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Old 10-07-2012, 19:15   #31
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Re: 8 Amps from a 400w Inverter ?

One note. The inverter itself uses a little power to do the magic. So add 5 percent for the inverter on the DC side.... cig plugs on inverters should always be cut off and hard wired, generally with larger wire,then provided with it...to the DC buss with fuse...
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Old 10-07-2012, 19:32   #32
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Re: 8 Amps from a 400w Inverter ?

I have a 400 W interverter running off a 12 V outlet and it does the same thing. My MAcbook Pro pulls 5 to 6.5 A DC when it is charging. If it is the middle of the day, the laptop will usually charge with my Kyocera 130 W panel bumping the voltage way up, or with the alternator of course. At night, with the battery at 12.65 V or something, it will not work.

I suspected voltage drop or crappy connection, but after following this thread, it really does seem like it is very sensitive to voltage drop.

Soon installing a bigger inverter, heavy gauge wire and fuse, but it is a long run and I just haven't gotten around to the cabling.

Practically speaking, these inverters have real limits when run through a cigarette style outlet.
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Old 10-07-2012, 19:44   #33
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Re: 8 Amps from a 400w Inverter ?

A lot of questions and valid points have been brought up in regard to the power use and output of an inverter. Maybe a good time to summarize the two most significant.

1. Power losses due to inefficiencies. Basically no machine is perfect so there will be losses in the conversion of the DC to the AC. So the inverter may use 100 Watts DC to make 80 Watts of AC output.

2. Power factor. To avoid getting too technical, suffice it to say that some types of AC loads (like inductive loads) are inefficient so they will draw more power from the source than they actually use. So the inverter may be putting out 120 Watts to give 100 Watts to the load.

Also, inverters will put out short term peak power higher than their rating and most cheap inverters will not put out their full rating for long term.

Except in extreme cases, if you are just doing a rough calculation of the size inverter you need to power a few AC loads and how many DC amps you will need to feed it you can use the simple formula of Watts = Volts X Amps.
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Old 10-07-2012, 20:25   #34
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Re: 8 Amps from a 400w Inverter ?

Ok, so I have another question, how much real world benefit would I see by getting a dc-dc converter for the gadgets?

I probably need 9v and 20v and I think I'd like 5v and a nice regulated 12 that would not vary much with an engine start or a charger hooked up.

oh, and I'd like it to be 'marine' and like $30 too

Does such a thing(s) exist? And secondly, is it worthwhile vs just running 5-6 powerbricks off a 2000 watt inverter and maybe spending the $$ on another solar panel?

I gave up on looking for a 32" class TV that had an external power brick. I found *one* a westinghouse and it had a TERRIBLE picture.
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Old 10-07-2012, 20:50   #35
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Re: 8 Amps from a 400w Inverter ?

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Originally Posted by xymotic View Post
Ok, so I have another question, how much real world benefit would I see by getting a dc-dc converter for the gadgets?

I probably need 9v and 20v and I think I'd like 5v and a nice regulated 12 that would not vary much with an engine start or a charger hooked up.

oh, and I'd like it to be 'marine' and like $30 too

Does such a thing(s) exist? And secondly, is it worthwhile vs just running 5-6 powerbricks off a 2000 watt inverter and maybe spending the $$ on another solar panel?

I gave up on looking for a 32" class TV that had an external power brick. I found *one* a westinghouse and it had a TERRIBLE picture.

A lot of gadgets will have a DC charger available, like laptops with a car charger. If these are options for what you want to run then use them.
Generally more efficient that converting DC to AC with an inverter to plug in a charger that converts the AC back to DC to run or charge the gadget.

And you can find generic DC/DC converters for various voltages for the gadgets that don't have their own and also still come out better that using the inverter.
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Old 11-07-2012, 01:03   #36
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Re: 8 Amps from a 400w Inverter ?

