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Old 06-19-2019, 10:36 PM   #1
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A/C or DC based electrical system

Given the option, would you prefer to have an AC generator tied into the electrical after the inverter (obviously) or a similar watt DC generator tied into the batteries before the inverter?

Im thinking the advantage of AC is that in the event the inverter fails i can just turn the generator on. The possibility of an out of phase electrical system scares me though if it doesnt play well with the inverter while charging the batteries in normal operation.

While the advantages of DC would be no fear that the generator sine wave would go out of sync with the inverter sine wave, i would have an “inverter generator” so generator rpm could vary based on load instead of having to hold a constant rpm to maintain the 60hz sine wave which would also save fuel under low demand. The batteries im looking at are lifepo and I believe have an internal controller that would prevent overcharging. (Powerbrick brand)

The generator would serve to power the electrical past what any solar panels could provide (duh) but the coolant system would include liquid to liquid heat exchangers to also provide heat and hot water. That would take me from 33% efficiency to up to 66% if im in fact utilizing the full coolant heat. So i plan on running it about half an hour every 4 hours in cold conditions.
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Old 06-19-2019, 11:25 PM   #2
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For long term off grid boondocking without frequent gennie use, my preference is direct DC as much as possible, focus on efficiency, keeping Ah per day as low as possible,

minimize use of load devices requiring inverters, only connection to shore power is the charger.

If your bus is on shore power a large proportion of overnights, or you're running aircon / genset many hours per week anyway, then that strategy is not so required.
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Old 06-20-2019, 12:06 AM   #3
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Sorry, should have added it would be full time live in and 48v because i dont want to pull massive amps for the equivalent wattage a 12v system would require. Basically for safety reasons. Smaller fuses, lighter wiring thats cheaper and easier to oversize. Im still in the planning stages and I dont necessarily want the generator and batteries close to the power closet. I would like to put the generator in the back under the bus but the power closet up front by the kitchen and front door for easiest access.

So dc appliances is out of the question. Also I do apartment maintenance so i could get free (older) appliances to offset some of the inverter cost. I also plan on using native 48v batteries not 12v tied to get 48v so that’s another reason 12v wont be wired in at all.

I guess my basic question is “do I want to directly charge the batteries off the generator or do I want to directly power the house and charge the batteries off the charger.

It seems like it makes more sense to push dc amps, let the batteries charge and still invert the leftover power. Less conversion overall because I wouldn’t be going from ac to dc back to ac. On top of the batteries not being limited to a 1800w charger (120v 15 amp). A 250 amp dc generator at 48v is 12,000w.

But am I missing something?
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Old 06-20-2019, 07:59 AM   #4
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If you're looking to be as self-sufficient as possible, cobbling a DIY DC genset together is the way to go to get your storage bank recharged quickly while off grid.

The efficiency issue compared to an AC genset is probably not that significant, once you're burning lots of dino juice daily anyway.

Go to LFP for high CAR, no need to charge for 6+ hours for bank longevity as with lead.

Check out Victron and Magnum for very nice high-capacity combi inverter/chargers.
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Old 06-20-2019, 10:23 AM   #5
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But am I missing something?
You could step down the 48V to 12V where needed. I have a 24V system, am floating a 48V battery bank, and would still want both 12V and 24V capabilities.


There's these fanless step-down transformers (24V or 48V -> 12V) on Amazon that go up to 480W, I have one. Those could be placed at the appliance for DC appliances.


The problem with inverters from my POV is the single point of failure. No getting around that, so nothing critical in my rig is going on AC.
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Old 06-22-2019, 07:26 PM   #6
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Thanks for the help, yeah DC does make more sense.

I'll think about the transformers. Not sure what i would want to run on dc though.
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Old 06-22-2019, 07:31 PM   #7
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Old 06-22-2019, 08:20 PM   #8
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You could step down the 48V to 12V where needed. I have a 24V system, am floating a 48V battery bank, and would still want both 12V and 24V capabilities.


There's these fanless step-down transformers (24V or 48V -> 12V) on Amazon that go up to 480W, I have one. Those could be placed at the appliance for DC appliances.


The problem with inverters from my POV is the single point of failure. No getting around that, so nothing critical in my rig is going on AC.
If you just have a couple devices where you can't find good native DC versions, no need for one big inverter, cheaper to get multiple smaller ones, easy to carry spares.

DC-DC changing voltage, proper term is converter.

The US RV industry uses that term for AC-DC rectifiers / PSU / chargers too, very confusing.
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Old 06-22-2019, 08:24 PM   #9
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The adversion to dc is 1. I can get free ac appliances, and 2. Ease of parts/ replacement.

