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Author Topic: To generate or to alternate that is the question, or at least A question...  (Read 1320 times)
Geom
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« on: February 22, 2014, 10:38:16 PM »

Ok here's a somewhat esoteric question that I believe will have real-world effect on my setup.

The bus I'm buying currently does not charge the house systems off of the engine alternator.
It does however have a very nice Onan 7.5KW generator on board.

My questions are as follows:

I have the option of reenabling the bus' ability to charge the house batteries. Doing so will require the purchase of additional relays, obviously at some modest cost (~$400 with wires etc). But do I want to?

Here is my logic,

Both the gen and the engine are diesel powered, drawing from the same source.
Both devices have the ability to convert mechanical energy, by burning fuel, to produce electricity.
Any amount of electrical generation is going to require appropriate fuel to make into electricity -whether that's the generator spinning its own "alternator" to make electricity, or the added resistance on the bus' alternator to produce additional current.

So my thought is that the generator is a MUCH more efficient machine at converting diesel into electricity than the bus' engine. Generating electricity is only a convenient side-effect of what that engine is really doing, moving a 30000 lb vehicle.
So I think I would be better off just running the generator while en route to recharge batteries and all that, and forgo the bus' charging system.

Additional point:

An average bus alternator (through some searches) is able to put out about 350-400Amps @12V (=4200w) and not all of that would be usable to charge the house batts. A good amount of that is probably running the 12V bus systems and I would guess maybe about 250-300A is actually available. So that's 3600W (assuming 300A).
So the generator, running at half power (its most fuel efficient operating state) is able to produce slightly more than that @3750W and significantly more than that at full power.

I realize doing so would add more hours and wear to the generator, and I guess I'm ok with that? It is probably a more expensive device to repair and replace than a bus' alternator. But again they're designed to work and lets face it, it'll most likely be idle most of the time; certainly when we're hooked up to a pedestal.

The one definite positive the additional charging option would give me, is exactly that, an additional option; which is always a nice thing Cheesy  anyway....

So what are your thoughts?

Thanks, as always
George
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« Reply #1 on: February 23, 2014, 01:42:14 AM »

I feel it is a simple answer to this;

While traveling use the bus alternator as using the seperate generator requires additional fuel and not needed.

When parked, use the diesel generator when needed to charge batteries.

Also  consider your type of batteries, Lead acid, Gel, AGM, they each take a different DC voltage.

Dave M
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« Reply #2 on: February 23, 2014, 03:49:07 AM »

The logic is unassailable however there is one flaw. If you use the coach alternator to charge the house when driving your generator becomes your back up power supply. If you rely solely on the generator you have no back up except shore power.

When your batteries are charged there is little or no load on the alternator and it happens seamlessly with no input from you. However if you rely on the generator you either have to set it for auto-start or you waste fuel and generator hours unless you manually start and stop the generator as needed.

I have a 270 amp @ 24V 50DN and it is more than ample even if I choose to be an energy hog. My actual loads are much less than what you forecast even with my bus over the road heat and air with the big blowers for the condenser and evaporator.
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« Reply #3 on: February 23, 2014, 03:51:57 AM »

I feel it is a simple answer to this;

While traveling use the bus alternator as using the seperate generator requires additional fuel and not needed.


But charging via the alternator also uses additional fuel - the question is, which uses more?

If you were being really clever you could have a switch which only put the extra load on the bus engine when it was idling or coasting etc. How significant the extra fuel usage is when charging the bus batteries is I don't know - my instinct is that they would quite quickly reach 'float', after which the extra load on the bus engine would drop right off. To me personally running a generator whilst the bus is in motion just seems 'wrong', but I know it is something which many of you guys are forced to do by the need to run your AC.

Jeremy

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« Reply #4 on: February 23, 2014, 05:21:15 AM »

Depending on one source if a second is so easily available seems short sighted.  Adding an ability to charge the house batteries cost me $40 for a manual switch and about 4 feet of cable, so essentially free given that I needed a switch anyway (house bank to inverter disconnect, just got a three way instead of a two way).  The bus uses essentially no electricity except for whatever lights or fans are on, so about all the power from the alternator is available, presuming the original OTR air conditioning isn't in use.  Charging the house bank also takes quite a small amount of power, in the greater scheme of things, figure maximum 20% of the 20 hour AH rating and that only at the start of a deep discharge charging cycle.  Bottom line, I believe charging the house bank from the main engine alternator is a pretty high priority.  I only run my generator to run a high current load like the AC or a coffee maker, almost never to recharge batteries unless I am parked for several days without external power, which almost never happens.

