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Old 06-26-2009, 01:04 AM   #1
Join Date: Jun 2009
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Default DC pwr for AC

Good evening Gentlemen. I like a few others am a newbie Maxum Guy, 2400 SCR and love the boat. It gets better every day I take it out. We are Florida people and have just survived the latest tropical heat wave. Which raises the issue. We can only get AC on shore power. Nice but doesn't cut it. I know about the Whisper Honda gens but dont like the idea of tripping over the box, tying it down, storing it and coming to the marina to see it grew feet.

Not trying to re invent the wheel so has anyone tried the following:

Install a Hi Output, say 140 -170 Amp heavy duty Alternator. Mercruiser 3.0 with the puny little 55 or 65 amp stock unit doesn't cut it.

Run that into a deep cycle #2 batt or even a third batt, that feeds a 2000 - 2500 watt inverter. Wire the inverter output to the AC panel with a double pole (?) AC switch to select between shore and inverter power. Voila. AC while the motor is running and with a good batt system, a couple or three hours of AC on the DC inverter. (Talk about optimism) Any hints would be appreciated.

This is a great forum with lots of info and great to be aboard. I see there are a few former Naval Persons admitted by their lack of decorum.

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Old 06-26-2009, 01:48 AM   #2

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Default Re: DC pwr for AC

welcome to the zoo...I used to be from fla...grew up around P-cola and the east I know the type of heat you fire up the AC on an inverter would take a huge battery bank....that is to fire it off and keep it running for more than a few the only thing you can do is the shore power or a portable genny...I don't see any way around the math....ampsxvolts=watts...or amp hrs at so many watts will only last so long....then your gonna have to recharge that batt. bank up the best thing I can suggest is to get some huge 6vdc golf cart batt's and a monster inverter..either way I think it would be very cost prohibitive for a small 24ftr......

maybe someone else can chime in and offer some suggestions that I don't know about..

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Old 06-26-2009, 04:59 AM   #3
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Default Re: DC pwr for AC

SP is correct, anything longer than a few hours will require a battery bank changer and genny.
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Old 06-28-2009, 09:14 PM   #4
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Default Re: DC pwr for AC

Our 3000W Inverter takes 200 Amps at 12V to supply a 13 amp 240V Device

We have a bank of 4 110Ah batteries and it can soon flatten them plus a 65 and a 100 amp alternator connected to a charging controller to feed the juice back in, It does work but unless your going to have the engine running all the time it wouldn't really work

Do you know what current your AC takes when working?
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Old 07-01-2009, 06:12 PM   #5
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Default Re: DC pwr for AC

Thanks for the feedback. My calculations are based on a Marine Air 5000 or less BTU unit that purportedly draws 3.5 amps ac while running. Bumping the figure to 5 amps, that translates to 600 W or so. Figuring a 50% margin, a 1000 watt inverter would be the minimum. That translates into about 90 Amps DC (there is some loss so a 1500 W inverter would probably be required.)

So. If I use a 140 Amp alternator that should be enough alone to run the ac and boat voltages (Figuring about 25 Amps DC to run the boat, engine, instruments etc.) while the engine is running. I am postulating that most of the use would be underway with the engine turning and the Alt producing power.

Plug it up to 100 Amps draw and the batteries alone would require at least 300 Amp hour capacity to run the ac on batt for a couple of hours anyway. (Need a battery isolator in there somewhere) I think the hardware, Alternator, Inverter, Batteries etc are pretty much off the shelf items and the wiring would need some #2 AWG for the battery, inverter draw.

Just wondering if anyone has had the experience of trying this method just to see if it works.

Thanks for reading and any replies. JDF.
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Old 07-01-2009, 07:14 PM   #6
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Default Re: DC pwr for AC

That should work fine be sure to get a pure sine wave inverter with a large surge start up capacity
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Old 07-01-2009, 07:23 PM   #7

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Default Re: DC pwr for AC

I run my Marine Air 5k btu off a honda 2000 generator. It runs fine off the genny since it draws only about 5 amps (don't forget the water pump load) but it requires close to 20 amps to start up. My genny can't do that on eco mode. I wanted to find some kind of capacitor or something to help with the start load and found this document. I haven't tried anything yet. Xantrex's site has a few faq about running AC off an inverter.


Technical Note
512-0037-01-01 Rev 1
Operating Air Conditioners or
Refrigerator Compressors
We do not have specific model test data regarding compressors. However, it appears there is considerable
variation in start surge from model to model of air conditioner or fridge compressors. In our experience the
newer small rooftop-style air conditioners (5000–7000 BTU) used on RVs in some cases actually draw only 8
amps running (with moderate start surge), and can usually be operated by a PW1500. The older models may not
be as efficient, and may draw a much higher surge, causing even a PW1500 to trip into overload. The PW2500
inverter in most cases can operate air conditioning units up to 11 A/10,000 BTU. The DC wiring and battery
capacity must be optimal to provide the large start surge without starving the inverter of power during a surge.
A three-foot-high, bar-style fridge, which draws up to 2.5 A, in most cases can be run by a PROwatt 800. Full
height fridge rated from 2.5–5 A can usually be operated by the PW1500. The PW2500 would be more suitable
to operate compressors up to 5–8 A.
The compressor amp/watt rating does not tell the whole story. The compressor spec plate rating is usually the
max running draw. The start surge can range far higher; the rule of thumb is 5–10 times the spec plate rating. If
the compressor demands more surge than the inverter can provide, the inverter would go into overload and shut
down the AC output until the overload is removed, and the inverter reset (switch off, then on again). In a
borderline situation (i.e. inverter surge capacity is just short of compressor surge demand) the addition of an
inductor, or filter choke of 5 mHenry/10–30 amp rating may be sufficient to buffer the surge spikes and get the
compressor started. The inductor would be wired in series in the hot lead between inverter AC output and
load/compressor (see diagram below). The start surge typically lasts less than two seconds until the motor’s
magnetic field and motor impedance rises, which lowers the current demand from the inverter to a manageable
level for continuous running.

1998 2700 SCR
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