watt hours and mAh?

My point from a few pages back. For the sake of this thread, we can assume that the regulator is 100% efficient. That is, power in = power out, or P1 = P2. Since power = current * voltage, and P1 = P2:

I1 * V1 = I2 * V2

I1 is our unknown, the current into the "black box". V1 is the input voltage to the black box, 3.7 volts. V2 is the voltage out from the "black box" or the voltage into your DC in jack on your laptop(19 volts). I2 is the current from the "black box" into your laptop. Using your numbers(which we will assume to be accurate for this example), we have 79.92Whr / 3.5 hours = 22.83 watts. 22.82 watts / 19 volts = 1.2 amps. So I2 = 1.2 amps.

Rearranging the old equation:

I1 = I2 * V2 / V1

or

I1 = 1.2 amps * 19 volts / 3.7 volts = 6.17amps.

As would be expected, more current goes into the "black box" then comes out.

If you knew the efficiency of the "black box" you could find a more accurate answer using:

efficiency = energy out / energy in

rearranged to:

energy in = energy out / efficiency

So instead of P1 = P2, we would have

Pin = Pout / efficiency

which would give

Iin * Vin = Iout *Vout / efficiency.

 

Please don't take this personal, but what do you do for a living?

 

I won't take it personal, but I might take it personally. After all, this is quite a personal question- how could it not be taken personally?!

If I tell you, you aren't going to listen to anything more I have to say. Maybe if I write a really long post and put the answer at the bottom you'll just skip over it.

I could say that I'm retired- in truth, I don't have a job. But seriously, I am not retired; I am simply unemployed.

I am a high school student. When I am not in class or busy doing school work, I spend my time learning on my own or working hands on with electronics.

 

Don't worry, just be good with yourself and don't try to debate everything with everyone. You do have some concepts but other are missing, which knowledge you will acquire while in college.

 

Are you speaking about missing concepts pertaining to the discussion of this external battery? I certainly understand that I don't know everything, but I believe everything that I have written about the capacity and drain of this battery is corrent as is the reasoning behind it and the math.

However, I did make a mistake about op-amps. Ideally, op-amps have low output impedance(but still very high input impedance). You can use them to drive small loads, but your average IC op-amp is rated for about 20ma of output current and still has relatively high output impedance(along the lines of 100 to 1000 ohms). Using any real op-amp will still result in a significant portion of the energy being wasted as heat if you try to use them to drive something directly with an op-amp.

But this question about batteries and capacity is a relatively simple problem and can be solved with very basic fundamental equations as I have shown. I am sure of that.

 

Yes, I was talking about battery capacity. You are saying that we cannot go from Wh to mAh or from mAh to Wh, or even worse, you said that mAh is useless, and I'm saying that you are WRONG.

A well established formula is:

Wh/V = Ah

So, the 74Wh will give you 20,000Ah if you are using the 3.7V as an output, but if you are using 19V which is more standard for a laptop, then you should have 3,894mAh at your disposition.

Examples.

http://www.camerahacker.com/Definitions/Battery_Capacity_Conversion.shtml

http://www.apexbattery.com/co-waampandv.html

 

OK. I derived that formula pages back if I remember correct.

OK. This is correct- this is the simple way to find the equivelant mAh rating of this battery.

Now if you want to estimate the battery life for your laptop, you use the equivelant mAh that you just calculated in combination with the current draw of your laptop at 19volts.

The problem here is that you have just proved that the mAh rating of this battery is useless and your example shows the point I've been making.

You used Whr to calculate equivelant mAh. That equivelant mAh can be used to estimate battery life. Although an actual battery cell has constant mAh, you can only use that constant mAh rating if the battery cell is wired directly to the device consuming power(voltage out from cell = voltage into device and current in = current out). When you through a voltage regulator into the mix, (current in != current out and voltage in != voltage out) so this does not work.

I don't remember what my exact words were about mAh, but they were probably a tad extreme. Charge capacity(mAh not to be confused with energy capacity[Whr]) isn't completely useless, but it is often misleading and in certain cases like this one, you cannot use mAh like you normally would.

Judging by your post, we're clearly not on the same page(pun intended). Perhaps it would be helpful to start over and/or figure out where the misunderstanding is?

 

I think our disagreement comes from your lack of practical experience, I'm definitely older than you and mAh is extensively used in the industry and it does not confuse people. I don't care if this universal device has regulators or not, at the end of the day it will work as a battery, that's why is called "external laptop battery", so all the equations will apply.

You are confusing this device with a charger, and yes, a charger has a current pump and it needs to be handle differently from a regular battery.

 

A "charger" is nothing more than a voltage regulator which uninverts an AC sine wave. This battery device clearly must have a similar voltage regulator.

I don't know how old you are or anything about you, but it is quite possible that I have more experience than you in terms of work even if you are older than I am and/or have been working with electronics longer.

