Question on amp draw from batteries

[Crispy]

Hello all !

I am in the design phase of a modest system with a 10' axial. This will be my first and I am approaching this as a hobby and will be happy with whatever I can harvest from the wind. No lofty expectations here. My strengths would lie with the fabrication aspects of the project and my biggest weakness would be on the electrical side.

Thus, my question which will probably seem very silly to most of the very knowledgeable people here but important for me to size my battery bank correctly to match my goals.

Ok, suppose I fire up the inverter and light up a 120 watt light for an hour. So that is 1 amp hour, 120 watts / 120 volts. Now my question is, from a 24v battery bank will I also be using only 1 amp hour, or 5 ( 120 watts / 24v = 5 amps )

I suspect the latter is the correct answer, but wow, that means if I put a 3000 watt load on the 120v side I'll be pulling 125 amps on the dc side...... just seems like a lot and perhaps if true a good argument for 48 volt and lower amps?

With a goal for battery power to provide 4kwh on the AC side and accounting for only half the rated capacity of the battery as useable, I would need a total rated capacity of approx close to 400 amp hours for a 24 v bank, correct? 4000 / 24v = 167 amps + 20% for inverter loss = 200 amps x 2 ( half  rated capacity) = 400 amps total rated capacity.

Please comment if I am on the right track here.  Thank you ,   Crispy

[ChrisOlson]

Yes, typically as a rule of thumb with a 24 volt system your inverters pull 5 DC amps for every 1 amp of AC load.  At full surge load they will pull more amps.  We have dual Xantrex SW+ 4024's series stacked for 120/240 volt split phase power and they pull 720 amps from the battery bank at full surge load (28.8 kW surge) for 11 seconds.

48 volt does not make one bit of difference, except for the wiring going to the inverter(s).  For the same battery bank wired either 12 volt, 24 or 48, every battery in the bank still has to deliver the same amount of amps for the same load.

Example: consider four 12 volt batteries with a 480 watt load:
- wire all the batteries parallel for a 12 volt system and every battery has to deliver 10 amps and you have 40 amps to the load
- wire the batteries series/parallel for a 24 volt system, and each battery has to deliver 10 amps and you have 20 amps to the load
- wire the batteries series for 48 volt, and each battery still has to deliver 10 amps and you have 10 amps going to the load

So the only consideration for various voltage systems is the size of the wire from the bus (or bank) to the inverter, which is usually very short and not a major deal, whether the system be 12, 24 or 48 volt.
--
Chris

[Crispy]

Thanks for the confirmation Chris. I take it that in your examples where in all 3 cases you say each battery delivers 10 amps that you consider series connected batteries as 1 battery? I never thought of it that way, but I can see it. No different than a single battery with it's individual cells, which could each be considered a battery, connected in series to make 1 battery.

Either way I can see it would be close to $3K to store half my daily 15kwh usage off the grid....... unless your lucky enough to know someone who sells good batteries cheap. What batteries have given you the best value and performance?

Thanks for taking the time to answer. This forum is awesome. The cumulative knowledge here blows my mind.

[ChrisOlson]

That is basically correct - four 12 volt batteries in series would be akin to a big 24 cell 48 volt battery.

Batteries are definitely not cheap for good ones.  When you compare off-grid living to having grid power, off-grid is much more expensive.  As an example, we have 24 Rolls-Surrette T12-250's, 2,400 ah.  Our bank is about 58 kW and cost us $9,600.  Our typically daily usage in our off-grid home is 22-24 kWh.  Assuming we get 10 years from our bank, the cost for batteries per year is about $960.  With grid power at 15 cents/kWh, that's 6,400 kWh of grid power per year, or 533 kWh/month.

We generate another ~1,400 kWh/year with the generator.  We run our generator about 400 hours per year for standby (low power days), plus load support (our inverters have the capability to sync with the generator and power heavy loads with both generator and inverter power).  Cost/kWh for generator power is about 50 cents, so that's another $700 a year, which would buy 4,800 kWh off the grid at the above price.

If you total the above numbers, you come up with 11,200 kWh/year, which is about what the average home uses in a year in the US for electricity (@ ~30 kWh/day) - JUST for what it costs us for batteries and generator fuel every year.

You have to have a different reason than saving money to live off-grid.
--
Chris

[Crispy]

You have to have a different reason than saving money to live off-grid

Yep, I see that! This is a hobby albeit an expensive one. I was the kid who wasn't satisfied with my store bought crystal radio, I had to make my own. My grandfather worked for Allis Chalmers as an electrical engineer back in the day and was on the design team for the turbines at Hoover dam. He infected me with the "bug" with all the cool gadgets in his workshop and gifts like erector sets etc.

How 2 magnets can attract or repel each other with such force still mystifies and sparks my curiosity at the ripe old age of 54 

Chris

[ChrisOlson]

OMG - your granddad worked for Allis?  In Milwaukee?  There has been so many changes over the years.  Allis-Chalmers was there since about 1860.  They bought everything in site - Monarch, Advance-Rumely, Buda Engines, and on and on.  They built Big Allis - a one terrawatt generator that I think went to New York.  And then the economy went down the tubes and they hit on hard times.  Going from Deutz-Allis to AGCO and in 1999 they closed the doors in Milwaukee - after almost 140 years.

Another sad story is McDormick-Deering, which became Farmall, International-Harvester, then got bought out by J.I. Case and became Case-IH, then merged with New Holland and became CNH (Case New Holland) - and finally today is owned by Fiat.

Two of the landmarks in the agricultural industry that stood for over 100 years, then went down the tubes and became foreign owned.

I got two tractors that I bought at auctions that are worth 10x more today than they cost new - a Allis-Chalmers CA with a wide front, and I got a Farmall C with a narrow front.  They are no longer useful for modern farming, but I still drive them around from time to time and use them to pull a wagon or something.
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Chris

[midwoud1]

Chris.

 You still have  John Deere and Caterpillar,Boeing, Halliburton. They will never be hijacked . I hope.

   - Frans -

[ChrisOlson]

Deere & Co. is basically the only one that has withstood the test of time in agriculture.  Caterpillar built the Challenger line of tracked tractors (I got Cat Challenger 45), and they also got involved with Claas in Germany and built the Cat Lexion combines.  The combines were basically a German Class Lexion with Cat power and drivetrain.

Caterpillar made history with the MT875B.  The Challenger MT875B is the largest production agricultural tractor ever built.  The MT875B broke the world record for the most land tilled in 24 hours on the least amount of diesel fuel burned/acre ever done, and the record still stands today after 20 years.



But then Caterpillar ended up selling that to AGCO-Allis.  Today AGCO builds the Challenger tractors and Challenger combines, which are still Lexion with Caterpillar power and a AGCO sticker.

Not to hijack the thread, but the two big ones - Allis-Chalmers (in West Allis) and Case-IH (in Racine) used to be Wisconsin, USA-based companies, and I've been to both factories over the years, when they were still running.  Case-IH is still headquartered in Racine.  But Case-IH is today owned by CNH Global which in turn is financially controlled by Fiat Industrial in Italy.  When I hear about somebody who used to work for Allis, that's like a piece of history from the good old days.
--
Chris

[Crispy]

Chris,

He actually worked for a company called S Morgan Smith in York PA which was then acquired by Allis. He retired in 1965.

I live 10 minutes from New Holland in Lancaster Co PA. I believe they may have changed hands again, now known as New Holland Agriculture. That company was started by a local guy welding up implements for local farmers in his garage and grew into quite a large company.

I remember reading a thread where someone mentioned about Ghurd hanging around the Amish...... has me wondering if he may be a neighbor