I was going to mention the DC-DC converter that was last mentioned. You can find DC-DC for virtually any voltagges used by electronics. they are cheap (around $15-$20). Anything you have with a brick, including TVs, can be run directly on 12VDC with the proper DC-DC converter. On tvs, the brick is usually internal and it may be a bit hard to find the actual DC operating volatge. I found that the Sharp Aquos sets are 12vDC and now have everything on our boat running directly on 12VDC. Makes life a bunch easier.
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Old 11-07-2012, 01:41   #37
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Re: 8 amps from a 400w inverter?

Quote:
Originally Posted by ArtM View Post
Voltage drop is a function of the source supply (the battery or alternator), not of the line. If the battery does not drop the voltage, then the line will not drop the voltage either, at least not until it begins to fail.
.
ArtM this is very wrong and is going to confuse people.
Voltage drop in wire is very real and important in boats. It is what determines the size of wire needed for most applications.
There are tables in any good boat electronic book that list the voltage drop in cable for different distances and current. Most electrical installations on boats require these calculations to determine the minimium wire size.

The wiring for boat cigarette lighter plugs is too thin to support the high currents necessary for the inverter to supply 400w which as others have said is over 30A

In adittion there is voltage drop over any connecter or plug. This is harder to quantify, but it is significant for cigarette lighter plugs and sockets due the poor design.

The bottom line is you cannot get anything like 400w out of an inverter (or anything else) that is plugged into a 12v cigarette lighter plug. This is the cause of the OP problem.
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Old 11-07-2012, 04:24   #38
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Re: 8 Amps from a 400w Inverter ?

Some of us might benefit from a quick reading of “Ohm’s Law & Boats”

"Ohm's Law & Boats"
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Old 11-07-2012, 05:49   #39
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Re: 8 Amps from a 400w Inverter ?

Thanks Gord - enjoyed that-

csh
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Old 11-07-2012, 05:57   #40
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Re: 8 Amps from a 400w Inverter ?

Great article Gord.
I think an understanding of this would answer 90% of questions in "electrical batteries and solar".
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Old 11-07-2012, 08:46   #41
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Re: 8 amps from a 400w inverter?

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Originally Posted by noelex 77 View Post
ArtM this is very wrong and is going to confuse people.
Voltage drop in wire is very real and important in boats.The wiring for boat cigarette lighter plugs is too thin to support the high currents necessary for the inverter to supply 400w which as others have said is over 30A.
So your saying that the alarm on the inverter is from the supply side voltage dropping, and not from overloading on the demand side?

And this is why the OP cannot come close to utilizing his 400W capacity, yet the fuse never blows? Because the inverter detects the voltage drop and shuts down before reaching maximum current on the input line?

Further, are you suggesting that the voltage drop is related to the current demand? That the line can provide full voltage up to a certain current, then will drop below an acceptable voltage beyond that current?
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Old 11-07-2012, 09:08   #42
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Re: 8 amps from a 400w inverter?

Quote:
Originally Posted by ArtM View Post
So your saying that the alarm on the inverter is from the supply side voltage dropping, and not from overloading on the demand side?

And this is why the OP cannot come close to utilizing his 400W capacity, yet the fuse never blows? Because the inverter detects the voltage drop and shuts down before reaching maximum current on the input line?

Further, are you suggesting that the voltage drop is related to the current demand? That the line can provide full voltage up to a certain current, then will drop below an acceptable voltage beyond that current?
Yes, I also said this back in post #29. Gord posted a bilge pump example in his link of how voltage drop affects circuits as well.

No it doesn't stay constant then just does a step function drop. As you draw more current you get more voltage drop. Wire has resistance, there is always a voltage drop in it if there is current flow. The question is where does it become significant? For some non-critical circuits it is accepted practice to allow up to 10% drop. This could be enough drop for the inverter to turn off. It is a safety feature of the inverter to not damage batteries. It doesn't know what the true voltage of the battery is.

The cigarette plug on bigger inverters is only meant to be used when you're not going to use the whole capacity of the thing. As someone else posted, read the manual it tells you this.