Edit:  If you figure an average (not peak, but over a day of travel) charge current to the house bank might be 20 amps or less, at 13 volts nominal that's 260 watts, or about 1/3 hp.  You might do a theoretical calculation of fuel required, but in reality that's about the difference between a 10 mph and a 12 mph headwind, or running 56 mph average instead of 55 mph.  As near free as something on a bus can be...

Just my way, as the saying goes...

Brian
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« Reply #5 on: February 23, 2014, 08:25:48 AM »

George,

I agree with Bryan. Our bus is set up similar with a single 40$ switch that combines or disconnects the house and start batteries.  Quite honestly designing the system so that there was only one bank for house and start batteries would be ideal.  We leave both systems connected most of the time.

I think something to think through - is the alternator A. "charging" the house batteries or B. actually "running" the house system.

As Brian said previously - in real world application there is a ton of juice available at the alternator without sacrificing any power/fuel from the engine.  I cant see where running your genny would save fuel while running the bus engine at the same time.  Unless (as Jeremy stated) you have AC units to run.  (I've also read posts here about someone running AC off an inverter hooked to the alternator).

If you keep most of the stuff you want to run while rolling on DC power that will keep from having to run the inverter while rolling and keep everything on a single electrical system.

-Sean








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« Reply #6 on: February 23, 2014, 08:46:18 AM »

If I run my generator while going down the road for A/C (I do not have an engine run A/C), my mileage drops 1mpg. On paper it seems that either running the big engine or running the generator you'd burn the same amount of fuel. But-remember, when the big engine is running, all you're doing is exerting a bit more power to charge the batteries to an engine that is already spinning with all it's friction. As compared to starting the generator that has it's own friction to overcome. Use the jumper relay. I have a 2 position switch on the dash that I can either have it switched through the ignition, or if the bus batteries are dead, will use the deep cycle batteries to power the relays, and an off position.

Since I have a 300amp 12vdc 50DN, I bought 2-150amp continuously rated relays (look like Ford starter relays), and strapped them together with heavy metal straps to create 300amps continuous rating. I have also started the bus on them-something you can't do if you're only charging from the generator. Good Luck, TomC
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« Reply #7 on: February 23, 2014, 10:08:37 AM »

Ok, great answers! Thank you.

So it looks like I overestimated the amount drawn from the alternator, that is not available to charge. @Jon, you're right, the bus really shouldn't have that much of a DC draw while running. There are no spark plugs, the coach AC air system is inop, and there is really little else that would draw much DC besides lights and some minor instrumentation/accessory load.

Fortunately the rest of my math should work. After some searching it looks like a 50DN @12V puts out 300A peak. So if we assume all of that is going to the charging system, then the alternator is putting in 3600w vs the 3750w from the gen at half power.

I do agree with you guys on having an additional backup and that is really the primary reason I'm considering reenabling the bus charging system. Having more options is always a good thing Cheesy

@Jon, you're also absolutely right about managing the charging while in motion. I hadn't considered that. The alternator will vary-off when the batteries are charged, automatically, and thus consume little to no additional fuel. As where I'd have to manage that manually using the generator.

@TomC, that's good info on the mpg drop running your gen. That's very good to know. So it looks like the gen is using about 1/2 gal per hour.

One additional point I hadn't considered is that the alternator is dumping pure 12V into a 12V system, so there is minimal loss there. As where the generator has to generate that same power @120V which is then passed on to the charger which then converts it back down to 12V. So there is bound to be some efficiency loss there, through heat and EM waste; in addition to the mechanical loss of spinning another crank (the generator) vs utilizing the existing inertial momentum of the big bus engine already spinning (as @TomC mentioned). 

So in summary, it sounds like reenabling the bus charging system is a good idea for redundancy purposes and is also perhaps a bit more efficient (or at least a wash) in terms of fuel use.

