# Homework Help: Batteries and Charging Capacity

1. Feb 10, 2004

### AshleyF708

I have a homework problem that asks me to find the charging capacity of a battery. I wasn't sure how to find this or what equation to use. I wasn't sure if the charging capacity was an equivalent to the capacitance. Can someone point me in the right direction?

Note: I am given it's a 12 V battery that drains completely in 24 seconds.

2. Feb 10, 2004

### faust9

Your not given a circuit or any resistance value? To rate a battery you need to know the current draw, or some initial current and a general formula for rate of discharge. Rechargeable batteries are rated by type (NiCad, NiMH, Led-Acid), By voltage (9v, 1.2v, 1.5v), and by current rating (600mA-h, 12 A-h). If you know the current draw on the battery you can calculate an amp/hour rating. Then, using the amp-hours-out=amp-hours-in your done.

Last edited: Feb 11, 2004
3. Feb 10, 2004

### AshleyF708

It was a multistep problem where resistance was found to be 0.027 ohms in a car battery that is possible of cranking out 450 amps in a cold engine. Then the part I was stuck on was finding the charging capacity if the engine was cranked for 24 seconds it was completely drained.

4. Feb 11, 2004

### AshleyF708

So is a battery considered a capacitor? And if so does finding the charging capacity imply finding the capacitance? If not, what sort of value am I looking for for the chraging capacity(i.e. energy, potential, electric potential energy)?

5. Feb 11, 2004

### AshleyF708

Anything of the slightest bit of insight? Am I helping myself if I find the charged cranked out in those 24 seconds? And then use that charge to find the Work done? Am I on the right track at all?

6. Feb 11, 2004

### AshleyF708

Any words of wisdom at all???

7. Feb 11, 2004

### turin

OK, well, I'll give my non-expert opinion, since I see you're still crying for an answer.

I don't think that the battery is much like a discharging capacitor for the purpose of this question. Unlike a capacitor, I'm pretty sure a battery is designed to maintain the terminal voltage until it is unusably drained. I am making this guess based on the assumption that the battery generates electricity by a specific chemical reaction. In the reaction, the sulfuric acid combines with the lead or whatever, and this leaves a certain amount of activation energy (~12 eV total for the series of plates). So, I think that the current comes out steadily from the ~12 V terminal (minus the drop across the internal resistance), until there is effectively no more charge left (you can probably assume that there are no activation effects other than the constant voltage, so that the battery can drain to 0 charge seperation). Then If it takes x sec for the battery to run out of charge at y amps, the charge capacity would be xy Ampsec. That is my feable conjecture. I seems too simple, though.

8. Feb 11, 2004

### chroot

Staff Emeritus
What is "charging capacity?"

Do you mean "how much charge can the battery hold?"

If so, charge is measured in units of current times units of time. The amp-hour, for example, is a unit of charge.

If the battery can supply a solid 450 amps for 24 seconds, then it is capable of storing 10,800 C of charge.

- Warren

9. Feb 11, 2004

### turin

How valid do you consider this assumption?

10. Feb 11, 2004

### chroot

Staff Emeritus
Ordinarily, it's not very valid -- but there's no information in this problem (at least as given to us by Ashley) that would enable us to make a better estimate.

- Warren

11. Feb 12, 2004

### Mr. Robin Parsons

As had been pointed out, not enough information to answer, either the question, or to speculate upon just what it is you need to resolve that Question.

As for batteries, as is pointed out above many varieties of battery, some differing in how the develop reserve amperage capacity, a car battery is a chemical process that 'rebounds upon use' in simple words, the voltage that is obtainable at the terminals, will rebound back up to it's "full charge" reading simply by waiting for the chemical reaction to develop (re-up) the voltage....but you will have used up some of the batteries "reserve amperage capacity"....

