Battery Backup Help Needed 2

[Amato]

Hi,

I have a circuit which needs a battery backup for 120 hours. The circuit on average draws about 10A and operates at 24 VDC. The main input power is at 460 VAC which is stepped down to 24 VDC using a power supply. That in turn connects to a UPS and then a battery bank. Can someone help in choosing a specific battery for this type of application?

Using standard batteries would require 3 closets full of batteries, I have looked into generators but my customer doesn't want to use a fossil fuel (even solar) as a backup option which eliminates then propane, liquid gas or diesel. I am looking for a more efficient way (in terms of space) if it exists.

Thank you.

Amato

 

[DRWeig]

Amato,

With no generator or other energy input source, you're looking at a really huge battery bank, as you said. That's almost 30 kWh of energy to store -- the batteries typically found backing up large solar systems, large sealed lead-acid types, are readily available up to about 1200 AH at 2 VDC, so you'd need 12 of those to just barely meet your goal -- an another 12 to be on the safe side. You'll need to design in some current limiting for charging purposes, too. Big room.

I say it's time to talk your client into a generator, maybe find him a source of grain-based ethanol so he won't be burning fossil fuels....

Just my opinion. Let's see what the others have to allow.

Good on ya,

Goober Dave

 

[Amato]

Hi Dave,

I have begun the process of trying to talk my client into something more space efficient because they simply don't have the room to store those batteries anyway. I appreciate you backing up my numbers in terms of size.

I am/was secretly hoping that there is a product out there that I might have missed that can perform those duties and not require the space that I am coming up with in my calcs.

Thanks guys for your comments.
Amato

 

[DRWeig]

You bet, Amato...

I guess I'd have to say that if there were a really good and compact method of storing energy like that, we'd have seen it in the electric cars by now.

You might look at Tesla motors -- their car battery arrays are based on Li-ion cells, and are pretty small for the application. Still the size of half a car though.


Goober Dave

 

[IRstuff]

3 closets? Sounds a bit much, but that depends on spare capacity, etc. On the face of it you need 1200 Ah at 24V, which would be 30 80 Ah 12V lead acid batteries. While solar is an option, unless you have guaranteed sunlight, you'd still need all the batteries.

What exactly is the available volume? Your customer may simply be smoking something other that herbal cigarettes. This may shed some light on your options:

http://en.wikipedia.org/wiki/File:Energy_density.svg

Main article:

http://en.wikipedia.org/wiki/Energy_density

On the face of it, a conventional gasoline generator might be the smallest volume choice. Your requirement is 104 MJ, so something like 12 liters, or 3 gallons of gasoline would provide the required energy and cover any losses the conversion.

TTFN

FAQ731-376

 

[itsmoked]

There is no question that a generator and a propane tank would be vastly smaller. Cost less and require little maintenance as compared to LA batteries.

It sounds like your client is being a numskull unless this is for a submarine, but then, solar would be a bit of a problem there too.

What are you trying to run at 24V and 10A?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

For large-scale energy storage it's hard to beat flooded cell lead-acid. 12x 1400AH 2V cells, weight roughly 2 tonnes, cost you just don't want to know.

Ask your client whether his environmental crusade extends as far as refining nearly two tonnes of lead and a few hundred litres of sulphuric acid!


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[DRWeig]

Good on ya, Scotty!

We often forget to delve completely into the total cost of energy alternatives.

Goober Dave

 

[Amato]

Everyone,

I just wanted to thank you all for your input. All of you had very valid points and I do appreciate them.

What I am running are a bunch of electronics - Touchscreen, dual PLCs, networking, etc. etc. All that added up to 10A.

With respect to my client - they didn't eliminate the fossil fuels/solar ideas because they are against it environmentally but because of the location of the product. Unfortunately, I cannot go into detail in regards to that.

Thank you all and I will report back to you on what we ended up using.
Amato

 

[IRstuff]

OK, how about a fuel cell:

http://www.heliocentris.com/en/customers/systems/products/fuel-cell-systems/360-1440-w-fc-42.html

http://fuelcell.com/index.asp?PageAction=VIEWPROD&ProdID=265

TTFN

FAQ731-376

 

[roydm]

I agree with IRStuff, a fuel cell. Not all cells require hydrogen, I have seen Methanol/Water used to power forklift trucks and another that uses two liquid tanks to store energy which is then returned by pumping the liquids back through the cells.
Roy

 

[djs]

The story with batteries is even worse. If you have 1200 AH of load, you will need at least 2400 AH of batteries. You never want to discharge below 40~50%. Also the max charge level readily available on most batteries is 90%. So calculate your bank on varying between 90 and 50% of charge.

 

[jimkirk]

Depending on the location, be sure to specify whatever technology you use for temperature, age, et cetera.

I don't know enough about fuel cells, but battery performance has a strong temperature and age function. Besides sizing your battery for minimum and maximum charge/discharge as others have pointed out, capacity over temperature range and age can vary widely.

The good news is 120 hours is a very slow discharge for batteries. Look up Peukert's Law if you're contemplating lead acid.

http://en.wikipedia.org/wiki/Peukert's_law

 

[peebee]

I'd pick AGM (absorptive glass mat) VRLA (valve-regulated lead-acid) batteries over the flooded cells if space is a concern. 30kWh doesn't sound that large to me coming from the data center & telecom world. C&D (

http://www.cdtechno.com)

is a major vendor of these batteries, there are others too. I'd consider getting a UPS with 120v output and then rectifying that down to 24vdc only because you may find better parts availability that way.