I have a simple question and would like a simple answer.

[injin man]

I know this is going to be tough for anyone to answer without the
temptation to turn into a question, but give it try.

The scenario is this: I have a 3500 watt inverter (Outback GVFX 3524)
and I want to be able to run it at  the full 3500 watt output @ 120 Vac.
(for those that don't know the code, it's a 24v 3500 watt inverter)

The question: How much Battery would it take to accomplish this.
You of course can use any combination of batteries you choose to achieve
the needed input.

[bschwartz]

2 12v deep cycle batteries would do the job

That was the quick answer.

The real question is how long do you want to be able to deliver the 3500 watts for?

[injin man]

2 12v deep cycle batteries would do the job

That was the quick answer.

The real question is how long do you want to be able to deliver the 3500 watts for?

10 hrs.

[Ronmar]

Watts are Watts regardless of the source, so for a 10 hour run@ 3500W you need 35,000watts of capacity, plus another 15% to account for conversion losses turing 24VDC into 120VAC.  Since battery capacity is typically measured in Amp Hours, lets calculate it that way.  3500W @ 120V is a 29amp AC current draw.  That same 3500W out of a 24V battery is 150A DC current draw, plus the 15% the inverter needs for the conversion, or about 172A draw from the batteries.  A 172AH bank would power this load for 1 hour in theory, but since 1 hour is a long ways from the 20hour period that the AH rating is based on, you will loose a bunch trying to take the current from the batteries that fast.  I would guess that a 200AH bank would be needed to power that 3500W 120VAC load for 1 hour.  With that said, a 2000AH bank should do it for 10 hours, or a bit more, as 10 hours is way closer to the 20hour AH rating which most batteries are rated at.  

Now I am sorry, but i am going to turn it into a question:  But you do not have to answer them here, they are more for you to ask yourself...

What do you need to power that needs 3500W for 10 hours?  Maybe I am weird, but my average house load is in the low hundreds, not the thousands.  But I also do not require A/C...  If I were running from inverters and battery, I would do everything in my power to get that average load into the teens instead of hundreds!  When making your own power, or powering something by battery, as far as load is concerned, less is more...

The next question is how many times do you want to do this?  Batteries only have so many full cycles in them before they fail.  The shallower the discharge, the greater number of cycles at the reduced rate the batteries can handle...  The larger the bank AH size, the longer life they have for a given load.

Hope this helped answer your question, good luck...

"2 12v deep cycle batteries would do the job"...  That was funny:)

[injin man]

It does appear that 2000 ah is the correct answer. While the questions always
lead to what it is you want to run for most folks it's a matter of how much
can I get for X. After much research into inverters that are transformer based
the 3500 watt is about the most appealing to handle a variety of loads. The next
issue is whether to use PV to charge the Bank. The Boat Store suggested 120%
for Inverter losses.

[Lloyd]

The big alligator, and I mean really BIG at this kind of amperage draw is the Pukert factor. So at that kind of draw for 10 hrs..I'm not so sure a 2000k bank is going to do it.

Plus You have to factor in the need not to draw the bank down past 50% SOC, bc if this was done on any kind of regular basis the bank would only live a couple of years at best. AND that's a lot of money.

To determine the PUK-FAC. you need to tell us what bats are being used to build the bank from.

A 2000 amp hr bank rated by the manufacture at  20 hr rate, based on a 1.3 Puk-Fac and 190 amp load.

A 2000 AmpHr bank at 190 amp draw has Calculated Battery Capacity = to 415Ah and will result in:

1. Full Discharge in in 2.181 Hours
2. Reach the manufacturer's recommended maximum of 80% Discharge Time in 1.745 Hours
3. Reach 75% Discharge in 1.636 Hours
4. Reach 50% Discharge Time 1.090 Hours
5. Reach 25% Discharge Time 0.545 Hours

To charge the bat back up to full at 75 amp charge rate is going to take just about 40 hrs from a full discharge, that is based on 50% eff. factor during charge. During bulk charge, the eff. will be near 80% but as it reaches asorption and finally into float the eff goes way down, so the average to full charge will be somewhere close to 50%.

All of these numbers are based on the average FLA, a VRSLA, will do a little better in all categories, maybe 25 % across the board. A true Gell-Cell will do even better, maybe 40% across the board.

A VRSLA will be about twice the cost of a FLA bank, and a Gell-Cell bank will be over 4 X's the costs.

Lloyd

[injin man]

Lloyd, I didn't really want to delve into the micro side of the battery debate
rather the reality and economy of what's not said in the hype surrounding
renewables and AE systems. I've been working out the imaginary versus the pratical
for a while now, mainly with 3 types of Batt's a large 12v AGM, a large commercial
12v wet cell and a very large 12v Gel Cell. Solar Charging is probably the best
for all intents and purposes but again the volume issue will rear it's ugly
head and cost will get in the way of doing it up right. So at this point I'd say
the loads are limited to Plugs, Lights and Refrigerator, no where near the
the output of the inverter. What drove my inquiry was a fellow at a place in Cali
that's having problems (his customers) aren't getting the goods on the delivery
end of the inverters he selling them. My guess is that the Batt Banks can't keep
up.

[Lloyd]

Lloyd, I didn't really want to delve into the micro side of the battery debate
rather the reality and economy of what's not said in the hype surrounding
renewables and AE systems. I've been working out the imaginary versus the pratical
for a while now, mainly with 3 types of Batt's a large 12v AGM, a large commercial
12v wet cell and a very large 12v Gel Cell. Solar Charging is probably the best
for all intents and purposes but again the volume issue will rear it's ugly
head and cost will get in the way of doing it up right. So at this point I'd say
the loads are limited to Plugs, Lights and Refrigerator, no where near the
the output of the inverter. What drove my inquiry was a fellow at a place in Cali
that's having problems (his customers) aren't getting the goods on the delivery
end of the inverters he selling them. My guess is that the Batt Banks can't keep
up.

Good answer,

BC if you really wanted to dicharge at that rate for 10 hrs, it would take a 20,000.0 AmpHr bank to reach the a battery saving 50% SOC.

Lloyd

[injin man]

Yes if you consider all the factors that say Nigel Calder throws at you,
it's easy to see how powering a complete house full of stuff would really
be expensive. The goal is to find the entry level system for a DIYer to get
his feet wet and not be disappointed with a mickey mouse system. There
are simply to many branches in this tree to get the total perspective but
for us to have a realistic idea of capacity generally speaking will go a long
way toward budgeting for your first foray into being secure with regard
to becoming self sufficient.