quick question about amps

[zbblanton]

ok so i need at least 18amps for a project im doing,the project will run off 12v dc

so i want it to run off 8 AA 1.5v batterys, 2500 mAh each battery, so the question is will that 2500mah become 20,000 mah when i put all 8 AA together in series

[802Chives]

  18 AMPS?  18A X 12V = 216W!  So do you think you can power 216W (slightly more than the launch version of Xbox360) off from 8 AA batteries?  Not very likely.

So when a battery says it has 2500mAh basically that is the total energy that is stored in a battery.  technically according to this the battery should be able to provide 2500mA in one hour, however this is not a true statement, as AA batteries cannot source 2500mA for very long if not at all, espeacially 1 hour.  If you were drawing 1mA it may last 2500 hours but at larger draws, the batteries will not live up to this specification.

There is another couple of terms that come into play when calculating battery life and the important one is C or discharge rate.  C is a term that works with the mAh specification as it is a normalized discharge rate according to the capacity(mAh).  Some Lipo batteries have 10C ratings so that means that a 2500mAh battery can be discharged at 25000mA for roughly 6 minutes.  These calculations are always best case scenario, so dont expect to ever achieve those numbers because there are other factors that can take away from battery life like pulsed power draw and leakage.

This discussion could take many pages to cover everything in depth, but here are some definitions that can get you started on this type of calculations:
http://stuff.mit.edu/afs/athena/activity/e/evt/summary_battery_specifications.pdf
At very best you will only get 4-6 Amps from a string of AA batteries depending on who mad them

Answer to your quesiton:
 the capacities dont add the way you want them to with batteries in series, they add in parrallel.  When added in series the voltages add up but the capacity will remain the same but at that voltage level.  if you us 3 strings of 8AA batteries I would suspect you could run your project for less then an 1/2 hour, maybe a little more depending on how smooth your draw is.

If I were you I would look for batteries with a higher discharge rate like a Lithium style battery that has a 10C rating, or better yet a big deep cycle 12V car battery. 

there really is no replacement for testing when it comes to battery life becaus there are so many variables that come into play like usable voltage.  In battery specifiacations a lot of times are measured by running the battery down to the dirt like 0.1V how many devices you know of that drain and entire battery? most I know of leave a AA at around 0.7V at best.

[zbblanton]

thanks for all this, so what your saying is that the batterys couldn't power the device very long?

all i need is it to power the device for maybe 3 or 5 minutes.

[802Chives]

basically I am saying that AA batteries wont be able to provide enough current or Amps. If you use only one string of 8 AA batteries you probably wont be able to get more than 3-4Amps @ 12V.  I think that probably you may be able to do it with 4 strings of 8 AA batteries, cuz the more strings you have in parrallel the more current you will be able to source. and if each string gives you 4 amps then 4 strings could give you 16 Amps.

putting battery in series makes the voltage add, putting them in parrallel makes the available current add, so you need a series parralell battery setup if you want to do this with AA batteries.

I suggested a lithium Ion battery pack because some are able to discharge at >10C (rather then a AA's 1.5C) so if you had a 2500mAH battery you would be able to draw 25A for roughly 6 minutes.

I cant really describe all of the factors in one thread, if you want to get together with me on MSN I can work through the design with you and help you come up with battery for your project.

[zbblanton]

ok yea that would be great i think i added u, if not msn: zbblanton@yahoo.com