Question about batteries


by LBrandt
Tags: batteries
LBrandt
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#1
May24-12, 01:26 PM
P: 63
Question about battery amp hours

Hello,
A particular manufacturer has two Lithium-ion batteries available for use in a cordless tool. One battery is rated at 2.4 amp hours, and the other is rated at 1.5 amp hours.

Given that both batteries are brand new and fully charged, is the following statement true?

When each battery is used on the same tool, each battery will provide the same power, torque or whatever you might choose to evaluate its strength on startup as the other battery, the only difference being that the 1.5 amp hour battery will run down faster than the 2.4 amp hour battery.

Or is this statement true?

The 2.4 amp hour battery will provide more power, torque than the 1.5 amp hour battery.

I am not interested in whether the 2.4 battery will last longer, only whether the two batteries will provide the same power upon startup.

Thanks,
Louis
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sophiecentaur
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#2
May24-12, 03:56 PM
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This looks like a question for the Homework and Coursework Questions forum.
But-
The answer will have something to do with internal resistance, I think. The question is whether Energy capacity necessarily relates to internal resistance. That could require some specialist knowledge.
LBrandt
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#3
May24-12, 04:18 PM
P: 63
Thanks. Well, I might be overstating the question, since I really didn't intend for it to get into quite that technical a direction. I merely want to know (in general) whether the lower amp-hour battery would (initially) provide the same power as the higher amp-hour battery. My assumption is that it will, but I wanted the advice of the members of this excellent forum.

Kugan
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#4
May24-12, 04:40 PM
P: 3

Question about batteries


Hmmm... I thought I might offer something here just in simple terms.

If you have 2.4amp*hr and 1.5 amp*hr then initially say the first second

there will be 2.4*3600 = 2.4*3600 Coulombs/s and 1.5*3600/s so per s there is more charge flowing through assuming V is constant then P =iV the 2.4A*h will provide more power. The initially part is what you are concerned with I believe?

Not sure if I helped :S
Khashishi
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#5
May24-12, 04:45 PM
P: 832
If you stick both batteries in the same flashlight, they should both shine as bright initially, except one will last longer. If you get into the nitty gritty, there might be small differences, but that depends on the battery details.
LBrandt
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#6
May24-12, 04:48 PM
P: 63
Thanks. Yes, the "initially" part is what concerns me. But my instinct (and of course I humbly realize that I could be wrong) is that both will have equal power, but one will last longer than the other. In the case of the specific batteries that I am concerned with, they are both 18 volt models. If I am wrong, then the 2.4 amp hour batteries would have two advantages over the 1.5 units, in that they would provide more power and for a longer time.
Gordianus
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#7
May24-12, 04:54 PM
P: 217
Formally, the battery with higher amp-hour figure has less internal resistance and may provide larger initial current.
However, I think a well designed tool wouldn't resort to this difference because it would mean the tool is stressing the battery too much.
Thus, I think the initial performance is the same but the larger battery lasts longer.
LBrandt
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#8
May24-12, 05:04 PM
P: 63
Ok, so two of you (Khashishi and Gordianus) say that both will have the same initial power, and one of you (Kugan) says that the higher amp-hour battery will have greater initial power. Hmmm.

