Invertor Output Voltage

Submitted: Tuesday, Jun 06, 2006 at 10:20
ThreadID: 34667 Views:3230 Replies:5 FollowUps:18
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Over the weekend i installed my 300w Dick Smith invertor into the back of the vehicle. It used to sit under my seat and plug extended to the cigarette outlet.

decided that i wanted to make it safer and more permanent so i ran some cable to the back and fixed the invertor in.

I get 12.4 VDC going into the invertor but only 175v coming out. It used to work fine but in ever tested it before so i dont know if this is normal. Does it need a load to increase output voltage?

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Reply By: Member - Troll 81 (QLD) - Tuesday, Jun 06, 2006 at 10:28

Tuesday, Jun 06, 2006 at 10:28
Hi Alan

You will need a pretty thick cable going to the back, what are you running and where is it coming from?
AnswerID: 176976

Follow Up By: Alan S (WA) - Tuesday, Jun 06, 2006 at 10:44

Tuesday, Jun 06, 2006 at 10:44
I ran 17 amp cable from the Aux battery, total distance is less than 3 meteres. BAttery voltage is 12.5V and at input into invertor it is 12.4 vdc so there is a little voltage drop.
FollowupID: 433000

Follow Up By: Member - Jeff M (WA) - Tuesday, Jun 06, 2006 at 11:09

Tuesday, Jun 06, 2006 at 11:09
What guage is the "17 amp" cable as that really means nothing. If you are getting 0.1v drop with no load on the inverter I would be concerned about that. My 450w Sinwave inverter was running off 4g cable (normal car battery cable) as it _Affordable_Storage_Drawers.aspx well over 40amps at full load. (I also got the cable for free which helps). Now I run a second 100amp/hour in the back as well so I have just hooked the battery and inverter up to the existing 4g so it runs almost direct off the 3rd battery now. ;-)

If you are only going to be using the inverter for small loads and/or when the vehicle is running the voltage drop may not matter too much, but for efficiancy it would be good to get rid of the voltage drop as much as possible.
FollowupID: 433003

Follow Up By: Member - Jeff M (WA) - Tuesday, Jun 06, 2006 at 11:13

Tuesday, Jun 06, 2006 at 11:13
Oh yeah the other thing to remeber is if your inverter has a "peak" load rating (which I'm sure it does) you need to account for that in your cable size. ie Mine has a peak rating of 1500w which is about 125amps. Obviously this will only be for a few seconds when you fire up somthing that requires a large load to start like a motor in a fridge or powertool or similar. You will find if you are running a laptop off it that they can have a resonable load at startup too. The batteries take full charge, the hard drive and DVD drive spin up the fluro in the backlight start etc etc etc. When I was running a Dick Smith 300w to run a laptop in another vehicle I had it hooked up to a ciggy plug and had to remove the battery to start it up then replace it once it was going, otherwise the cable couldn't supply enough amps to the inverter and it would just konk out.
FollowupID: 433006

Follow Up By: disco1942 - Tuesday, Jun 06, 2006 at 14:42

Tuesday, Jun 06, 2006 at 14:42
17A looks a strange figure for cable ratings – generally the ratings are a multiple of 5. IF the 17A figure was on the cable real – this is the highest current that can be drawn without the insulation melting or the wire fusing. It is not the current that can be drawn without significant voltage drop – this figure cannot be quoted on the packaging as voltage drop depends on the length of cable. For more on this subject see Site Link

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FollowupID: 433044

Follow Up By: geocacher (djcache) - Wednesday, Jun 07, 2006 at 23:08

Wednesday, Jun 07, 2006 at 23:08
Careful about applying input and output wattages and voltages in the same sentences. How long do you think the fuse on the input (which won't be more than 20 or 30A) is going to let it draw the suggested 125A for? The 1500W peak is referring to an output rating. Not the input. If it was even getting close to that for an instant the current draw would blow the cigarette lighter fuse rather than allow the inverter to "konk out".

I = 1500W / 240v = 6.25 Amps output.

To get that in a process that isn't 100% efficient - and may not be even 80% efficient will require maybe as much as 9 or 10Amps continuous input.

