AA battery power rating

[Uman]

I am designing a circuit that is powered by 6 AA batteries (9 volts). Due to the circuit load, there is a concerned about the power draw on the batteries. How do I calculate the the circuit continous run time until the battery voltage drops below 7 volts. The circuit will draw about 100ma when energized.
Thanks for any help,
Jeff in Florida

[audioguru]

Hi Jeff,
Energizer has a pretty good web site. They have detailed datasheets for their batteries but be careful because a disclaimer says that they are subject to change. Their main site is here:
www.energizer,com

I am assuming a 20 degrees C temperature, its performance is poor when it is very cold.
The spec's for their Energizer alkaline consumer AA cell shows it discharging in about 13 hours to a 1.17V voltage (7V/6) with a 100mA current. Their AA lithium cells (very expensive) last nearly 30 hours!
Your load is probably resistive, not constant current, so the 100mA will drop with the battery voltage. Therefore with a 15 ohms load (1.5V when new at 100mA), your battery life is about 18 to 20 hours.
The datasheet is here:
http://data.energizer.com/datasheets/library/primary/alkaline/energizer/consumer_oem/e91.pdf

[AJB2K3]

on average AA's are 1.2 V
so 6 = 7.2V.
to get 9V uses 6 high quality AA's or8 standered(use a meter to check voltages)

[ante]

AJB2K3,

Alkaline cells are 1.5V!
NiCd cells are 1.2 Volts!

[audioguru]

on average AA's are 1.2 V

Yeah, nearly dead ones! (non-rechargeable kind)

Ante is correct, they start life at 1.5V (or even 1.6V).

[trigger]

batteries always start at 1.5V (some I have measured up to 1.7V)and then gradually down to 1.2V... and then dead.. so if you really want to power the circuit with exact 9V... AA or AAA are not fitted your application.

try to use the 9V cells.... or more AA/AAA cells...

For 100mA current.... it really depends on the power rating (check the datasheet or the graph on it) on the batteries.... say for example, alkaline AA cell is approx. 1000mAH (i.e. provide 1000mA an hour), divide this parameter with the current drawn and you will get your system will survive for 10hours..
This calculation isn't the exact figure as the cells are usually not the same figure and this calculation assumed the circuit is constantly drawing 100mA... but it just give the goood estimation.

[quantum]

If AA and AAA are 1.5 volts, whats the difference?

[ante]

quantum,

The difference is the capacity (mAh) and the physical size of the cells.

[quantum]

So in other words, them AAA do not last as long as those AA? ;) :o

[audioguru]

Hi Quantum,
Simply compare the size and weight of AA and AAA battery cells.
They are full of chemicals, not majic. Of course the biggest one will supply more ma/hr. (This does not apply to their "almost empty" Ni-MH D cells, that only have a little AA cell inside).

You could also compare their datasheets that I posted for the exact details. See how much more capacity that an AA cell has than an AAA cell. See if the difference in weight and volume agrees with the difference in capacity. Also look at the ratings for a alkaline D cell.

Compare the difference in capacity between different chemicals (and therefore price). That's why I mentioned that the lithium cell (very expensive) has such a high capacity.