charging from alternator effects on fuel milage

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TMG51

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Adding a positive cable and solenoid from your existing alternator back to your house battery? I don't think that does affect your mileage. Someone correct me if I'm wrong, but, I think the house battery is just taking surplus that would be generated by the alternator anyway and so the resistance is the same.

Upgrading to a bigger alternator or adding a second alternator would increase resistance, and therefore reduce mileage... though I don't know if it would be in any appreciable amount.

Either way it's an option worth having.
 

firebob

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1 hp will put out around 160 amps at 12 volts. Alternator technology changes a lot over the years.
Some of the stuff made after 2010 would put out 12.3 volts or so at idle and running the stock radio and fan on high would use up more power than the alternator would put out.
Until you get some RPM's you do not get any good voltage and amperage. There can be a good amount of voltage drop by the time you get to your house bank with the wire size, connectors, relay, and stuff. You can also get a good amount of voltage drop on your negative side if your running a "body ground" and not running your ground all the way back to the engine.
 

John61CT

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In addition to what I said here: https://vanlivingforum.com/showthread.php?pid=263695#pid263695

There are alts designed to put out a high % of their powerful output at low rpms, say "fast idle".

And regulators + add-ons that will give you fine control over the load imposed on your engine, e.g. Balmar's belt manager feature.

Plus people can and sometimes do insert controls to let you tweak the field current manually if you want more or less engine output available for propulsion vs servicing the charging load demanded by your banks - see Stern Wake's clever hack.

IOW it's complicated. Basic rule of thumb IMO would be: in normal circumstances, don't let your Alt(s) consume much over 10% of your engine's power output.

But OTOH, do try to make sure you're able to charge up your banks as quickly as possible if you're taking short trips, or even running a motor for only that purpose.
 

SternWake

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According to this site, Each 25 amps the alternator has to make, requires one HP from the engine.

https://alternatorparts.com/how-much-horse-power-does-an-alternator-make.html

So A deplted battery bank sucking up 60 amps is certainly more of a load on the engine and will affect mileage

I can feel it in my gas pedal when my alternator is making 65 amps for the depleted battery, vs when the thing is full.

My engine requires 8.2 amps to run at Idle and about 12.2 amps at 2000 rpm, to fire the ignition and fuel pump.

Lights and blower motor on high are about an additional 30 amps, in addition to whatever the battery needs at 14.X volts, or whatever voltage I choose.

At hot Idle my alternator maxes out at 50 amps or so, So Hot idling with lights and blower motor on high, can max it out It is rated at 120 amps and this appears accurate, but not when super hot, and not until about 2400+ rpm

Alternator juice is not free. the more amperage it has to make to maintain voltages at 13+, the more HP it requires from the engine
 

Mobilesport

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SternWake said:
Alternator juice is not free.  the more amperage it has to make to maintain voltages at 13+, the more HP it requires from the engine

It would be nice to get some real numbers at how much is spent at the gas pump.
Like fill the gas tank and go on a drive charging a depleted battery bank with 14.8 volts until you
reach 85% state of charge , then fill the tank and note how much $ it took to fill the gas tank.

Let's say it took 3 hours of driving  to get to 85% state of charge , so now 
go on a 3 hour drive with no charging of house battery and after 3 hours fill the tank again and
note how much $ it took to fill the gas tank.

How much more did it cost for the drive when you where charging ?
 

TMG51

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When I wrote my above reply, I was assuming we were talking about charging during driving you would otherwise be doing (e.g., I'm driving at 80 MPH on the interstate for an hour and a half today).

If you would not otherwise be driving and/or have to rev your engine higher than you otherwise would to keep up RPM and charge, well then, it seems pretty self explanatory that this has a great effect on your economy.
 

SternWake

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MS,


There are so many influencing variables in such a test, I would not contemplate even trying it unless I was paid well, and given precision testing/measuring instruments.  Without precision, one could Will the results to see what one wants to see.

It is what it is. 
 Get my depleted battery to 14.7v pronto and hold it, you SOB alternator.  Check. 
 Meanwell, Check.  
  198 watts of solar, well you can at least hold absorption voltage for the corect time even if you cant get there fast. Check.

If I go from 16.0mpg on the highway at 65mph to 15.0mpg, for  30 minute duration of getting my 40% battery to 80%, So be it.  Could slow down 5mph and get it back.

City driving is too variable.

80% to 100% SOC never takes the same amount of time either, unless every cycle is a "lab" cycle, perfectly recharged under the same exact  controlled conditions from the same depletion level as All the cycles before.

MS Since you generator recharge I guess you are curious as to whether you are better off running the generator powering meanwell/powermax rather than employing the alternator.