So what if it is extensively used? If it didn't confuse people, then why does this thread even exist? Why are there countless articles on the internet on the subject of mAh?

Yes, but you need to be careful in your application of equations.

For example, before you said that your laptop draws 22.xx watts at 11.1 volts, which equates to ~2 amps and with this 20,000mAh battery that will give you 10 hours. That is an example of how you cannot apply an equation.

At this point I don't even remember what it is that I'm trying to prove to you.

 

I won't tell you my age, but believe me, I have been doing electronic way much longer than you.

They just missed to specify the voltage for both values, the mAh AND the Wh.

You were trying to prove to me that you know better

 

Whr. doesn't depend on output voltage from the regulator- no matter if you plug a 19volt laptop or a 16 volt laptop or a 5 volt usb device, you will still get the 74Whr. That's the beauty and importance of the Whr rating.

When you put a power regulator between the battery cells and the power load(read, laptop) the mAh of the actual cells starts to become less meaningful. Instead, you must calculate the equivelant mAh rating like you did in the last power you did some math(74Whr / 19 volts = ~3.7Ah = ~3700 mAh).

The manufacturer could have given the equivelant mAh ratings for each output voltage, but that would just confuse people- they would think that they would get less battery life if they plug a 19v laptop than if they used a 16v laptop, and the mAh rating would be rather unimpressive at 19volts compared to the average 11.1volt internal battery.

I am not. If I wanted to do that I would have lied when you asked what I do for a living and I would tell you about all the things I have designed and built.

 

I found funny your loud reflection so I made a joke, don't be too sensitive. However I must say that you perseverance is a great quality, don't loose it.

Conclusions for the OP question:

1. Wh/V = Ah, VAh = Wh.

2. mAh w/o V, is useless.

3. Wh and power data can gives us the battery life in h.

4. 3.7V won't definitely meet the voltage requirement for today's most popular laptops.

5. 20,000mAh is for 3.7V, and 3,894mAh is for 19V.

6. This external battery stinks for the price

And I still claim that universal power supplies are not good for laptops

 

mAh is useless without voltage AND Whr. If you have either of these two, mAh can be used, but if you have Whr you might as well just forget about using mAh.

Where does this fit in? Of course you can't power a 12 volt or 16 volt or 19 volt laptop with 3.7 volts DC. This point is not clear, it seems like you are claiming that this battery will not be able to power any normal laptop.

Battery cells have constant mAh rating. This point should talk about the important of equivelant mAh and how to calculate it.(this point and point one should be combined)

Sure. That's a matter of opinion, but I certainly wouldn't buy that battery.

My revised conclusions:

1. Battery life ultimately depends on two things: how much power the laptop needs, and how much energy the battery has stored. The energy capacity of a battery is rated in Whr; this is the important number. mAh does not directly influence battery life- the energy capacity(Whr) of a battery depends on both mAh and voltage.

2. Energy capacity of a battery is constant, regardless of whether that battery is part of a complex circuit or not. Battery life can be determined by dividing the Whr of the battery by the power usage(in watts) of the device, or: Battery Life(in hours) = Whr/W

3. If you are dealing with a battery like this that has multiple output voltages and for some reason you need a mAh rating, you need to find and use the equivelant mAh rating. As a result of the energy capacity(Whr) of a battery being constant, the apparent mAh (what you will actually get changes) changes when the output voltage changes.

4. In this case(where there are multiple output voltages) the mAh rating of the actual battery cells means very little and should only be used by people who know what they are doing.

5. You can find the equivelant mAh rating of a battery by dividing the Whr rating by the output voltage: Whr / V = Ah[equivelant]. Multiply Ah by 1000 to get mAh. However, it is easier and safer to use Whr for calculations rather than mAh.

6. In the end, all this mAh/voltage/current/Whr/watt nonsense boils down to energy(Whr), and power(watt). Everything in this thread that uses mAh, voltage, or current is a reworking of power and energy. Working directly with power and energy is easier and less complex. Always work with energy and power if you can.

Some additional points and conclusions within this thread:

7. This battery does not offer as much as would be expected in terms of additional battery life and probably isn't worth the price.

8. The 74Whr rating of the battery is a reasonable number and is likely correct. Taking the size of this battery and the energy density of current lithium technology, 74Whr makes a lot of sense.

9. There is more to math and science than knowing formulas and plugging in numbers. Knowing when and how to use equations is arguably just as important.

10. I need a coffee.

11. Op-Amps are magical.

12. I don't like drawing pictures.

13. So called "black boxes" are bad.

14. WirelessMan doesn't like to read long posts. Note that this is the last point- there's a reason I put this at the end.

15. All the equations used in this thread assume standard units. Power is in watts, current is in amps, voltage is in volts, charge capacity is in Ah(amp-hours instead of milli-amp-hours), energy capacity is in Whrs.

 

I agree Wh can be more useful than mAh as it is a specific quantity. But because battery manufacturers for the most part insist on putting mAh on the battery instead of Wh or mWh we are forced to know these equations to get the Wh?

