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My laptop battery has finally taken the plunge and I'm looking online for a new one. However I'm getting lost in all of the specs of the batteries that are out there. I'm looking to gain a better understanding of what effect each spec has on the overall life of the battery (meaning length of hours that I can operate on battery power).

Some questions I have:

  • What do Watts equate to?
  • What do mAh (milli Amp hours) really mean?
  • Does voltage have an effect on the life of the battery? (I understand that laptops can support multiple voltage types reference)
  • Do 9 cell battery have a greater battery life than that of 6 cells. If so, by how much?

Give to me what I should look for in a battery that will affect the battery life the greatest. Some questions I have in this regard:

  • Are Watts or mAh are the greatest factors to look for, or if it's a combination of the two, or something else entirely different.
  • How much difference is there between two values of the factors (i.e. 4800 mAh vs. 7200 mAh)

It doesn't have to be these exact numbers and in fact I don't want specifics but rather a general rule of thumb)

I am NOT looking for "This is the best battery to buy" or "This is the best company to but from". I am NOT looking for just links to websites that I have to sift through. If you post a link, please summarize what it says. I AM looking at a greater understanding of the technology behind it.

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Since there is already great answers here, I'll just drop a comment to add: more important it seems is the reputation of the replacement battery's maker. i.e. I've purchased less expensive batteries before that claimed to have as good, or better, specs than the OEM. Only to find out the battery was really a piece of junk. Most consumers aren't going to have the equipment required to verify the specs and even if we did, getting a refund from some of the companies that sell low cost batteries may be a challenge. Stick with a premium brand replacement from a well known supplier. –  Tyson Nov 3 '14 at 0:42

4 Answers 4

As far as battery technology for a laptop, what you've got is what you're stuck with for the most part as the charging electronics to different battery technologies are internal to the laptop itself.

That said, you've asked good questions about the terms used to identify batteries - mAh (yes, milliamp hours) and W (watts).

Watts are a standard way to measure power output of any electrical device. The best measure for a battery would be in Watt-hours, measuring actual energy capacity of the battery. (Watts = Volts * Amps)

A second-best measure would be the mAh measure which would indicate how much current the battery can supply over time at the specified voltage. (I used this figure frequently when selecting AA rechargeable batteries.)

I've not heard about voltage variations between batteries. Essentially the most efficient battery will be the one with the voltage that best matches what the computer's internal electronics were designed around. Any excess will be frequently lost as heat as the internal electronics convert the voltage to the proper level (which is probably not a figure specified by the manufacturer - they're more likely to supply a figure denoting the range of voltages the laptop will accept).

Bottom line for most batteries is that shelf-life is still a factor, so look for a reputable dealer with an indication of when the battery was manufactured since an older better specified battery may not have the capacity of a new battery with poorer specifications.

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this is the type of answer that I'm looking for. However, you state that the best measure is "Watt Hours" but don't give a way to calculate that... is this generally given when shopping? Secondly I'm assuming that the higher the mAh is the better. Is there a major difference then with a let's say 4800 mAh rating and a 5200 mAh battery? –  KronoS Oct 25 '10 at 16:42
    
@KronoS - you're right - Watt-Hours isn't a typical spec. So with the example you gave, as long as the voltages are identical, the 5200mAh will have about 8% more capacity than the 4800mAh battery. –  JDB Oct 25 '10 at 18:52
up vote 4 down vote accepted

This is what everything related to a Laptop Battery means:

Milli-Amp Hours (mAh) and Amp Hours (Ah)

1 Ah is 1000 mAh. So if you see a battery that states it's 7200 mAh it's just 7.2 Ah. (I'm guessing this was done as a marketing ploy). A 7200 mAh battery will supply 72 milli-Amps (7.2 Amps) for 100 hours or 1000 milli-amps (1 Amp) for 7.2 hours. In order to fully determine how a battery will do with your laptop, you must find out how much amperage your laptop uses and then back calculate the amount of hours that battery will supply the laptop. In short this means that the higher the number the greater amount of supply

