Electrical Math question

[tb-av]

Here are the pieces and ratings. Also my measurements. Trying to figure what I am doing wrong on the math.


300 Watt Sine Wave Inverter
10-16vdc input range
120V - 300W output.


Cordless Tool Battery Chargers
1-85W
1-95W
1-50W

When I plug them into the house power I measure just about 1 Amp draw from each.

My math... 120V - 300W = 2.5Amp

So I was thinking if I plug the Inverter to 12V Batt. Which I did. Not being charged, just a free standing Batt.. I figured if I plugged two of those into the inverter I would be drawing near 2 Amps.

However, when I plugged just the 85W rated one in and put a battery on charge it only drew about 5.5Amps DC after a surge to about 7Amps DC. So it seems I could run two of those chargers well under 20 Amps from the battery.

What I wanted to figure out was circuit draw, wire size, fuse size, etc..

My problem is I'm doing something wrong DC Math side.

If I go to a calculator and use 12V - 85 Watts = 7 Amps, that makes sense, but that's for the battery charger. Does the inverter simply pass the wattage through? It is not marked for wattage draw.

It seems like to me Wattage from the Batt. Should be Inverter+Charger
I measured the Inverter ON at 0.2Amps DC so maybe it's just insignificant?

IIRC the Blue Sea fuse panels will take 30Amp fuses so it seems like this Inverter with maybe three chargers would still be plenty safe. I'll probably run two and then some lower power chargers for AA, Camera batts., a Laptop feed as well hopefully, etc..

So if my testing and reasoning is correct or close. It seems like I can build charging station off that 300W Inverter and run it off one 30Amp Blue Sea circuit with the correct sized wire for distance which will be very short.

Does anyone see anything wrong or dangerous with this?

 

[TowMeOut]

Inverters are not 100% efficient. That's why they get hot. If you're lucky a "good" one might be in the range of 95% but the cheap ones are more like 75-85% or so.

That means that your 300W inverter (output) will put out that amout of power but draw quite a bit more. So....your calculations 300W = 120V x 2.5A on the output side are correct. However, in order to generate that amount of power you're looking at 300W = 12V x 25A. Once you factor in this efficiency loss youre' gonna be right at 30A draw.

You should be OK but the best thing would be to get chargers made to use a 12VDC input if possible. Starting with 12VDC, converting it up to 120VAC, then back down to some charging voltage is a lot of conversion and you lose a lot with each process.

 

[Prommy]

You might get current spikes if you switch everything on at once. If you're operating near the fuse's threshold, best to power on items one at a time.

Also, be wary that some cheap inverters do not always output clean sine waves, and some electronics do not take kindly to it. Unfortunately, the best way to determine the quality of the AC output is an oscilloscope, which is not something most have readily available. Perhaps someone has done the work on this issue for your brand on the internet.

 

[fourwd1]

Moving to OT.

 

[tb-av]

Guys, I'm sorry. I made a reply to this and apparently it did not go though.

I wanted to thank you for your input.

I do have scope and will scope the Sine wave. I paid for a sine wave over 10 years ago so I hope that's what I got. Never thought to check it.

I'm going to hook everything up to a stand alone Batt and put the amp meter on it then switch in all my chargers.

Again, sorry, I thought I made a rather detail reply a couple days ago. I do appreciate your input. Several things I had not thought about.
[MENTION=1857]fourwd1[/MENTION] - didn't mean to cause you undo work.

 

[fourwd1]

[MENTION=1857]fourwd1[/MENTION] - didn't mean to cause you undo work.

not a problem