Onan Quiet-Diesel Gen 12V lead

[Geom]

I'm working on my initial design for a lithium based battery storage system.
I don't plan on using solar for quite a while, and would like an alternate source of 12V to charge the batts; when not hooked to a pedestal.
It occurs to me that I have a great 12V power source already on-board -the Onan 7.5KW genny installed.

But currently our genny provides 110V to the inverter/charger, which in turn converts that (back down to) 12V to charge the lead-acid batts we have.

Does anyone know of a (pseudo-legitimate) way to tap directly off of the 12V leads from the genny itself, and run that 12V directly to a separate 12V battery management system, bypassing the genny's on-board inverter?

As it stands I think there's A LOT of loss in converting the 12V the genny naturally produces, to the 110V it inverts it to then and pushes to the charger, which then converts that back down to 12V.
If I can skip those 2 middle inversion/conversion steps, I think I can substantially increase overall system efficiency.

I'm not even sure if Onan would allow this. There is a lot of "intelligence" built into it to manage a 110V output (with shutdown if there are issues with that). So it might be unhappy if I tap directly into the 12V side (before its inverter).

Thoughts??

EDIT: It's also possible that I'm misunderstanding the output cycle of the Onan and it could be a straight AC generator; outputting an AC voltage naturally (ie no 12V to 110V inversion step).
But I'm not sure if the generator does that.

[Geom]

LOL, yeah I was completely misunderstanding the output of the gen.

These generators output an AC signal naturally, that is governed by engine speed (in sync with either a mechanical or electronic governor).
So there is no 12V cycle involved. The way it's currently configured is the most efficient output for it, and a 110V to 12V conversion step is necessary.     

[Lin]

A generator of that size is, of course, massive overkill as a battery charger even if you could find a way to tweek the efficiency.  We do use ours for that purpose when dry camping but generally hope to be supplying other electrical needs at the same time-- heating water, running AC or heat, microwave etc. But as the batteries charge and the charge rate goes down, it really gets ridiculous to run the generator to top off the battery bank.  We have a small Honda 1000w generator that we can use to do this.  However, even a small temporary solar setup would be preferable.

[Iceni John]

If you were planning to buy PV panels anyway, why not get them now?   They're about as cheap as they'll ever be, and the general consensus of opinion is that PV prices may begin to rise again soon, now that some large Chinese PV manufacturers have gone out of business.   I paid only 81 cents a watt  -  unless you're buying pallet-loads you probably won't get much lower pricing now for USA-made panels.

John

[Geom]

Yeah that generator is a bit of overkill as a battery charger, but I plan on installing high capacity batteries (~12-15 KwH).

I had considered getting one of the Honda gens (2Kw) because they're super quiet and sip fuel. But I like the idea of being single fuel and all electric. It simplifies things, for what I'm trying to accomplish. But it's still on the table as an option.

My thought on PV is that it's not quite there yet. At least not for me.
Plus I'm not in a position to install those on my roof yet. As that will require some advanced planning. There are issues with where we park, how much shade we get, whether to install a sun tracking system, how to mount that, how many panels to get, how to mount them to maximize output and maximize heat dissipation (those things generate quite a bit),etc, etc, etc. I don't want a bunch of panels floating around in my bays (that can break, in addition to consuming a bunch of space) that I'd have to manually deploy on the ground. And even if I deploy a 1000 watt panel system (at considerable cost, let alone space) I'd realistically be looking at getting 3-5 Kw out of it, at best, during an entire fairly sunny day.

So for me solar isn't quite there yet. I'm certain it will be, but not right now. The technology is rapidly improving and efficiencies are increasing almost month over month. A 250w panel is quite easily attainable now as it was untouchable 24 months ago. So solar is in the pipeline but not for a while yet.

So in the interim, we still intend on boondocking; in a bunch of different places.
So my thought is invest the money in a high capacity, high efficiency, high output battery bank; which I would need anyway with PV. My onboard lead acids are sucking fumes, weigh a ton, and really can't keep up with our demands. With lith, I can fill them up in the morning or midday or whenever (probably during a time when we'd be cooking and using other peak loads like a water heater) using a predictable source of power (i.e. a generator). And then run quietly off of the batteries for the remainder of the day. A 15kw system would more than adequately allow us to run whatever we might use during the day to cook, bathe, use computers, lights, fans, even mild AC/heat use, etc and then just repeat the process again the next day.

The onan uses .4 gallons of fuel at half load (~3.5kw) and 1.07 gallons at full, per hour. It's pretty clean and fairly quiet-ish. I can have my entire battery bank topped off and be done in a couple of hours or less (depending on previous use) at little over a gallon or two of fuel. With a 165 gallons of onboard fuel I'd run out of several other resources, long before fuel. And if we went the Honda route, the 2kw can run for hours on a (1.7 gal) tank of fuel and is virtually silent.

So while I don't like burning diesel, it seems to me to make the most sense and ironically the more efficient path.
Now talk to me in 12 months when diesel is at 5 bucks a gallon, and I might have a totally different opinion on the matter 

[TomC]

I know that L.A. Freightliner (where I used to work) sells 160amp alternators for about $160. Why not buy two of those and make your own 12v charging with either a small 6hp gasoline or Diesel engine. Good Luck, TomC

[Lin]

I have sometimes thought about installing a couple of solar panels to the roof of my Jeep.  That way it would be secure and could be parked in the sun while the bus was in the shade.  It would only need one cable to run to the controller in the bus.  The bulk charging would still be done mostly with the generator in the AM, and the panels would do the rest at leisure.

[bigred]

For what it's worth,running a gen-set with little or no load is extremely hard on it!!

[Geom]

Tom,

While that's an intriguing proposition, especially for the wanna be hacker in me; I just don't think I can assemble anything as cleanly, or efficiently, let alone quietly as the Onan or the Honda. 

Lin,

I love that idea!! I can probably even use the "luggage rails" to fashion a quick mount/dismount bracket.
I can place the car anywhere, without worry about being under shade.
The luggage rack is somewhat elevated, allowing for airflow and heat dissipation behind the panels.
I might even be able to get 2 x 250W panels on the roof.
Then just run a single cable to the controller inside the power-bay.
First time I drive off with that cable still attached, though, will kinda suck, LOL.
Guess I'll have to come up with a way to have a quick disconnect, like they do at gas station pumps 

Will have to run this idea by the "committee" and see if she approves having 2 solar panels strapped on top of the suv!
I just have to convince her it won't look goofy at all, but rather futuristic and cooool, LOL 


I did do some additional math on fuel consumption and it seems I've been giving the Hondas way too much credit, at least on fuel consumption.
According to their stated output and runtimes:
The Onan will produce ~7300W/Gal of fuel. While the Honda (2000i) will produce ~5300W/Gal of fuel.
And the Honda will run for 3.5 hours to do so, while the Onan can knock that out in an hour.
So for 15-17 gallons, we can boondock for up to 2 weeks and have all the power we could reasonably need.
And if I couple that with a 2-panel PV system, I'd obviously need less, and can implement that (once approved ) whenever.

I'm starting to like this plan 

[Lin]

Geom-- An added benefit to the panels on the toad roof would be that everyone will think you have a solar powered car.

The thing about the Honda vs the diesel for charging the batteries is that, since the charge rate gets so low when topping them off, that it would be better to spread the fuel use over several hours.