12W / 12V = 1A

Continuous = 24hrs/day x 4days x 1A = 48amp-hrs.

Depending on how many cycles you want the battery(s) to last before they are destroyed, if infrequently, you could get by with 50% draw-down.

So you need 96Ahrs of 12V battery.  That's about 60lbs.
http://www.trojanbattery.com/Products/27TMX12V.aspx

That would be the most efficient result.

If you use a UPS it would need to have at least twice this capacity due to it's own added consumption and double conversion losses.

I'm not quite sure how having a monstrous UPS is going to be better than having a less large battery and a small wall charger.  You probably couldn't even find a UPS with the utilization profile you need.

What is this, maybe we can suggest other solutions.
 

Keith Cress
kcress - http://www.flaminsystems.com

Isn't 24 hours * 4days & 1A = 96 amp-hours?  If so, then if I limit it to 3 days instead of 4, then the 105 AH battery you linked would result in 69% draw-down.  Since this is for unexpected emergencies, would such a battery survive, say, three such discharge cycles?

When you say a "small wall charger", are you referring to something like this?

http://www.pplmotorhomes.com/parts/rv-converters/rv-battery-charger.htm

And if the battery is being charged, can my embedded device be powered from the battery simultaneously?

My custom embedded device is a black box with supersonic (10-300 kHz) multi-channel audio amplifier, A/Ds, and small Linux embedded computer.  It needs to listen continuously for events that occur during shipping and storage; i.e., the primary locations are warehouse and tractor-trailer.
 

I was just testing your math..
 
I was writing 24 then used 12 in my head.

Yes, if you use a flooded battery it will last probably 15 to 50 of these 69% draw downs.  The key will be how soon it gets recharged.

If you did that to gel cells they could last substantially fewer cycles.  And if you actually flatten a gel cell one cycle would be likely.

A flattened flooded might last 2 or 3 but would possibly have a serious reduction in capacity; like turn into a 50AH.

As for a charger.. Ah yes the charge and run gambit. To be honest I have never seen it work very well.  Batteries need a certain regime to be long lived. You have precharge, float, bulk, and cutoff areas.  It is very tough to have a charger make sense of a battery's state of charge when there is a load hooked to it.

When I do this I set up a system that has just a small charger with full cutoff capability that keeps the battery charged. I also have a small power supply that can provide the 1A at 12V.  Something like a cord wart or a wall wart.  

To this I add a relay that is held closed by shore power. The N.O. contacts, (presently closed because AC is available) runs your load via the cord wart.

When AC is lost the relay drops out and the N.C. contacts hook your load to the battery.  A charger keeps the battery cut-off so you have no boil off issues.  The charger has a clear mission it understands and keeps the battery perfectly topped up.

Add a fuse between the battery and everything else as the available fault current is stupendous.


Charger; I use these all the time, they're very good.  They also come with a bolt-on plug swappable fused connector.  You can leave a battery on them indefinitely and they take good care of it.
http://batterytender.com/product_info.php?products_id=4

Cord wart: http://www.mouser.com/Search/ProductDetail.aspx?qs=sGAEpiMZZMtpkqKkT5w3unIPqQ29rfWGA3onOrf0FVw%3d


Relay:  Whateveh SPDT 120VAC coil. Socket?


 

Keith Cress
kcress - http://www.flaminsystems.com

Laptops are designed to run and charge at the same time.  It's really just question of how the functions are partitioned.  All that's required is that the load can be powered directly from the wallwart, instead of through the charger.  This obviously requires a more complex design, but it's routinely done for lots of applications.

Unfortunately, there are lots of other applications where cheap is the order of the day, so if your battery is tango-uniform, you can't even run your shaver directly off wall current.

TTFN

FAQ731-376: Eng-Tips.com Forum Policies

Yes it is done all the time but those are generally engineered systems where the simultaneous load is well characterized and often there is some intelligence in the loop.  Laptops are a good example of that.

A lot of laptops run the charging in the laptop essentially doing what I described above, powering the laptop independently from battery charging.  They use semiconductors instead of relays.

Keith Cress
kcress - http://www.flaminsystems.com

For my 100 hours of 12v/12w, I'm trying to decide between an off-the-shelf solution, and a mostly-off-the-shelf solution.

Option 1:
The off-the-shelf solution was suggested by the "UPS Selector" at apc.com
http://www.apc.com/tools/ups_selector/index.cfm

1. APC Smart-UPS XL 2200VA 120V Tower/Rack Convertible
$1150, 137 lbs
http://www.apc.com/resource/include/techspec_index.cfm?base_sku=SUA2200XL&fnl=4606,2&fnl_basket=4606,3c

plus

2. APC Smart-UPS Ultra Battery Pack 48V
$789, 280 lbs
http://www.apc.com/resource/include/techspec_index.cfm?base_sku=UXABP48&fnl=4606,2&fnl_basket=4606,3c

and just use my device's regular AC adapter.

Option 2:
For the mostly-off-the-shelf solution is, the centerpiece is what appears to be a "build your own UPS" in a box

1. 12VDC Battery Back Up Module
$44.95
http://www.lindelectronics.com/cgi-bin/store/shop.cgi/!ORDERID!/other/BB1280-1098/dbx_gen_detail_product/

plus a battery, such as

2. 12 Volt 98 Ah Gel Cell Sealed Lead Acid Battery
$193.95, 72 lbs
http://www.batterymart.com/p-12v-98ah-sealed-lead-acid-gel-battery.html

plus a way to plug the 12v charger into a wall

3. Wagan Tech 9903 AC to DC Adapter 5 Amp Socket Output
$19.99
http://www.jr.com/9903-ac-dc-adapter-socket-output/pe/WGN_9903/

Obviously I'm leaning toward option #2, not only for weight and cost, but also because it has the flexibility to be powered by either 110VAC or 12VDC.  Am I missing something?  Is there a complication with Option #2 such that I should go for Option #1 instead?
 

Option 2;

Your #3 is not going to charge anything since 12V is substantially below a "Dead" gel cell's voltage.

Why not use a charger like the one I showed you above?

Keith Cress
kcress - http://www.flaminsystems.com

It looks to me like the Lind Electronics "12VDC Battery Back Up Module" solves the problem of simultaneous running and charging, whereas a charger-only cannot.

I mentioned component #3 of option #2, the AC to DC Adapter, only because that Lind Electronics "12VDC Bettery Back Up Module" requires a 12VDC cigarette lighter socket.