And for those who are not mathematically gifted (myself) I will write the formula including the missing step that the mathematically gifted just consider a given.

Wh/V=Ah, Ah*1,000=mAh

The add was using the mAh rating in a deceptive manner playing on peoples lack of understanding in my opinion. Many people would look at their 4500mAh to 6500mAh 6 cell rating and think wow! This thing will power my notebook (12.6V) not for 3hrs but 12hrs (say 5000mAh vs 20000mAh?) Now I know, for comparing my battery to this. I take 74/12.6=5.873*1000=5873mAh. Not as impressive.

People (companies) will not always give you information in the best most useful way. When they don't it is frequently to their advantage. Knowledge is power.

 

PowerPack, the NBR convicted felon, is back!

Thank you, PowerPack. Yes, these factors are something that I don't even have to think about, and through the progress of this thread I realize that I have left that important bit out of my posts recently. I'm going to go back and edit them.

A better way to make the comparison would be using Whr. Instead of finding the equivelant mAh of the external battery, calculate the Whr rating for your battery if it isn't already given. For example, take the 12.6volt 6500mAh battery from your example to find that 12.6 volts * 6.5Ah(6500mAh) = ~82Whr. By comparing the 82Whr battery you currently own to the 74Whr battery external battery and you will see that it holds less than your existing battery.

Comparing Whr instead of equivelant mAh may not seem very different, however the Whr value carries more meaning than the equivelant mAh rating. This becomes more important as the situation becomes more complex.

For example, say that you have found two nice external batteries and would like to compare them, not only to each other but also to your internal battery to see whether it is even worth buying an external battery. If you find the Whr of each battery, you can compare them directly. If all three batteries have different output voltages, then you will need to find two equivelant mAh ratings for each battery to compare to the other two.

 

On a side note, I think this thread deserves an award for having more text within 7 pages of posts than any 7 pages of any other thread on NBR.

 

We can thank you and monito.

 

Yes, if you read through this thread from the beginning you can see how my habit of writing exceptionally long posts has slowly "rubbed off on" WirelessMan.

 

I have, I had some extra time to read and not post the last couple of days. Read every word worked thru the math and got it. Were is monito? Is he hiding? He needs to come in and agree or disagree.

I so much want to use mAh in the indirect way we use TDP? Higher TDP generally indicates higher power consumption. I am so tempted to want to say higher voltage indicates higher power consumption and that would correspond to lower mAh less battery life. But I won't as I think I already see flaws. And I am already aware of for example people using 220V lines for air conditioning to save power vs 110V.

I just wanted to write something wordy in the spirit of the thread. JPZ you are rubbing off.

 

Ok, I was wondering if I should buy this battery, I did not get the Wh, mAh, mWh, VI, Coulombs, Joules, McAin, Obama, etc.

They just told me that I can buy it at any food store and that I would just need a nail and a cent, can someone tell me how do I do to calculate how long it will last this battery?

 

Monito how big is it and how much does it weigh? We must start somewhere?

 

Oh, that's an amazing deal! You better jump on it right away! See, this device uses special magic op-amps that create free energy- this battery will last forever and you never even have to charge it!

I suppose this means you have nothing more to contribute to this thread?

 

Good job with all the explanations, JPZ. I was able to follow your main point from your very first post - but maybe that's because I've taken an Analog Circuits course in college. In any case, you surely know more than the average high school student.

Also, I've noticed that laptop batteries are usually advertized with their Whr rating, rather than the mAh rating, which is almost exclusively used in the cellphone industry. If you go to Dell's website and try to configure laptops, you get options for 56Whr (6-cell) and 85Whr (9-cell) batteries. I don't know about recent batteries, but the ones for the old Inspiron in my signature mention both the Whr rating and mAh rating on them.

So, to conclude from your conclusions, if I may, the Whr rating is the best indicator of battery life! Maybe it will help the OP if he decides to return.

 

As big as a regular lemon, and as heavy as a (thinking) hmmmmmm.......as heavy as a lemon too. Perhaps is a lemon, what do you think?

I think we are done, the OP left 100 years ago, so now we just need the fish, I have provided the lemon. Let's eat it while is hot before the mods close this mind powering thread.

Good night fellows!

 

Maybe... just Maybe... if he doesn't get scared by the 7 pages of in-depth text.

EDIT:

You're making me hungry... maybe I should go eat dinner.

 

is there a website that can convert this

 

Hi.

example 14.8volts x 6600mah = 97680 milliwatt hour

 

What do you want converted? We can do all the math for you and give you explanations and justifications if necessary. Just so you know, Google is capable of making just about any conversion or solving any problem that any other website can do.

If you're talking about "conversions" related to the device the OP linked, chances are no website can do the math for you. The device the OP linked is more complicated than the normal kind of battery you just stick in your laptop.