Thanks again!
George
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« Reply #8 on: February 23, 2014, 01:20:12 PM »

When you charge both banks of batteries you have the option of a single alternator charging each bank through some type of isolator, or having a second alternator dedicated to the house. Regardless you do not want to tie the chassis and the house battery banks together because if one set is discharged, for emergency purposes you can use the other bank in its place. Factory conversions address that in a number of ways and it is a cheap bit of redundancy.
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« Reply #9 on: February 23, 2014, 01:24:11 PM »

Tom is on the right track - the flaw in the math (genset is cheaper than alternator) is that the mill is already operating and ALL the inefficiencies of the mill already exist - the additional load of the alternator to charge and maintain the house bank while underway is a de minimis factor in computing the additional fuel use - also the amount of HP generated for a certain RPM (through to the final drive ratio via gearing) has been engineered to develop more HP than is normally required to move the vehicle down the road, partially to provide reserve HP and partially to power accessories such as the alternator - I don't see how you could (when you take into allowance all of your genset operating costs on an hourly basis) operate the genset for even twice the cost of scavenging the mill via the alternator FWIW
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« Reply #10 on: February 23, 2014, 02:51:07 PM »

That makes sense. The engine is already running either way, and its inefficiencies, to itself, are accounted for. As where spinning an additional engine (generator) will, indeed, produce its own inefficiencies. That additional engine, however, is specialized and tuned to do nothing but produce power.

That alternator will add some additional load to the engine. That load will translate to additional work required from the engine, that work requires fuel. Kind of like how my AC pump reduces the mileage in my car, when it's on. It's gotta get that 300amps from somewhere.

I'm confused on the spare HP. Any HP (i.e. energy) produced by the engine will have to go somewhere. It's either going to the wheels or alternator or other accessories or as heat (resistance), right? It's not going to be "stored" anywhere (leaving the batteries out of it as they're just a product of the already accounted for alternator output).

In any case, it is sounding more and more to me that having this capability is, if nothing else, handy (and seemingly also more efficient and cost effective).  Smiley
I'm looking into adding the necessary relay to reenable it.
I do also like the idea of having the option of using them as start assist batteries, if needed.
« Last Edit: February 23, 2014, 03:00:08 PM by Geom » Logged

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« Reply #11 on: February 23, 2014, 03:43:16 PM »

As your batteries charge, they will require less amperage.  A good deal of charge time will be under 10 amps at 12v.  So you need less than 120 watts of power.  It is not highly efficient to run a 7500 watt generator to get 120 watts.
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« Reply #12 on: February 23, 2014, 03:51:07 PM »

I wonder if an engine's alternator can properly charge house batteries at all.   Assuming that the house batteries are true deep-cycle type, don't they require a different charging regimen that starting batteries?   For example, good solar charge controllers not only charge in three distinct phases (Bulk, Absorption, Float) which an alternator doesn't do, but can also be set for specific types of deep-cycle battery such as golf-cart (e.g. T-105), L-16, AGM, etc., with each having a slightly different voltage and time for each charging phase to ensure they're all charged as effectively as possible.   Aren't vehicle alternators and their associated voltage regulators optimized for starting batteries only?

John
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« Reply #13 on: February 23, 2014, 04:24:01 PM »

I wonder if an engine's alternator can properly charge house batteries at all.   Assuming that the house batteries are true deep-cycle type, don't they require a different charging regimen that starting batteries?   For example, good solar charge controllers not only charge in three distinct phases (Bulk, Absorption, Float) which an alternator doesn't do, but can also be set for specific types of deep-cycle battery such as golf-cart (e.g. T-105), L-16, AGM, etc., with each having a slightly different voltage and time for each charging phase to ensure they're all charged as effectively as possible.   Aren't vehicle alternators and their associated voltage regulators optimized for starting batteries only?

John


You can buy multi-stage alternator regulators specifically for the job - here's a datasheet for one such device (actually the model I've bought myself):

http://www.shop.sterling-power.com/acatalog/proreg.pdf

Jeremy
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« Reply #14 on: February 23, 2014, 04:51:27 PM »

I see by your signiture line that you are running a 6V92 and V730.  Thats is what I have in my RTS and here is my setup (see pic).  The large alternator is a 160 amp 12v truck alternator that costs less than $200 to charge the house batteries and any 12v items I have, the small alternator is a 45 amp 24v alternator just for the starting batteries and any 24v items I still have in the bus.  I don't run the generator while driving unless I need more than one A/C going.  The 160 amp alternator going through my Trace SW2512MC alternator produces more than enough 120v amps to run one air conditioner while driving.  The load impact on a 350 HP 6VV92TA running a 160 amp alternator is minimal, and it runs off the V730 accessory drive.

BTW, these two alternators are the replacement for a 50DN running off the back of the engine.

« Last Edit: February 23, 2014, 05:03:43 PM by Geoff » Logged

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