There is no comparison on ac vs dc appliance availability. Even led lights are just easier to be ac for ease of replacement.
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Old 06-22-2019, 08:28 PM   #10
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My main thing was just if if was better to charge the batteries off a dc generator. Seems like the answer is yes.
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Old 06-22-2019, 10:15 PM   #11
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Neither is "better" objectively except maybe the noise factor. What kind of unit you have in mind?

Yes personally I prefer a DC genset, but that's because it fits my preferences and values. I'm surprised if you do, seems inconsistent with yours.

An AC genny is easier to find, likely cheaper if you aren't into DIY self-sufficiency.

You need a big mains charger anyway for when shore power is available.
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Old 06-22-2019, 10:19 PM   #12
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Better would be charging the batteries with the full power output instead of being limited to a comparatively low amp charger.

No one wants to run a generator longer than needed wasting fuel.

"Houses" are AC, batteries are DC. The line becomes where do you do the conversion. To me right before consumption seems like it makes the most sense. Again partly because I dont like the idea of multiple sine wave sources. If the generator doesnt play well with the inverter then an expensive inverter will blow and ill probably still need to replace the offending generator. Or worse ill keep blowing appliances and not figure out why.

Maybe im paranoid but that isnt even a factor if generation is all dc
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Old 06-22-2019, 10:44 PM   #13
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What's your intended use for your skoolie?

One advantage I see to an AC generator is that you can pretty much swap the generator output for shore power with a simple switch - either input will work, then downstream, you're generating the DC power supply. It makes the wiring simpler. Plus, it's easy to get a new generator, or plug in to a different electrical outlet. If you make a DC generator, then you only have custom replacement options - you can't just buy a new one off the shelf if it breaks.

If you're not/never planning on hooking up to shore power, then it doesn't matter.
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Old 06-23-2019, 12:03 AM   #14
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Better would be charging the batteries with the full power output instead of being limited to a comparatively low amp charger.

No one wants to run a generator longer than needed wasting fuel
I did say big charger.

How fast the bank charges has little to do with the amps rate the source puts out, more to do with the chemistry's Charge Acceptance Rate.

With a lead bank getting to 100% Full takes 6-7hrs no matter what.

You only want to do the first couple hours max off ICE, need solar for the rest of the long tail.

For AGM about .4-6C is ideal, FLA can be .15-.20C.

With LFP no need to get to Full, but could get there in a couple hours with enough amps available.
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Old 06-23-2019, 10:33 AM   #15
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my system has 4x 220AH AGM batteries and a 3kW inverter / charger. The charger will put out about 110 A.

Most of the time all my charging is provided by the Solar panels but for those dark days, sometimes it needs a little help
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Old 06-24-2019, 08:25 PM   #16
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Use case is full time live in, shore power seems like it wouldn't be worth the expense. Not enough time would be spent near a plug. I live in AZ, wintering in quartzite sounds good, or just empty federal land.

I guess an AC generator only hooked up to a charger would serve the same purpose as a dc generator with the emergency use of eliminating the inverter if needed.

Definately something to think about.
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Old 06-24-2019, 09:34 PM   #17
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Large amp shore power charger is needed anyway if you run off a normal AC genny.

An inverter is only needed for load devices, completely separate issue how your bank gets charged.

You need some solar if you want the bank to last, at least a couple cycles per week need to get to 100% Full, takes 6+ hours to get there.
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Old 06-24-2019, 10:34 PM   #18
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"Hardlined" generator, if you have to store it on the bus you might as well leave it there. People build "garages" well what better place to put the generator than a sealed off, vented properly garage.

My point on "still the same as dc" was that i wouldn't hook up generator ac to the "house". It would go through a dc conversion first. I really dont want any source of ac besides the inverters.
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Old 06-29-2019, 03:22 PM   #19
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For long term off grid boondocking without frequent gennie use, my preference is direct DC as much as possible, focus on efficiency, keeping Ah per day as low as possible,

minimize use of load devices requiring inverters, only connection to shore power is the charger.

If your bus is on shore power a large proportion of overnights, or you're running aircon / genset many hours per week anyway, then that strategy is not so required.
/\ this hit the nail on the head.

Also I might add and energy conversion from AC TO. DC With a inverter will result in a loss of energy from it being less than 100% efficiency
No matter which way you go always use low wat devices like led lights to get longer time from a batt charge
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Old 06-29-2019, 06:27 PM   #20
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Do not use electricity for anything involving heating, cooking etc, use engine waste heat, propane or solar for that.
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