To ask the question in a different way, does 2.4 amp hours mean (1) that I can produce 1 amp of power and maintain it for 2.4 hours, or (2) that I can produce 2.4 amps of power and maintain it for 1 hour? If the second is correct, then a 2.4 amp hour battery would produce more initial power than a 1.5 amp hour battery, but if the first is correct, then they would initially produce the same power.
sophiecentaur
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#9
May24-12, 05:27 PM
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Quote Quote by LBrandt View Post
Thanks. Well, I might be overstating the question, since I really didn't intend for it to get into quite that technical a direction. I merely want to know (in general) whether the lower amp-hour battery would (initially) provide the same power as the higher amp-hour battery. My assumption is that it will, but I wanted the advice of the members of this excellent forum.
I think that it is actually necessary to get technical about this if you want a definitive answer. The capacity of a battery will depend on factors other than the area of the 'plates', which will be the main factor in how much peak current can be delivered for a given voltage drop.
Battery technology is very sophisticated and I doubt whether there is anyone on this forum who could (or would be prepared to) tell us all the true facts about modern batteries. It could be one of the biggest business areas in the future and worth a lot of money. Hence the secrecy.
256bits
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#10
May24-12, 06:02 PM
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To ask the question in a different way, does 2.4 amp hours mean (1) that I can produce 1 amp of power and maintain it for 2.4 hours, or (2) that I can produce 2.4 amps of power and maintain it for 1 hour? If the second is correct, then a 2.4 amp hour battery would produce more initial power than a 1.5 amp hour battery, but if the first is correct, then they would initially produce the same power.
Neither of your statements is correct. Power is volts x amps.

Batteries have several ratings.
One of them is the nominal voltage at the terminals for a fully charged battery, in your case 18 volts. This is related to the chemical reaction taking place within the battery. A typical dry cell outputs 1.5 v. To get a 9v dry cell, manufacturers stack 6 of the 1.5v cells in series.

Another is the amp-hour rating, which is the capacity of the battery. You can think of it as a crude rating of how long a batery will provide acceptable voltage and amperage so your device can operate to your satisfaction. A battery with a higher amp-hour rating will last longer than one with a lesser rating. This would be related to the size of the plates and /or numer of plates connected in parrallel.

In your case the 2.5 amp-hour battery will last longer than the 1.5 amp-hour battery.

Another rating might be short circuit amperage output, which would also be related to the area of the plates connected in parallel. Manufacturers do not list this for most batteries as a short circuit could heat up and destroy your battery. One place you will find a somewhat related figure is the cranking amps listed for car batteries. Your battery does not list such a figure, as it is manufactured for a specific tool perhaps, and is not meant for high current loads I suspect.

You can think of the amp-hour ratings as follows. If battery A has X amp-hour rating then 2 of the same batteries would have a 2X amp-hour rating - ie 2 batteries would last twice as long as 1 battery if they were in a configuration to output the same voltage and amperage to your device.
rcgldr
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#11
May24-12, 06:19 PM
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What matters is the voltage that the batteries output for a specific resistance. Assuming a constant resistance, most batteries will experience a slight decrease in voltage over time as the batteries are discharged. If the initial voltage between the two batteries is identical, then there could be a slight difference in voltage after a few minutes of usage.

However, the powered device itself might have circuitry to consume a nearly constant amount of power, reducing resistance to compensate for the slight decrease in voltage. In this case, the choice of battery won't matter as long as the battery can output the required power.

Note that the amp hour rating for a battery assumes a reasonably small load (reasonably high resistance). For a large load (low resistance), the capacity of most batteries will end up less than the rated capacity.
mrspeedybob
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#12
May24-12, 06:55 PM
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The O.P. Did state that the batteries in question are lithium-ion. Lithium-ion cells are easily damaged by excessive charge or discharge rates, therefore in consumer applications they are almost always (I know of no exceptions) packaged with some electronics which govern charge/discharge rates. Since your 2 batteries are designed for the same power tool they will be regulated to produce the same power output. This has nothing to do with the chemistry of the battery, but with the little microchip packaged in the case with it that decides how many amps are allowed in and out.
rcgldr
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#13
May24-12, 07:54 PM
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Quote Quote by mrspeedybob View Post
The O.P. Did state that the batteries in question are lithium-ion. Lithium-ion cells are easily damaged by excessive charge or discharge rates, therefore in consumer applications they are almost always (I know of no exceptions) packaged with some electronics which govern charge/discharge rates.
In the case of a consumer product like a power tool battery pack as mentioned in the original post, I would assume that maximum discharge rate isn't very large.

