Calculation for Battery sizing for Emergency lights

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ddderek

Member
What are the calculations used for determining battery size for emergency lighting?

How does battery amp-hour convert for determining what size needed for say two 12V 12W Halogen lamps?

120V source voltage, need 90 minute coverage...
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
Most people usually just buy a Lithonia ELM2. The manufacturer has preformed all the calculations, and the fixture and battery are listed as a unit.

But if you really want to roll your own, I think you need to know the allowable voltage drop your battery can take. That, in turn, will come from the IBC 1006.3 which says that the initial emergency illumination of 1 Footcandle can drop to 0.6 footcandles at the end of the 90 min. period.

Or I guess you could just size your battery for a minimal voltage drop during the 90 min. If you go that route, and you have a 12 volt battery, you will need 2 amp for 90 min. That would be 3 amp hours.

So you need a battery that has a certain voltage drop if you discharge it for 3 AH. (You should be able to get a battery discharge curve from cut sheets.) Again, the voltage drop needs to be limited so the light output doesn't drop ot more than 60% of the initial required illumination.

Edit: Changed the calculation for 2 lamps.

Steve
 

ddderek

Member
More Specifics...

More Specifics...

I guess I need to be more specific...

I have one unit using (12) 12V, 33Ah @20Hr. rate batteries: PRC-1235L
(This takes care of several individual lights)

I have two stand alone units with two 12W, 12V, Halogen Lamp heads on each. They each have a 12V, 40Ah@20Hr. rate battery: PM-1240

The large unit had a few batteries fail, I will be replacing all of them at one time. My question is: Since the stand alone units take a single battery, and the voltage is identical, and Amp-hours close... How would one of the 12V, 33Ah batteries perform in the stand alone units instead of scraping all of them, (good and bad) and buying additional 40Ah batteries?
 
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steve66

Senior Member
Location
Illinois
Occupation
Engineer
Each lamp will draw about 1 amp (12W/12V = 1 amp).

2 lamps will draw about about 2 amps, and running for 1.5 hours, that will be about 3 amp hours.

There are still a few unknowns here: Your discharge rate is closer to a 10 hour discharge than 20 hours, so you're 33 AH battery will actually be somewhat less.

And we don't know exactly how much the voltage on the battery will drop. Even if we knew that, I'm not sure how we would figure out the amount of lumens the lamp would loose by not running at full voltage.

But, after all that is said and done, you are actually only using about 10% of the battery capacity. Based on that, I would think the 33 AH battery would work just fine, and would probably last a lot longer than the 90 min run time.

One other issue to note - I'm not sure you would maintain the UL listing of the fixture with a different type of battery.

Steve
 
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