results to compare charging vs. not charging. 

 

[Henry J Fate]

Thanks for the input. Just an idea I had to try and up the dreaded 8 mpg on the 454.

How does the alternator respond to the varying loads? Is the excess power dumped somewhere or does the alternator vary power generation as needed?

 

[Back2PA]

Is the excess power dumped somewhere or does the alternator vary power generation as needed?

As needed

 

[Gene50]

A 90 amp alternator charging at 14 volts is putting out 1260 watts which is the electrical equivalent of about 1.6 horsepower

Reducing the amperage/watts output by half will not be noticed more than a pimple on an elephants butt. 

 

[Henry J Fate]

If it is as needed then I suspect this may be a fun project. I am mostly on flat roads here in the Florida winters. It seems the only time I experience a hill is an overpass. I drive with all of the mentioned habits to attain good mpg. It appears I may have the geographics and habits to give things a good test.

I will need to figure out how to toggle the charging voltage to the house batteries from the alternator. I would prefer to be able to toggle the alternator charging without interrupting normal 12 volt house power. Any help with that would be great. I will look in my documents to see if there is any wiring diagrams of the charge circuit from the chassis.

Does anyone have an idea of how that may be done on a 2001 Winnebago p32 chassis?

 

[Gary RV_Wizard]

The alternator doesn't have to do anything to manage "as needed".  It's inherent in the physics of the power generation. The greater the amp load, the greater the resistance to spinning the armature inside the alternator, so the more horsepower needed to keep the drive belt turning. There is a minimum, of course, needed to overcome the friction in the belt and armature shaft, but the rest of the horsepower load varies with the amps produced.

That 90A alternator in the prior example isn't always producing 90A, so the 1260 watts is a peak rather than constant.  The output power is only a portion of the horsepower the alternator consumes, but it's the max you could save by cutting the amp draw.  So if your RV is using 200 hp to maintain a steady 60 mph (that's fairly typical, by the way), you might save 1.6 hp or less than 1%.  At 8 mpg, that might get you a 0.08 gain.

 

[Henry J Fate]

Hahaha. How would you know about pimples on an elephants butt? I am concerned.

 

[Gary RV_Wizard]

In most class A coaches, the house batteries get charged simply by closing a big relay that connects the house system to the chassis system while the engine is running. The source of the voltage to trigger that relay is a wire off the ignition that is active whenever the ignition switch is ON. Stopping that charging is as simple as interrupting that wire.

 

[Gene50]

Hahaha. How would you know about pimples on an elephants butt? I am concerned.

Well I?ve never noticed one of them so I figured it would be an apt comparison

 

[Henry J Fate]

Would that be the same relay activated by the momentary switch for emergency starting?

 

[Back2PA]

you might save ... less than 1%.  At 8 mpg, that might get you a 0.08 gain.

I suspect this may be a fun project.

LOL. Henry, you must be out of projects, I'm not. Stop on by, I've got a list if you want to lend a hand  ;D

 

[ChasA]

Yes, the same relay (solenoid). Find the wire that comes from ignition and put a switch in it.

 

[Domo]

You could empty your water/gray/black tanks and only fill your fuel 1/4 of the way to reduce weight...


Reducing wind turbulence from roof protrusions plus adding a "nose cone" could reduce wind resistance pretty quickly and we all know how head winds kill our mileage...



Seriously though, I think driving better/smarter at the vehicle's sweet spot, not swerving, not accelerating up hills, etc. will give you more benefit.


You do present an interesting question where someone could calculate how many amps have to go into the coach batteries, convert to watts and then figure the horsepower lost to generate those amps and equate to overall mpg. However, despite the potentially interesting but doubtfully rewarding outcome, I think it'd be more work than it would be worth...


Thanks for making us think about it.

 

[Domo]

A 90 amp alternator charging at 14 volts is putting out 1260 watts which is the electrical equivalent of about 1.6 horsepower

Reducing the amperage/watts output by half will not be noticed more than a pimple on an elephants butt.

I'm betting the elephant know it's there! ;D

 

[Henry J Fate]

If the same solenoid is responsible for that emergency start momentary switch then it wouldn't seem to be much of a project at all. I have a full time switch to replace that momentary. From there all I would need to do is severe the ignition wire from activating the solenoid. I would probably prefer that design anyway. I will look into setting that up.

Elephants are pretty smart. About 20 years ago I was in Asia and watched an elephant paint. He didn't just paint. He painted a beautiful picture of flowers. I watched him do it no lie. After he was through, the trainer (who was human BTW) offered the painting for a price. I should have bought that painting but because it would have created issues with my travel, I passed on the offer. Someone else bought it while he paint another.

I should have asked the elephant if he knew when he had pimples but didn't think of it at the time.

I must now figure out when I can add the switch modification to the coach and post the results. I will do it soon.

 

[Mark_K5LXP]

Unless you start your trip with the batteries discharged, the alternator won't be doing anything anyway.  If you want to run the numbers you could check charge current into a discharged pack and compute your HP, 742 watts per HP.  A "hard" charge for a house battery is about 50A, so working that out at a 14.2V alternator output would be 14.2V at 50A ~1HP.  With batteries >80% SOC this current will be less, to nearly none for full batteries.  If you can detect a 1HP difference in your performance or mileage it would be purely psychological.  Contrast that to driving with 500lbs less water or other cargo, and maintaining tires at the proper pressure.  There's no magic bullet to gaining a lot of power or economy, most anything you would do is pretty minuscule which is why most people don't bother.  You can go through many measures and not move the needle very much.  You're moving a big heavy box down the road and there's an energy price to pay for that.

Mark B.
Albuquerque, NM