Battery Life Duration for Specified Voltage Drop

Thread Starter

tvist21

Joined Jun 11, 2021
20
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?
 

drjohsmith

Joined Dec 13, 2021
394
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
 

peterdeco

Joined Oct 8, 2019
481
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.
 

BobTPH

Joined Jun 5, 2013
4,906
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
 

Thread Starter

tvist21

Joined Jun 11, 2021
20
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.

1646236417543.png
 

BobaMosfet

Joined Jul 1, 2009
1,979
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.
 

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