P = IV = 12.5 * 10 = 125W

It's probably not a true sine wave inverter, so a multimeter on AC volts isn't necessarily going to report the same voltage you expect.

A 240v AC output from the wall socket at home is made up of a sine wave that is peaking at 339v. Because it is a sine wave the multimeter reads the RMS value of 240v.

RMS stands for Root-Mean-Square, RMS is a standard method for determining
the effective value of a varying or alternating voltage. The RMS value
is the DC voltage that would produce the same amount of heat as the
voltage you are measuring, if you were to apply both voltage across
identical resistors.

The value produced by a square wave that is 50% duty cycle and peaks at 340v if measured as RMS will be about half that at 170v.

You are probably chasing a problem that doesn't exist, but is rather a function of the fact that it's a cheap square wave inverter.

If it drives what you want it to drive, don't worry about it.

FollowupID: 433393

Follow Up By: Member - Jeff M (WA) - Thursday, Jun 08, 2006 at 00:01

Thursday, Jun 08, 2006 at 00:01
I run my inverter through a 70amp circut breaker. Watt's is watts isn't it? I mean if you want 1500w or 10 amp at 240v out then you have to put that in @ 12v. 1500w @ 12v is 125 amps.
FollowupID: 433410

Follow Up By: Member - Jeff M (WA) - Thursday, Jun 08, 2006 at 00:20

Thursday, Jun 08, 2006 at 00:20
Oh yeah, the Dick smith 300watter in question is a square wave, it's not even modified sinewave. The 1500w peak unit I was talking about is a pure sinewave.
FollowupID: 433416

Follow Up By: Alan S (WA) - Thursday, Jun 08, 2006 at 08:42

Thursday, Jun 08, 2006 at 08:42

In regards to your laptop i would have thought starting it up without the battery would put more load on the invertor, as if the battery is in this would take the load with the invertor just nneding to chare the battery?

The wire i run is through a 30 amp fuse. And the system is just for charging small nbatteriers etc, cammera, toothbrush, kids dvd player etc. My laptop _Affordable_Storage_Drawers.aspx at a max 4.9amp @ 18.5 volt which is well within the invertor and cable capacity.

Although on this i am assuming that the 300w is the output rating @ 240v which is a low amperage draw, and not 300w at 12volt which is extremely high amperage.

FollowupID: 433440

Follow Up By: Member - Jeff M (WA) - Thursday, Jun 08, 2006 at 09:37

Thursday, Jun 08, 2006 at 09:37
"In regards to your laptop i would have thought starting it up without the battery would put more load on the invertor"

Nah mate, some laptops may be different, however this particular one was an older one called an "emerald aci" that I was using purley for ozi.

It actually had 4 cables. - and + for charge, and - and + for mains power. When you powered it up off mains power the laptop switched over to mains AND used the power supply to charge the battery independantly. The laptop didn't actually charge the battery at all, the power supply had the charger built into it. You've got to remeber that laptops do not use Led Acid batteries and that LiIon and all these new polima batteries probally can't be charged while being used like a car battery can. Maybe some can be charged in use, but in order to get a full charge the basic charging circut in the lappy needs to know the unloaded voltage of the battery to estimate a state of charge. Also heat would be an issue, in these new polima batteries heat is the major problem, ergo running a load and charging them at the same time on a "fast charge" cycle may not be desired by the manufactuers.

On my old work NEC that I used to use in the car for watching DVD's for the kids, that was similar. It would draw about 3 amps with the battery out (as it wasn't being charged) and nearly 5 amps with the battery in. If you're at camp trying to consever battery power, that's a fair bit of difference over the length of a movie.
FollowupID: 433450

Follow Up By: geocacher (djcache) - Thursday, Jun 08, 2006 at 09:53

Thursday, Jun 08, 2006 at 09:53
Jeff M

As you state, watts is watts but I wasn't working it out for full rated current. Just what was going to be required to run a 150W laptop off his 300W inverter.

To get 1500w out at 240v from your inverter will require more than 1500W at the 12v input. Efficiency losses will dictate that.