Watts and Watt Hours

Watts (W) and Watt-hours (Wh) is a measurement of power that can be given over time. Meaning that a 100 Wh battery will supply 100 Watts for one hour, or 50 Watts for 2 hours, and so on. Since most battery manufacturer do not post a value of Wh or mWh, a simple way to calculate mWh is to take Milli-Amp hours (mAh) and multiply it by the Voltage output of the battery [mAh x V = mWh; Ah x V = Wh]. This means that the higher the number the greater amount of power supplied per hour

Cells

The amount of cells has an effect on the overall battery life, in that there are more cells to supply power. More cells will only result in higher values of either V or Ah and thus affecting the over all Watt-hour value.

Making Sense of it all

No one is really going to calculate exactly how much a battery is going to power their laptop. To make a simple comparison of all the different types of batteries is to best compare Watt-hours (Wh), since this takes into effect both voltage and milli-Amp hours, and doesn't matter the amount of cells within the battery.

Note: most, but not all, batteries have the same voltage for a particular model of laptop, so if your feeling lazy a look at simply mAh sometimes can be sufficient

Finally you must realize that this comparison is only valid for two batteries that are of the same age. An older battery with a higher Wh is going to perform less than a brand new battery of equal Watt-hour value. Also known to affect battery life is the surrounding temperature of the battery.

References:

1 2 3 4 5 6 7

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KronoS, you asked and answered yourself your question ... And by the quality and amount of information it seems you already knew the answer when you asked. If you knew the answer, you should have answered faster, and not wait for the users to lose their time answering the question... If this was not the case, never mind. –  Lombas Oct 25 '10 at 20:20
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@Lombas I did not know about this technology at all when I asked the question. I did research and presented what I had found. It was your "loss of time" looking that helped to prompt me where to look for answers. I urge you to please continue to learn about this and add to what you or I have already discovered. I have not yet accepted any answer for such reason. –  KronoS Oct 25 '10 at 21:14
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I thought the point of having a Q&A site with focus on voting is to have multiple answers and let the community vote on the best answer - not to have just a single answer. –  Sathya Oct 26 '10 at 13:35
  1. All batteries run on chemicals inside it. In notebooks, they always use Lithium batteries, so you don't have to worry about the technology (chemicals inside it).

    What you can worry about is the capacity of the battery. In large batteries which include those for notebooks, you will always find the capacity in 'Ah' = Ampere-Hour not in 'mAh' = milliampere-hour. The bigger the number, the greater the capacity of the battery. More capacity, more time you can use your notebook.

    The problem is that most notebook manufactures doesn't tell you the capacity in 'Ah'. The only information you have is the number of cells inside the battery. More cells -> more chemical -> more time you can use the notebook.

    What the value actually means is that with a battery of 3 Ah (for example), you could drain 3 Amperes of current for 1 hour. So if your notebook drains only 1 Ampere of current, you battery would last for 3 times longer. But you mustn't worry about the calculations, because you will never know exactly how much your notebook drains, to make the calculations.

  2. Voltage doesn't correlate in any way to the capacity of the battery and doesn't interfere with the lifespan of the battery,

  3. Watts is a measure of power, not capacity, but you can use it to ascertain the capacity of the battery, because Watts = joules/second = energy/time. So again the bigger the number, the better.

For a deeper understanding (if you want), you can go to wikipedia.

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+1, but there are also Lithium-Polymer batteries, which are beginning to get more popular (you can get them on some ThinkPads and MacBooks). This is a good website for anybody that wants to learn more about batteries: batteryuniversity.com –  paradroid Oct 25 '10 at 16:50
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Also, I don't think you can call the Ah rating the capacity of the battery, as it would depend on how many Volts are being drawn. I think Wh would be the capacity. –  paradroid Oct 25 '10 at 16:55
    
