Inverters

So? Why not parallel-up the batteries at 12V? It's only a question of cable size....

12 Volts battery is no going to work on a inverter that is made for 120VDC input, Yes if you change inverter, and battery is made for that capacity yes it can last long.

But from the designers point of view higher is input voltage better is efficiency.

"What battery is going to last very long with 250 amps passing through it." A big one! 3 kW is a fair amount of power from a battery. For the length of time you want it to last you can work out the amp-hour rating needed.

Cheers.......Codey

all the others gave the general picture...if it were just one battery, the current would be excessive for sure, and the power dissipation was a consideration... P=R(I)^2

you could had more batteries or increase the voltage on them, so the inverter would not had to drain current, and you could still mix the solutions.

but for big powers there are not miracles (not that I know of...)

You need a couple of batteries connected in parallel to supply 3KW inverter that is capable of delivering 250 Amps. That is if your load is about approaching 3KW.

Remember, 3KW is the maximum rating of your inverter, you could still use it at lower loads with no problems with batteries as long as discharge current capacity of the paralleled batteries are proportional to whatever load your inverter carry.

It is possible a 12 volt block battery type was given - without adding, say, " Quantity 4, in series"! Battery cells are made up to 1000s of Amp hours, 250 amps for 10 hours is not a problem - with enough space and money!

<Shakes head>

You cannot get 120V out of a 12V battery no matter how "big" it is! You need 10 batteries in series connection to get 120VDC. (yes, I'm aware you technically could use a series for voltage doubler circuits, but that would cost more and fail more).

If you have batteries capable of 100 amp-hours each, and you want to deliver 250A (although I'm not sure where that number came from) for 2 hours (SWAG to keep it simple), you need 5 batteries in parallel to be capable of that.

So to get 250A at 120VDC for 2 hours from 12V 100 A-H batteries, you need 10 series sets of 5 batteries = 50 batteries.

But 120VDC will not get you a 240VAC output from an inverter! 120VDC will only net 84VAC from an inverter. To get 240VAC from an inverter, you need 340VDC (look up "inverter RMS factor"). That is 29 (call it 30 for fudge factor) 12VDC batteries in series. But before you go off calculating the SIZE of the batteries, determine how long you want the inverter to run without power input from elsewhere. So far if you do just one set of series batteries for 360VDC, your 3000W load will only need 8.33A, so a 100A-H set of batteries will net you approx. 12 hours of run time. You may be able to get away with smaller batteries if you don't need it to run that long.

Or you could use the 84VAC output and make a custom transformer to step up to 240VAC. I wouldn't, but you could.

Or go buy a UPS where all this is done for you and tested, and the battery charging circuit is designed and monitored etc, etc. Why reinvent something that already exists? You cannot possibly build it from scratch cheaper...