FollowupID: 433455

Follow Up By: Member - Jeff M (WA) - Thursday, Jun 08, 2006 at 09:57

Thursday, Jun 08, 2006 at 09:57
Yeah sorry, geocacher , must have miss read your post. Yeah your absolutally right.
FollowupID: 433458

Reply By: hl - Tuesday, Jun 06, 2006 at 10:57

Tuesday, Jun 06, 2006 at 10:57

If that is a modified sinewave inverter, the voltage you read is correct.
Just plug in a 60 or 100W watt light, and you will see it runs at full brightness.
Most meters only read correct when the AC voltage is a pure sinewave.

AnswerID: 176983

Follow Up By: Alan S (WA) - Tuesday, Jun 06, 2006 at 11:00

Tuesday, Jun 06, 2006 at 11:00
Thanks, yes it is a modified sine wave. So it appears it is okay

Thanks for your help

FollowupID: 433001

Reply By: Redeye - Tuesday, Jun 06, 2006 at 12:15

Tuesday, Jun 06, 2006 at 12:15

It is difficult to accurately measure AC voltages unless you have a true RMS meter and a pure sine wave. Most AC meters measure the peak voltage and multiply this by 0.707. This is very inaccurate in most instances but more so with modified sine wave.

The only method to measure the voltage would to to use an oscilloscope or meter with a visual trace.

If it works OK on whatever requires the 240V than all is OK

AnswerID: 177007

Reply By: Mazdan - Wednesday, Jun 07, 2006 at 11:54

Wednesday, Jun 07, 2006 at 11:54
Alan, I suspect the cable you have used to proivide the power to the inverter is not capable of handling the max demand.

Your inverter is rated to 300w and as it is a 12V system that is going to be running it you need to divide 300 by 12 which comes out to 25...........that is the max load in terms of Amps that will be drawn through that cable, hence you need a cable that is capable of handling at least 25 A...........also ensure you have a fuse at the battery end that is capable of protecting that cable..........hence if the cable is rated to say 25A then a 25 A fuse should be used.

Yes the inverter is designed to spike on start up of some equipment but this is only for a few seconds if that and wont cause any problems to the cable or the fuse.

Having said that, this wont change what the output is in terms of volts........that has been adequately covered above by other posters...........but it will ensure you are not cooking your cables.
AnswerID: 177199

Follow Up By: Member - Jeff M (WA) - Wednesday, Jun 07, 2006 at 15:03

Wednesday, Jun 07, 2006 at 15:03
Mazdan, the peak load can and will still cause a problem if you are only using cable rated to the 300w RMS load.

1. As mentioned above, the "rating" on the calbe is only an indication of when that particular cable will catch on fire at, it is not an RMS rating.

2. If you try and draw 100amps through a 25amp rated cable you will suffer an extreme voltage drop. Yes, if it is only a few seconds it will not "catch on fire" nor will it blow the fuse (probally), however the voltage drop will activate the LVC (low voltage cut out) built into most inverters (yes the Dicksmith 300w has one, as I have the idendical unit in the garage as a spare) and will consequently cut out.

1. Turn on device.
2. Device Spikes a massive load from inverter.
3. Cable is crap and can't handle the load, hence causes massive voltage drop.
4. Inverter goes "BEEEEEP" and trips out, thinking that the battery is flat because of the voltage drop.
5. You scratch your head and think "wow, that's weird the cable is rated to 25amps".
6. You get some decent cable (like has been suggested in the first place) and rewire it.

FollowupID: 433219

Follow Up By: Mazdan - Wednesday, Jun 07, 2006 at 16:24

Wednesday, Jun 07, 2006 at 16:24
1. I did suggest at least 25A capability...........but even if using a cable that is suitable for a 25A max demand, its not going to catch on fire at that point........or maybe you were referring to something else???

2. Jeff I think you will find that the max spike is something like 500W for the inverter he is talking about.........mine is a 500W inverter and will only spike to 800W.........hardly 100A. However your point is still valid but should be overcome with a bigger cable as i suggested...........all I have done is try to point him in the right direction as nobdoy else was able to give him any guidance as to what he needed.

Personally for my 500W inverter I use a 16mm sdi which easily handles anything thrown at it with a 50A fuse.