@jason404 great link! @Lombas I can very easily find mAh ratings of batteries. –  KronoS Oct 25 '10 at 17:09
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Actually Volts are never drawn. The current is the one been drawn. The voltage remains relatively constant. The capacity of any battery is a function of how much electrical charge it can produce. The charge is measured by Coulombs. One Ampere = one Coulomb/second. 'Ah' in the one that makes sense for battery as the charge will be produced by the chemical reaction over time. 'Wh' is a measure of energy. Nevertheless both are valid ways to say how much energy the battery can provide. –  Lombas Oct 25 '10 at 17:15
    
Yes, drawn was not the best word to use, but when you think of voltage as being the cross section of the river, the flow being the current, you can see what I meant. –  paradroid Oct 26 '10 at 8:51

(spilling as an answer to another thread)
Shortly, the only importance with Volts is that one must buy a battery compatible with the laptop. And the only importance overall is the energy of the battery pack in mWh. But beware that the battery world is known for overstating energy, power and charge. Should you have to compare mWh and mAh, just multiply mAh by Volts to get mWh.

Now, if you're curious and want to make more conclusions... Some theory first

1 C(oulomb) is the electric charge of (really) many electrons ( 6 241 509 629 152 650 000).
It is a quantity of electricity, like a quantity (volume) of water.
1 A(mpere) is the current (like the flow of a river) of 1 C(oulomb) during 1 s(econd) = 1 C/s. (2 A = 2 C/s or 1 C/½s) A river may flow 1 m3/s.
1 mAh (milliAmpere × hour) is the quantity of electricity carried by 0.001 A during 1 hour. Hence, 1 mAh = 0.001 A × 3600 sec = 3.6 C.

The quantity of electricity is not an interesting measure. What matters in electricity to power a device is its energy. The energy is the quantity of electricity multiplied by the difference of potential climbed (Volt). Volts are like the height our river cascades down: energy is water mass × height × g.

Energy mW(att)h = mAh × V(olt).

A problem is that the potential (voltage) of a accumulator varies during charge/discharge. Not much for NiCd (1.1-1.2) but more for Li-ion (3.6-4.2). Hence, for the same current, more energy flows per mAh at 4.2V than at 3.6V or 1.2V. mAh does not account for that. mWh does and can be used to compare energy of batteries at or of different voltages.

Watt is not a energy, it's a power.
The power is the quantity of energy a battery is producing or able to produce per sec or per h. That is why one must "sum up" W during 1 h to get the energy 1Wh = 1W × 1h. A (very special or old) battery could have much energy but little power, that's be able to produce energy slowly, maybe because its internal resistance is high and current is heating it.

So much for the theory, now in practice...

Laptop battery packs are most usually made of 18650 or like Li-Ion accumulators.
The voltage of a 18650 varies from 3.6 when empty to 4.2 when full.
3×18650 are connected in series to produce 3×3.6=10.8 to 3×4,2=12.6 V.
But the battery could use 4×18650 in series, giving 14.4 to 16.8 V.
Unless there is a voltage regulator in the battery, that's all there is to it.

2 or 3 such accumulator series are connected in parallel (all + to + and - to -).
That augments the energy as well as the power by 2 or 3 but changes nothing to volts.
The total energy is the sum of (usually 6× or 9×) a single accumulator's energy.

    ┌─────█████───█████───█████─────┐
- ──┤                               ├── +
    └─────█████───█████───█████─────┘

Supposing each 18650 contains N mWh energy,
- e.g. a 3 18650 series will contain 3×N mWh energy
- the e.g. 2 series 6-pack will contain 2×3×N mWh energy
Supposing each 18650 is at 4V,
- the same series will measure at. 3×4=12V.
- the same pack will also measure at e.g. 3×4=12V.
Supposing you decide each 18650 can safely flow 3A.
- the same series will also let flow 3A.
- the same pack will let flow 2×3=6A.

Regarding "life", this term is a misnomer.
End of life is when you discard something, not recharge it.
The life of a Li-ion battery may be about 3 years after which it is too rapidly discharged. That's when the charge is kept at 100% or 0%, even if not used.
If the charge is kept between 30% and 70%, its life is about 15 years.

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