John
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Old 11-07-2012, 09:11   #43
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Re: 8 amps from a 400w inverter?

Quote:
Originally Posted by ArtM View Post
So your saying that the alarm on the inverter is from the supply side voltage dropping, and not from overloading on the demand side?
Yes (usually no alarm the inverter will simply shut down when the input voltage drops too low)

Quote:
Originally Posted by ArtM View Post
And this is why the OP cannot come close to utilizing his 400W capacity, yet the fuse never blows? Because the inverter detects the voltage drop and shuts down before reaching maximum current on the input line?
Yes
Quote:
Originally Posted by ArtM View Post
Further, are you suggesting that the voltage drop is related to the current demand? That the line can provide full voltage up to a certain current, then will drop below an acceptable voltage beyond that current?
Yes, but not full voltage, there is always some voltage drop in the wire with current.
The voltage drop in the wire is roughly proportional to the current. When the voltage drop causes the input voltage to go below the minimum threshold for the inverter it will shut down. This voltage drop depends on the wire gauge, quality of the socket etc but typically a 120w output would be a maximum for an inverter wired via a cigarette lighter plug.
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Old 11-07-2012, 09:25   #44
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Re: 8 Amps from a 400w Inverter ?

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Originally Posted by GordMay View Post
Some of us might benefit from a quick reading of “Ohm’s Law & Boats”

"Ohm's Law & Boats"
That's a very good article, but I could not help but notice that you recommended some name brand products, specifically the adhesive lined shrink wrap connectors. I have found that this particular brand is outrageously overpriced. Putting it in a "boating" package and tripling the price does not make the product any better. I happened to buy some of these types of butt connectors yesterday at a local auto parts store. They were priced at 15.99/50 or $32/100. The cheapest I was able to price the "Marine" brand you mentioned was 89.99 and at WM they were 99.99. At the auto parts store they are sold under the name of Weatherproof connectors. I have compared them to the "Marine" brand and I can find absolutely no difference. When I placed them side by side I couldn't even tell which one was which. When they are shrunk down you get the same extrusion of adhesive out of the ends you do with the "marine" brand. Also Adhesive lined heat shrink tubing is available on the internet from a variety of sources for much less than what you pay at "Marine" stores. In fact a four foot piece will cost you less that three 3 inch pieces at the Marine store. The internet route may not be convenient if you need it now, but if you're stocking an electrical kit, it's probably worth the trouble.

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Old 11-07-2012, 09:44   #45
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Re: 8 Amps from a 400w Inverter ?

Quote:
Originally Posted by Captain Bill View Post
That's a very good article, but I could not help but notice that you recommended some name brand products, specifically the adhesive lined shrink wrap connectors. I have found that this particular brand is outrageously overpriced. Putting it in a "boating" package and tripling the price does not make the product any better. I happened to buy some of these types of butt connectors yesterday at a local auto parts store. They were priced at 15.99/50 or $32/100. The cheapest I was able to price the "Marine" brand you mentioned was 89.99 and at WM they were 99.99. At the auto parts store they are sold under the name of Weatherproof connectors. I have compared them to the "Marine" brand and I can find absolutely no difference. When I placed them side by side I couldn't even tell which one was which. When they are shrunk down you get the same extrusion of adhesive out of the ends you do with the "marine" brand. Also Adhesive lined heat shrink tubing is available on the internet from a variety of sources for much less than what you pay at "Marine" stores. In fact a four foot piece will cost you less that three 3 inch pieces at the Marine store. The internet route may not be convenient if you need it now, but if you're stocking an electrical kit, it's probably worth the trouble.
Several members have also recommended genuinedealz.com as a good, low cost source for high quality wire, crimp terminals and such. Have not had a chance to try them out but they are on my list.

Agree that there is a benefit of West and other marine stores giving the convenience of having something when you need it now. Sometimes it's worth it to pay $5 for a $2 part if you can finish your repair and move on. This has been really an issue for me since the nearest boat store is over 2 hours drive from the boat, and I live in FL.
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