Again many variables would need to be controlled/considered/compensated for, for an accurate repeatable result.  
If you have no issues running the generator when driving, the 40 amp Meanwell or 65 amp powermax can certainly recharge your house battery bank faster from ~70% and ~55% State of charge, Simply because they will seek and hold the ideal absorption voltage, something that one cannot control with the alternator without other modifications.  Those % are assuming you alternator can provide 90 amps as you drive, and are also guess-timations.

Do you know How much amperage you alternator delivers into house bank as you drive?  What voltage it is allowed to achieve, and for how long?  Video it if you cannot see voltmeter and ammeter from drivers seat.

Long term trends could be noticed, If you wanted to notice and C/C/C the variables to some degree.

Perhaps some interesting reading:

http://www.mvtanglewood.com/2017/02/engine-alternator-or-generator-which-is.html


One other thing MS, if this thread is about ultimately saving money, well it comes down to Gas  consumed versus battery life in your scenario/usage.

How long did you get from your last set of golf cart batteries?  Not very long if I recall correctly.  200 watts of solar on the roof can not only save some load on the alternator, and Generator and 120v chargers, saving gas, but it will likely also extend the lifespan of the batteries by a good factor as it recharges all day long, at some rate, and/or mitigates the discharge from loads present during the day.

So it really comes down to, how soon does that 200 watts of solar pay off?

High amps in the morning when most depleted, and enough  Solar (hopefully more than enough) to reach and  hold absorption voltage all afternoon= happy long lived batteries one is not constantly fretting about or replacing every 8 or 9 months.


I could not imagine it being bright sunlight, winter or summer, and not having some solar to charge or hold at full charge while my fridge holds beer at 33f, and my Fans keep the internal temps no hotter than outside, while listening to music.
 

Ticklebellly

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Mobilesport said:
How much does alternator charging effect fuel milage?

Not much but really difficult to quantify.   A cold engine start in my Tin Tent runs the starter motor for under 3.5 seconds.   Once the motor is spinning at over 1500 RPM, the alternator takes less than 2 minutes to replace the energy used in that start.   I can follow a bit of this by watching the alternator voltage as I drive off.   Normally gets to 14.2 in seconds then drops to 13.8 pretty quick.   Smart regulator only applies higher voltage level to quickly bring the starter battery back up to full charge level.    All this controlled by voltage levels/readings.

For charging the house battery from the alternator, I use a D250S DC to DC charger jumpered across the starter battery.   Typically, I do not monitor the current into the 250S so do not know how long that charging takes on any particular day.    The state of charge of the house battery at the time I start the vehicle will determine how long the 250S takes energy from the alternator, as I drive.    An over night house battery draw for me is about the 12 AHs.   The 250S puts that back in a half hour time frame.

If you are using the starter battery for a dual purpose, at engine start, the alternator might need to put out bulk amps and that ramp will quickly fall off as the starter battery voltage comes up.   I have had both situations and not noticed much effect on fuel economy when using the alternator to charge a well depleted battery. As said earlier, pretty hard to put any accurate figures in an answer to the question.

Using the alternator to charge a battery has the effect of "Robbing" some engine power to do that work.   My other vehicle has some extra smarts in the starter battery charging system and it does not get any charge applied until its terminal voltage drops to 12.45 volts.    In this setup, full engine power is normally available as I drive and I have noticed when the charge cycle does happen.   Over lots of distance, the effect on fuel usage will be the same.
 

Ticklebellly

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TMG51 said:
..... I think the house battery is just taking surplus that would be generated by the alternator anyway and so the resistance is the same.

Demand always happens first and supply from the alternator happens second.   Demand from the alternator is supplied by the engine, in that sequence.   It is an example of the conversion of the form of energy.    You know,energy cannot be created or lost, only changed in form.   Alternators only put out what is being asked for by the load.   Check this out for your self by running the engine then turning on as many high current devices you can, like a heater and headlights and spotlights.   As the high current draw devices come on line, there will be a corresponding change in engine note as the engine supplies more power to spin the alternator.
 

TMG51

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Ticklebellly said:
Demand always happens first and supply from the alternator happens second.   Demand from the alternator is supplied by the engine, in that sequence.   It is an example of the conversion of the form of energy.    You know,energy cannot be created or lost, only changed in form.   Alternators only put out what is being asked for by the load.   Check this out for your self by running the engine then turning on as many high current devices you can, like a heater and headlights and spotlights.   As the high current draw devices come on line, there will be a corresponding change in engine note as the engine supplies more power to spin the alternator.

That makes sense of course, but again my thinking was in the scenario of driving at high RPM. If under little load then it makes sense the alternator would have to work harder to keep up with demand. If already driving at highway speeds, is there an appreciable difference to adding another load? My thinking was no.

