However you do need to take the resistance of the wires into consideration.
#4 copper cable, for example, has a resistance of 0.253 per 1,000 ft at 68 degrees F. .308 at 167 F. 1 amp current through this "resister" results in a voltage drop of almost 1/4 volt. 10 amps results in a drop of 2.5 volts. or about 20% losses. Your 12 volt circuit suddenly becomes a 9 volt circuit. So at low voltage you require large, expensive, wires to avoid significant voltage losses.
A 120 VAC circuit using 1,000 ft. of #4 wire with 10 amps flowing through it would have a voltage loss of about 3 volts, or about 2.5% so with conventional AC circuits you can use smaller, cheaper, wires.
All good to know. And I always note wire sizes in inverter specs. Always considered wire costs to be significant to reduce losses going to the inverter. But I've booster cable jumped a lot of cars, and just naturally know that cheaping out on cables is a bad move. I was looking at cables in the 1/0 - 4/0 range and it looked to me that jumping a step in cable diameter would easily compensate for the resistance increase jump from 3 to 6 feet, and still meet a particular inverter's cable specs. I just disagreed about routing heavy cables to my car interior for an non-permanent, "as-needed" inverter setup.
Another thing to keep in mind is that the output of the alternator might be 325A, but if the battery is down 2 volts that's ony 650 Watts output (less than 1 HP).
That's about the biggest *practical* unit I've seen also. They put those on the Semis when they used to plaster them with 500 marker lights and they were all incandescent.
Oh, and they did use the 300A ones on the new Disneyland Parking Lot Trams - a tractor with a Cat 3208T on Natural Gas, and a string of 7 passenger trailers with overhead lighting in all the cars. Just a little bit of alternator load...
Now that they've gone to LED's it cut the power draw by about 75%, and they can get by with the normal factory-option 'heavy' alternator in the 200A range.
The best way to make an Inverter Rig like that for your house is to get a larger gasoline engine, like perhaps a 13-HP to 18-HP Briggs with the flyweight speed governor for generator set use, find a sheave to fit the crankshaft, and couple that to a truck alternator and a big deep-cycle battery - preferably two GC-2 Golf Cart batteries in series.
The governor part is very important, you can't chance setting the throttle by hand - if the belt snaps and the load dumps from the alternator you can overspeed the engine and blow it up. The air-vane governor on a Lawnmower-duty engine will work kinda sorta, but they'll 'hunt' and 'surge' on you while working. But most small car engines that you would repurpose into a prime mover don't have any sort of governor built in at all - you have to add one.
For small overnight loads you can run off the batteries, but you want the engine running before you crank up the microwave oven, or start charging your cellphones and Laptop battery.
Put it on a cart with wheels, add a pair of 4/0 Truck-sized Jumper Cables (and a 300A or 500A circuit breaker in case of an "Oopsie!"), and you can boost-charge and start almost anything from a Semi to a D-9 Dozer to a Liebherr mining dump-truck or a Locomotive.
Note that when the engines get really huge they are often 24V electrical systems. Or they have an Air-Starter, and you need a gas-engine air compressor on a cart to give them a jump-start.
The devil is in the details, and what initially seemed like a slam-dunk no-brainer has become quite problematic. I'm looking to replace my (stolen) BlackMax with a tri-fuel -- just bite the bullet, and go thru all the sound-proofing, plumbing, and electrics to the panel. And put a chain on it... I'm still going to fool around with a big-ish inverter, just to see what can be made of it.
A good fraction of the Sandy victims will not have power 'til Nov 11 or so.... holy shit.... And more salt-in-wound: Linemen from other states have arrived, to a massive gas shortage, and are essentially immobile.... it just never ends.
I'd like to see the rectifier and regulator for a 500 amp unit!
When we got the latest rig (E One built on a commercial chassis with no electrical upgrades!) I stuck a spare LN unit I had on it. The original dept was always complaining that the low voltage alarm would go off and they had to rev up the engine to bring it up (which is SOP in most departments anyway, even the big units don't put out enough at idle to power all the lights on the new rigs)
Only real problem I had was the crappy wiring that I replaced with heavy duty stuff. Just bugs me to see low grade crap on an emergency vehicle.
I just did a couple quick calc's and here's what I got. Assuming a regular setup first with the battery sitting at 12.5 volts, which is reasonably well charged, and with an internal resistance of 0.08 ohms. V=IR, doing the math gives you 25 amps if the alternator spins fast enough to put out 14.5 volts while it's charging the battery. That's based on the 2volt difference between the battery voltage and the alternator voltage. Add a second battery with the same state of charge at 12.5v and the parallel resistance value calc's to 0.04 and the math gives you 50 amps from the alternator.
If the two batteries were down to 12.0 volts, which is a fair discharge level, you get a final value for both batteries charging of
62.5 amps. If those assumptions are in the ballpark it doesn't seem to me that adding a second battery that's 50% discharged would be that big a deal IF charging the batteries was the only thing going on. If you are driving around and using 30 amps to run the car, stereo, AC system then you are pulling over 90 amps and I suspect most standard alternators don't like cranking out that much power for very long.
FWIW, CoorsTek, a major ceramics company, is a different company from Coors Brewing. They used to be under the same parent company and the ceramic business derived a large percentage of its income from support services for the brewery but they separated in 1986. I remember back in high school in the late '60s thinking it humorous that the crucibles we used in the chemistry lab were labelled "Coors".
The 1" Sony VTRs we used at Ch. 55 (WACX), a Christian TV station in Orlando, Fl. were bought from Coors marketing unit where they produced their commercials.
Good analysis, altho you have to be careful of what no load terminal voltage of a battery really "means". A fully charged battery, afaiu, quickly develops "surface charge", equal to the charging device, so theoretically relatively little current actually passes thru, despite Ohm's law. I'll have to measure this myself one day to make sure....
But at some discharge level, the above sounds about right. I solved/simplified this whole thing with a bit of manual labor: I just hump the marine batt/inverter inside the house, and charge it with a charger, about once a week. Saves a whole lot of mucky-mucking around with car wiring/hole drilling, but..... you just can't believe how HEAVY those marine batts are..... holy shit.....
One thing I"ve learned from these varied discussions is, There is no free lunch. Having the car alternator charge a second battery is neat and nifty, but not at the trouble/expense of a new alternator. So I don't really mind humping that heavy marine battery, at least once a week.
And, it seems these newer cars indeed draw a lot of juice. My buddy has a DC clamp-on ammeter (yup, DC), and holy shit, with nothing running but the engine, the current is 20+ amps, and can quickly jump up to 40. So a second battery, esp. a discharged one, could really add to the load.
They make these isolator switches for second batteries, but really, too much electrical drama for me.
Electrical load can greatly affect mpg's, afaict. Consider that cruising 60 mph for a small car can occur at a mere 10 hp. Adding 1 hp worth of electrical load can then reduce your mpg's by a full
10%, over old non-electronic cars. Technology giveth, and tech taketh away....
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