I have a 8 AA batteries (12V total) that is powering a light that draws .0517A.
The light no longer outputs enough light when the battery voltage drops to 8Vdc.

Typically I'd calculate battery life via:
2.5Ah * 8 = 20AH.
Then:
20AH/.0517A = 386 hours.

Does this equation change if I want to calculate the duration until the battery voltage drops to 8Vdc?

 

Where does the 2.5Ah come from ?
manufacturers specify batteries via initial and end voltage ,
and at specific discharge currents,

As the battery discharges, the output voltage drops,

e.g. Alkaline batteries are normally specified from around 1.5 to 1,2 V

At 52 mA, with a decent type you should be ok,
but in my experience, as the discharge current goes up the Ah available decreases,

So a battery discharged at say 10 mA gives a better Ah than same discharged at 100 mA

 

Let's assume a perfect battery and round off numbers for simplicity. Good quality AA alkaline batteries are rated close to 2500maH. A 1mA draw would theoretically last 2500 hours. 2500/50mA (your led's) will run for 50 hours. BUT batteries are not perfect. The higher the current draw, the shorter the life. This can be found on a graph in the battery manufacturer's datasheet.

 

Batteries in series DO NOT add their capacity. 8 AAs in series is still 2.5Ah. So you are off by a factor of 8 even if the batteries really did last as long as rated at the given current.

Look for a datasheet of the battery. It will (should) have graphs of the voltage / time for different current draws.

Bob

 

Where does the 2.5Ah come from ?
manufacturers specify batteries via initial and end voltage ,
and at specific discharge currents,

As the battery discharges, the output voltage drops,

e.g. Alkaline batteries are normally specified from around 1.5 to 1,2 V

At 52 mA, with a decent type you should be ok,
but in my experience, as the discharge current goes up the Ah available decreases,

So a battery discharged at say 10 mA gives a better Ah than same discharged at 100 mA

I'm tasked with specifying how long a light will last so I'm trying to find the correct calculations to show.
2.5Ah capacity for a AA came from a quick google search but I found a datasheet showing that this rating comes with a discharge voltage value. This means my calculations need to be modified to account for only discharging to a certain value.

 

Your battery cells are in series which increases their voltage. But your calculation shows 20Ah(!) when the cells are in parallel.

The spec's for an Energizer AA alkaline cell shows 50 hours at 52mA.
An Energizer Ni-MH AA cell is rated at 2.45Ah

 

I have a 8 AA batteries (12V total) that is powering a light that draws .0517A.
The light no longer outputs enough light when the battery voltage drops to 8Vdc.

Typically I'd calculate battery life via:
2.5Ah * 8 = 20AH.
Then:
20AH/.0517A = 386 hours.

Does this equation change if I want to calculate the duration until the battery voltage drops to 8Vdc?

@tvist21 Who made the battery? You need to look at the datasheet.

As an example, I've attached a datasheet for an Energizer(tm) AA battery which can give you a rough idea of what you're battery may be capable of supporting.