P.S. Most car batteries are not '12' volt that is simply a generalized public perception. work in the car industries and you will find out that it is more around 13.5 Volts...something like 2.1 or 2.2 volts, per cell, 6 cells in a '12' volt battery....

12. Feb 12, 2004

### AshleyF708

Here I'll give you the question as written:

A starter motor in an automobile is an impressive piece of machinery. When a cold engine is started, this motor can draw 450 Amps of current from the 12 Volt battery.
a.) What is the effective resitance of the motor?
b.) If the starter motor turns for 1.30 seconds, how much charge is pumped by the battery?
c.) How much work does the battery do during this time?
d.) The starter motor can crank the engine for 24.0 seconds before the battery is drained. What is the charging capacity of this battery?

I had no trouble with a, b, or c. D was where I was stuck I already turned in my solution which I completely guess on but since there was still some conversing on the topic I decided to clear up the confusion I started.
I found the charge pumped by the battery in 24 seconds(app. 10800 c) and then I found the energy from multiplying that charge and the voltage of the battery(1.3 X 10^5 J). Don't know if it's right but it was a nice effort. Thanks for trying to help it just came a little too late :)

13. Feb 12, 2004

### chroot

Staff Emeritus
I'm still not clear on what your teacher means by "charging capacity." That just isn't a term normally in use. If you can clarify his terminology, that'd be a big help.

- Warren

14. Feb 12, 2004

### AshleyF708

Your guess is as good as mine. Personally I think he makes this stuff up but doesn't excuse me from homework assignments.
I guess I kinda looked at charging capacity as the battery's ability to hold on to charge. So I went on to figure out how much charge the battery can pump in 24 seconds which should the max amount of charge it can hold since the battery dies after 24 seconds. Then I took that value and multiplied it by the volts to find the max amount of energy that is possible to be pumped by the battery. That's how I interpreted charging capacity since any normal physics text or anything written about physics doesn't mention charging capacity.

15. Feb 12, 2004

### Mr. Robin Parsons

From what I read of your question, it is off, quite a bit, as a car battery deals with a reserve amperage capacity, and if it is draining completely, in 24 seconds, something is wrong with it...the only manner to discharge a battery, that quickly, is with a (fairly) large resitance, (rheostatic pile) and when you do that, a slight amount of passing time will allow the effective charge (voltage) of the battery to return, (chemical reactions take place) but the reserve amperage capacity has dropped signficantly, which, as an activity, simply means that the car starter motor will crank, and 'rebound' for less, and less, time...till dead...

It would probably help (more) if you could provide more info, but if it is unavailable....

In car battery technology 'charge capacity' can simply mean "when the specific gravity reaches a certain point", relative to temperature...but that is chemistry, now, isn't it...

16. Feb 12, 2004

### AshleyF708

I think what is possible and what isn't isn't the question here it was a homework problem that discussed I guess why batteries die in cold weather. But regardless he wanted how much charge is pumped in those 24 seconds which is drum rolll..... 10800 Coulomb's!!!

17. Feb 12, 2004

### chroot

Staff Emeritus
Mr. Robin Parsons,

It sounds like you have no idea how batteries work.

- Warren

18. Feb 13, 2004

### Mr. Robin Parsons

Well congratulations! cold weather "death" (reduction of charge) of a battery is due to the lack, or reduction, of chemical reactivity...hence less available 'reserve' amperage....

Chroot please explain your comment, as in, where do you derive that from?

19. Feb 13, 2004

### Mr. Robin Parsons

Based upon this (erroneous) statement, and the information available from this site http://www.dcbattery.com/faq.html...well, it would seem that, it is not me, who doesn't know about batteries....years of experiance chroot, and BOOK learning...so maybe Chroot, you should take that advice you tried to give me, the other day....remember it?

Last edited by a moderator: Apr 20, 2017
20. Feb 13, 2004

### chroot

Staff Emeritus
How exactly is 450 amps for 24 seconds NOT 10,800 C?

- Warren