Energy cannot be created or destroyed, but it can be wasted when not sent to an auxiliary battery.

I know that low RPM usage (idling) does little to charge a battery, so my thinking in terms of alternator charging was only when other high RPM activities are taking place.
 

jonsun

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a generator without an inverter the RPM is set to the voltage output and a load put on it doesn't change the rpm's. alternators are also constant voltage, varying current due to it's voltage regulator. So the pulley is setup for it to put out it's nominal voltage at idle. Putting a load on it doesn't require a faster RPM, just more throttle to maintain it as it wants to bog the engine down.

just how much is lost is hard to say....too bad mythbusters is no more

Im going with a shot in the dark to say it's about like running the AC. ~1mpg
 

DannyB1954

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An 800 watt generator can charge up a 100 amp hour battery using a 35 amp charger in about 4 hours. The generator burns about a gallon in 8 hours, so a guess would be it takes the energy of about a half gallon of gas to charge the battery.

If you are driving 60 miles per hour for 4 hours that would be 240 miles. If you get 15 miles per gallon normally without charging an extra battery you would use up 16 gallons of gas. So with the extra battery your mileage would be 240 divided by 16.5 gallons or 14.5 miles per gallon.

This is a guestimation. you could try to convert amphours to kilowatts and horsepower to kilowatts and do a bunch of math to figure out what amount of gas is needed to produce that amount of kilowatts, but it would still be a guesstimation.
 

jonsun

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my 1500w continuous generator burns .12gph @ 50% load.

If I lose .5 mpg from alternator

going down the highway at 60mph

@ 12mpg for 1 hr = 5 gallons
@ 11.5mpg for 1 hr = 5.22 gallons

= ~2x more fuel over a generator
 

John61CT

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Modern even stock amps in the US may be designed to be able to put out close to their constant-rated amps at relatively low RPM, ~2000.

Alts designed for the purpose as with emergency vehicles may do so at high idling.

But again, Volts output does not create any appreciable load on the pulley, only Amps when the battery pulls a charge.

There is NO current being generated, no energy "going to waste" unless the bank needs charging.

Switching off the field current and A/C gives all HP to propulsion no matter the state of the bank.

So switching the field current back on, but keeping your empty 800AH LFP or AGM bank isolated from the Alt circuit, will not make any significant change in your MPG.


On a boat this could actually be a life and death issue, there are often times it is essential to have all propulsion energy available for maneuvering in currents and hazardous emergency situations.

Such switches are less common on land since going slower is rarely dangerous, but you do see them in the big overlander unimog type setups.

To me MPG is the least important issue, getting most of your charge from solar or shore charging can save wear and tear on your engine.

If you're starting up a long steep mountain with an overloaded trailer, already at the limits of your drive train, e.g. overheating your transmission lubrication, and THEN close the connection between your high-output Alt and that big empty bank, your climbing speed will drop dramatically, you're burning more fuel, and the added strain on your drive train could at least in theory do real damage.

Flipping your AFD switch will have the same effect as opening the circuit combiner, instant relief as all HP goes back to the drive train.
 

John61CT

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But if your engine is oversized relative to your propulsion load, say a Ford F350 with a pop-top, adding say 7HP load to drive even 300A from the alt into a low SoC high-acceptance bank won't be a strain mechanically.

But you would notice a change in MPG, just not as much as say a Honda, since your setup is already inherently inefficient.
 

jonsun

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We were traveling once in an e350 box truck fairly overloaded. They turned on a 1200w space heater. It ended up breaking the belt and the alternator bolt off inside the bracket. Got it fixed and on the way home it happened again.
That's how much pressure can get put on one.
 

John61CT

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And some charging setups can pull even more current than that.

It's very common fitting an aftermarket HO amp that the existing belts setup just won't hold up under load.

Which is why tech like Balmar's "slow start" and "belt manager" exist.
 

Ticklebellly

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bardo said:
WThey turned on a 1200w space heater. It ended up breaking the belt and the alternator bolt off inside the bracket.

I have seen a home built wind generator with a mechanical gearbox, stopped dead, due to the "back EMF" created when a very large load was connected.
 

Mobilesport

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Sternwake wrote
"MS Since you generator recharge I guess you are curious as to whether you are better off running the generator powering meanwell/powermax rather than employing the alternator."

Exactly  ,  I've been running  some numbers and I came up with  this.

My Honda genset uses approximately .33 cents per hour in fuel to charge  the batteries 

I believe a traditional alternator would cost ballpark . 60 cents a hour to charge batteries while driving and
a setup like your 14.8volts adjustable regulator would cost quite a bit more in fuel then a traditional  alternator. 

The Honda would need replaced every (estimate) 6-7 years @ $1050 but i believe the fuel savings
would pay for it.
 

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