I would suggest the OP drops in to an auto store and asks for a cable that can handle 32A.........this may be called 4 guage in auto terms but Im not 100% on that......

I was having doubts about the likelihood of a voltage drop while using a cable capable of handling 25A even under spike and decided to check the book that came with my inverter. Jeff there is another protection device on my inverter that is more likely to be the culprit in a spike situation and that is whats called the Overload protection.........This means that large spikes will cause the inverter to turn off. Sometimes items with large inductive loads such as televisions can be used with an inverter by repeated on off switching of the inverter.

Hope all that helps.
FollowupID: 433235

Follow Up By: Member - Jeff M (WA) - Wednesday, Jun 07, 2006 at 16:32

Wednesday, Jun 07, 2006 at 16:32
No it's not the overload protection that is at fault, as I could not turn on a laptop with two thinner wire hooked up through the ciggy lighter. The laptop draw a little over 4 amps or around 50w. Change the wire, and WHAMO - worked perfectly.

My 450w Sinewave has a peak of 1450w... Well over 100amps.

I just went and got the POS dicksmith 300 watter out of the shed and it says on the cover:

High grade Thermal Protection - (woohoo, WTF)
Overload Protection
Battery Low Shutdown - (this is what I've been talking about)
Peak Power Surge Capable - (great, that tells us a lot).

Ok, so I was probally exagerating a little on the catching on fire part. But if you ran 25amp cable at 25amps for along period of time in a confined space (under the dash) I would put it past it....

FollowupID: 433237

Follow Up By: Mazdan - Wednesday, Jun 07, 2006 at 16:43

Wednesday, Jun 07, 2006 at 16:43
I didnt realise that the sinewaves were capable of such high spikes..............thats excellent.

I am actuallyu about to buy a second one to put in the bike trailer as I dont always use my fourbee to tow it but I cant really see the point in anything other than the basic el cheapo 500W........I only need it for charging batteries and running a few lights.........if I was running TVs etc then I would think again.
FollowupID: 433243

Follow Up By: Member - Jeff M (WA) - Wednesday, Jun 07, 2006 at 16:56

Wednesday, Jun 07, 2006 at 16:56
"I didnt realise that the sinewaves were capable of such high spikes..............thats excellent. "

They don't sell mine anymore, but this is the same brand, only a 250w and a modified sinewave not pure, however you can see the percentages common to this brand for surge/spike:

Site Link
FollowupID: 433246

Reply By: geocacher (djcache) - Thursday, Jun 08, 2006 at 10:01

Thursday, Jun 08, 2006 at 10:01
I've posted on this topic before, but I'll repeat it here because it's relevant.

You are basically throwing away perfectly good amp/hours by using an inverter to run your laptop.

The laptop power supply is converting 240v to 12v - 18v (for most laptops - mine is 15v) using a switchmode power supply. Anyone who's picked theirs up after it's been running a while will have noticed the heat generated.

The inverter is converting 12v to 240v to run the 240v to 12 - 18 v converter that powers the laptop.

Both devices generate heat. Both do this because the processes used are inefficient.

You can buy for between $70-200+ dedicated power supplies designed to run a laptop off 12v. The one I bought was $89 from Jaycar.

You use far less current this way (single conversion DC-DC) over the hours the laptop is on - therefore conserving battery life or allowing you to run the fridge a little colder to keep the beer and bundy happy.

And for the cost of the $89 laptop power supply you still have change from what you were going to spend on a crap (non sinewave) inverter to fill the fridge.

Oh and one is recent technology, and the other (if you've ever opened a cheaper DSE inverter you'll know what I mean) came off the chinese equivalent of the ark.

AnswerID: 177406

Follow Up By: Alan S (WA) - Thursday, Jun 08, 2006 at 10:35

Thursday, Jun 08, 2006 at 10:35
Totally agree, that was my first approach, however my laptop has a non standard input power plug.

The only option i have is to use a 240v invertor and plug the normal powersupply into it.

I did check with maniufacturer regarding warranty issues and other problems using a non sine wave inverteor and they have put it in writing for me that there is no issues with their product.

I dont intend to run it all the time and as we stay about half the time where 240v is available it will only get intermittent use.

FollowupID: 433469

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