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Old 03-07-2011, 02:18 PM   #15
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So do you recall what type of AC unit was used?

It may be similar in compressor size, so at least it has been done before.

I will make measurements and start experimenting to see what happens.

Tony & Lori
1989 Country Coach Savannah SE
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Old 03-07-2011, 03:49 PM   #16
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The recommendation to not go below 50% State-of-Charge on the Batteries had me wondering if it would be addressed, but it was finally noted in this Thread. AGMs can have more than the usual recommended 10% of total AH current slammed into them while charging vs. 'ordinary' Technology Batteries. Both these observations, unoriginal on my part, are readily out on the 'Net from RV and Solar folks, and from Battery Manufacturers.

Expedition folks who go out to the middle of nowhere in specialty-built Rigs sometimes charge at high Amp rates. One noted that it was necessary to put an Aux Fan on his otherwise-capable Alternator to not have it overheat under high load output. Especially at idle; Stoplights [in Urban areas]; etc..

I'm a clean [buff, then Isopropyl Alcohol], crimp, Silver solder w/Flux, and heat shrink kinda guy. IF you go with such Hi Rel connections, you can parallel 2, more-flexible Wires to handle high current. 2 - #6 = 1 #2, in round numbers, on the AWG Wire Resistance Charts readily on line. Those absolute value Wires aren't recommended; I'm just illustrating a principle I've used in Industrial Wiring. I use my Plumbing Torch on 'low' at my Bench. Clamping on Vise Grips is a good way to heat shrink Cable and not have heat conduct back up the Cable to melt Insulation.

For such high currents, I would fuse at both ends of the low voltage runs. Standard practice. Shorts can happen anywhere along the wire run, thanks to Mr. Murphy and his Law.

A 'soft start' Capacitor Kit can moderate inrush current demands of the A/C and make life easier for whatever size Inverter you choose after research. One fellow runs his Tools in his Fields off his Truck Motor via his 3 kW Inverter. Smart move. Good Inverters average 10% energy conversion loss. This is a fine point, but perhaps you need to design your System like this fellow running his Tools directly off the Alternator. The Battery Bank is kind of a 'energy flywheel' that is incidental to AC operation. So, with any Alternator I know of - save the Dual Alternator setup some folks use - you'd want to start the drive with fully charged Batteries. You'll be using the high output Alternator directly through the oversized Inverter, like the fellow using his Tools in his Fields. His Inverter is under the Hood somewhere. This upconverts Voltage immediately to 115 VAC; easy to run and manage. This entire idea is 'pay me now' [charge Batteries and pay a higher Utility Bill], or 'pay me later' [higher Gasoline usage with the 2nd idea]. Ain't no Free Lunch here anywhere...

BTW, the REAL high output DC Geeks are the ones installing VERY high power Stereos for these over-the-top Competitions they have. They REALLY know high output DC Systems, so look them up. I picked up a Mag on this whole scene and laughed myself silly. Some Stereos 'won' in competition by hitting 135 db while folks held the Car Windows in place to keep them from exploding out. Whatta Country!

I've seen 400 Amp Battery Switches around, so use 'big stuff' like that for your switching, obviously. Me, I would also run the finished setup at Home and 'feel' turned-off low voltage connections and wiring after ~1 hour run period or so. Summertime will raise temps, and the under-Dinette 'stuff' concerns me a bit...

To me, thinking through ordinary and unlikely failure modes is critical with the currents you're managing. I don't have an opinion either way on idea feasibility, other than a great idea that kills you or burns your Rig is not real 'green' either!

'Thermal runaway', where a connection heats, corrodes a tiny bit, introduces resistance causing more heat, is always a concern. So, with non-conducting 'Scotchbrite' cleaning Pad [my fav] and connection-enhancing 'gel', you'll keep the odds of success on your side vs. some 'Wily Coyote' scenario.

I can't speak for the Sine Wave quality issue - an important one - other than to say the true 'power factor' of any Sine Wave has to do with integrity of that Sine Wave. 'Partial', or 'stepped' Sine Waves, have less, or 'partial', power factor, proportional to how 'much' of the Sine Wave 'isn't there' at the Inverter Output. As with the Microwave experience above, you almost have to try the actual final setup because it's tough to calculate accurately. Just factor upwards on the current 'fudge factor' - especially on Inverter sizing req'd - and give it a go.

"In theory, theory and practice are the same. In practice, they are not." — Albert Einstein

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Old 03-07-2011, 09:14 PM   #17
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Do those agm's vent gas like a lead acid? I would be very concerned putting that much capacity for heat and chemicals in my living quarters. While I was trucking I installed an inverter to tun a microwave and the wires and inverter got very hot. With 3 large 12vdc batts I still had to run the engine to keep up.
I think you would need very good ventilation in that compment. And again I wonder if those batteries are approved for use in living areas.
As for a use why not use them for trolling motor batts on your bass boat?
2004 Dolphin Basement AC
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Old 03-07-2011, 09:58 PM   #18
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AGM Batteries

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RV Stuff

'Gel batteries and AGMs do have caps, but don't ever try to remove them. First, you'll violate the warranty. Second, you'll contaminate the inside. When it dies early, the dealer/factory will know you did this and will void the warranty. Also, if you overcharge a gel or AGM battery, the factory can detect that, too. Again, no more warranty. Gel and AGM batteries are expensive. Retailing at about $300 each in the Group 27 size, they are available for less than $175 plus freight from many alternative energy dealers. The best info source is SKP Noel Kirkby at "RV Solar Electric". He can refer you to a good dealer (RV Solar Electric Store). Check also with Back Woods Solar (Backwoods Solar Electric Systems) and West Marine (West Marine: Home Page). No matter what price, these are not $55 pieces of RV junk you get at a discount store and return for almost full credit because you screwed up.

So why bother with them? If they're expensive and so sensitive? Because they do a superior job in some cases. Venting to outside air isn't needed. (They can vent gas and fluid if you seriously overcharge them or do something catastrophically stupid.) Moot point, because you will take pains to never overcharge them anyway (won't you?). You can keep them inside the RV (or in a basement compartment). They stay clean and need very little service (other than a spray'nwipe with household cleaner and an occasional check for tight connections) and, more importantly, will be temperature consistent (more later). Piling a bunch of batteries on the tongue of a trailer or directly behind the grill of a Class "A" motor home is not a good idea. you just overweigh the front of the thing and end up with a wallowing vehicle. The back bumper is also bad for the same reasons, plus more, as you overweigh and flex the frame and create leaks (if you don't break the frame, hitch or the vehicle skin). These battery mounting schemes also lead to ultra-hot and ultra-cold temperatures'.
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Old 03-07-2011, 10:03 PM   #19
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There are AGM, then there are VRLA

There are many different types of battery construction.

The batteries here are telco grade VRLA, they are approved for use indoors, they will gass off if they are over charged, thus the rate of charge must be controlled.

I cannot speak to other batteries that some folks mention, just to the ones that I have.

The design here is to use trickle charge to bring up in preperation for trip, and have limited charge if higher rate of charge is to be applied.

The spec for these batteries is in the range of 1C for short periods, of a few minutes (C equals the labeled capacity of the battery), the reccomended rate is C/5, so with 70 amp hr battery the rate of charge can be up to 14 amps, with 8 in parallel the total is 112 amps before the batteries are at any risk of damage.

So the charger for short term could be direct connection to the converter or to the alternator, then while stored they can be floated at 1% C or 5.6 amp maximum, so battery maintainer will do the job with no worry.

Sorry, no bass boat...the Bayliner is V8 power, no trolling motor.

Tony & Lori
1989 Country Coach Savannah SE
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