However, in the case of lithium-ion battery packs used for radio control models, they often have a high maximum continuous rated discharge rate, such as 25C continuous for a 2200 mah 11.1 volt (3s) battery pack, which translates into 55 amps. There are high power packs like a 35C continuous, 5400 mah, 18.5 volt (5s) battery pack, which translates into a continous drain of 189 amps (such a pack would be drained in about 100 seconds at that rate. There are fast radio control powered gliders that only use the motor a few seconds at a time). Charge rate is much lower, that 35C pack maximum charge rate is 5C or 27 amps.
LBrandt
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#14
May25-12, 08:25 AM
P: 63
Quote Quote by 256bits View Post
Neither of your statements is correct. Power is volts x amps.

Batteries have several ratings.
One of them is the nominal voltage at the terminals for a fully charged battery, in your case 18 volts. This is related to the chemical reaction taking place within the battery. A typical dry cell outputs 1.5 v. To get a 9v dry cell, manufacturers stack 6 of the 1.5v cells in series.

Another is the amp-hour rating, which is the capacity of the battery. You can think of it as a crude rating of how long a batery will provide acceptable voltage and amperage so your device can operate to your satisfaction. A battery with a higher amp-hour rating will last longer than one with a lesser rating. This would be related to the size of the plates and /or numer of plates connected in parrallel.

In your case the 2.5 amp-hour battery will last longer than the 1.5 amp-hour battery.

Another rating might be short circuit amperage output, which would also be related to the area of the plates connected in parallel. Manufacturers do not list this for most batteries as a short circuit could heat up and destroy your battery. One place you will find a somewhat related figure is the cranking amps listed for car batteries. Your battery does not list such a figure, as it is manufactured for a specific tool perhaps, and is not meant for high current loads I suspect.

You can think of the amp-hour ratings as follows. If battery A has X amp-hour rating then 2 of the same batteries would have a 2X amp-hour rating - ie 2 batteries would last twice as long as 1 battery if they were in a configuration to output the same voltage and amperage to your device.
Ok, 256bits, I accept your statement that both of my statements are incorrect. Then please, in a single sentence, tell me (assuming that both batteries are used in the same tool) whether I will experience more initial power using the 2.4 amp hour battery or the same initial power as the 1.5 amp hour battery.
sophiecentaur
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#15
May25-12, 09:22 AM
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Quote Quote by LBrandt View Post
Ok, 256bits, I accept your statement that both of my statements are incorrect. Then please, in a single sentence, tell me (assuming that both batteries are used in the same tool) whether I will experience more initial power using the 2.4 amp hour battery or the same initial power as the 1.5 amp hour battery.
I think his answer was that there isn't an answer to your question. You don't specify enough about the batteries for a definitive answer to be given.
LBrandt
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#16
May25-12, 09:49 AM
P: 63
Quote Quote by sophiecentaur View Post
I think his answer was that there isn't an answer to your question. You don't specify enough about the batteries for a definitive answer to be given.
Thanks. Well, I'm not sure what else I could specify about the batteries. They are both manufactured by the same company. They are both Lithium-ion 18 volt batteries, and one has a 2.4 amp hour rating, whereas the other has a 1.5 amp hour rating. They are both designed to fit the same tool.

I realize that battery technology may have become almost a science in itself, and I certainly don't mean to be sarcastic, but surely a question such as mine doesn't rise to the level of theoretical physics.

Louis
Gordianus
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#17
May25-12, 10:17 AM
P: 217
I repeat my opinion. If we're speaking of a standard tool (screwdriver) and it has been well designed, in the sense it doesn´t overstress the batttery, the larger battery lasts longer. That's all there is to it.
LBrandt
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#18
May25-12, 10:26 AM
P: 63
Quote Quote by Gordianus View Post
I repeat my opinion. If we're speaking of a standard tool (screwdriver) and it has been well designed, in the sense it doesn´t overstress the batttery, the larger battery lasts longer. That's all there is to it.
Thanks again. I noted your answer, and I agree with it. It's just that others have made comments to the effect that the question cannot be answered. That's why I continued to post on the issue.


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