# battery capacity (easy ,but Im too stupid :-( )

Discussion in 'Homework Help' started by LunitaCanada, Jan 9, 2006.

Jan 9, 2006
2
0
Hi,
I can't solve this question, I'd be so glad if someone could help me solve out this question.

Question
Assume a 1 Amp/Hour battery is used on a cellular phone, and the cellular phone draws 35 mA in idle mode and 250 mA during the call. How long will the phone work if the user leaves the phone on continually and has one 3-mins call every day?????

How could you find the initial Amp of the battery??? and how long will the phoen work???? Thank you very much for your help.

2. ### JoeJester AAC Fanatic!

Apr 26, 2005
3,401
1,218
Show us how you tried to solve the problem.

Jan 10, 2006
613
0
If I said that in an Ideal world, a 1Amp hour battery can supply 1 amp for 1 hour OR 2 amps for 30 mins, OR 500 mA for 2 hours... does that help..??

Jan 9, 2006
2
0
Thank you very much.

5. ### n9xv Senior Member

Jan 18, 2005
329
1
Wait a minute! There is a little more to it than that. Amp-Hour rating means the amount of "constant" current the battery can deliver over a 20-hour period. The constant current level is comulitive, that is, the constant current over a 20-hour period must "add up" to the Amp-Hour rating. In your example, a 1-Amp-Hour battery can deliver a "constant" current of 50-milliamps over a 20-hour period.

Constant current over a 20-hour period = (Amp-Hour rating) / 20

This 20-hour period is a universal industry standard. I like to call it a battery-day.
There is an excellent calculator on the web that allows you to enter all the variables that exist in a problem like yours. Click on the link and give it a try.

Amp-Hour Calculator

Jan 10, 2006
613
0
Of course that is correct, some also use the 10 hour rating, (most use the 20 hour rating)... but I am assuming that for the purpose of this exercise in which the variables 1 AH, 35 ma, 250ma, and 1 x 3 min call @ 250 ma per day, the teacher probably intends a more straighforward answer, as without knowing the battery manufactures specified capacity for that battery at a 35ma or 250 ma discharge rate (something the teacher hasn't given), battery type (Lead acid, Lithium, Lithium ion, NiMH, NiCad, Alkaline, Carbon Zinc etc etc), an exact answer would be impossible.
I thought the term "In An Ideal World" would cover that.

7. ### n9xv Senior Member

Jan 18, 2005
329
1
The Amp-Hour rating is universal. The type of battery does not matter. He asked us to "assume" a 1-Amp-Hour capacity. You could apply the formula in reverse to find the initial Amp-Hour rating but that would involve monitoring different levels of current over the 20-hour period in order to find that "magic" constant current rate for the 20-hour period. Its possible, but obviously not practicle. Generally most cell phone batteries are in the 800-mAH to 1600-mAH capacity range. You should be close to the target with an average of 1200-mAH.

Jan 10, 2006
613
0
The teacher hasn't supplied a 1AH Battery to measure the 20 hour AH rating, which as I said earlier would depend on the type of 1Ah battery it is, just a few simple figures and probably expecting a simple solution. Lets not confuse the issue with Primary/secondary cell theory and charactoristics.

As I said before, work with 1Amp over 1 hour, 250ma over 4 hours etc. Its not correct battery theory, but this seems to me an exercise in Mathematics, not the capacity differences at differing discharge rates.

9. ### JoeJester AAC Fanatic!

Apr 26, 2005
3,401
1,218
This is the classical exercise in predicting how long the battery will last.

I've seen it in another form on the FCC General Radion Operators License exam concerning the survival craft radio.

You need to know the duty cycle ... we have 57 minutes per hour @ one rate and 3 minutes per hour at a different rate. The use rate is 5%, leaving the idle rate at 95%.

On a per day basis, the idle current consumes 35 mA * 24 hours * .95 or 798 mA.
On a per day basis, the usage current consumes 250 mA * 24 hours * .05 or 300 mA.

Adding those two total currents yields 1.098 Amps per day. We know the instructor gave us a 1 AH battery, so it won't last a day, so 24 * (1/1.098) = 21.85 hours would be the best estimate of battery life.

Now, reality steps in. That duty cycle is unrealistic for those of the x-generation, especially those with surgically implanted cell phones, or so it seems from observations.

10. ### n9xv Senior Member

Jan 18, 2005
329
1
I bet thats really a battery pack instead of a "book bag" they have strapped on their shoulders!