I have an HP 15-d053cl laptop with a 14.8V, 2620mAh, 4-cell battery (although HP's website says it's 2800mAh) and I'm looking for something to add to my battery life. I've found my same battery and an external power pack for the same price, but the power pack is rated at 16/19V and 5500mAh. I thought I remembered reading somewhere that the mAh rating on a laptop battery is the charge per cell, but I'm not sure. Is laptop battery charge really measured per cell, in which case a replacement battery would be 10480mAh and it would be the better deal, or is that the charge of the entire battery, in which case the power pack would be a better deal? I've also found a 14.4V, 2200mAh, 4-cell battery for half the cost of the other two. Would buying two of those give me more battery life than either of the other two?
4 Answers
The battery pack's capacity rating is for the entire pack, not just a single cell.
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3Brad is correct. If the rating is on the outside of the battery pack, then it's for the entire pack. Of course, if the pack consists of a single "string" of cells in series, then the mAh rating is the same for the pack as it is for each cell, because in such a configuration, only the voltages add. Most modern packs though are made in a series-parallel configuration, and the pack mAh rating is equivalent to the combined ratings of one parallel set. Mar 2, 2015 at 5:24
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mAh is a quick indicator of what sort of output a rechargeable battery is capable of. For example, 2000mAh means a battery can output 2 A(mps) for one hour at a given voltage.– astv25Mar 2, 2015 at 5:43
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@astv25 It is not quite that simple: at a higher current draw, less useful energy can be extracted from the cells. For more information, Does mAh measure how long a battery would last?. In a laptop computer, the difference would be most noticeable between a sleep state and fully on. Jan 12, 2016 at 19:52
There's no difference. A 4-cell, 14.8V lithium pack consists of four 3.7V lithium cells connected in series. Since they're connected in series, the same current flows through all the cells. So when you've drawn some number of mAh out of the pack, you've drawn that same amount of mAh from each cell as well.
Think of it like a line of men relaying heavy rocks up a hill. If you add men, you still move the same number of rocks, you can just move them up a bigger hill. The cells connected in series all carry the same current just like the men on the hill each move the same rocks. The four men let you move the rocks over a bigger hill, just as the four cells let you supply a larger voltage.
Keep in mind, mAh is a measure of capacity, not power. A measurement of mAh is like a measure of how many rocks the men can move. If each man can move X number of rocks, the row of four men can move X number of rocks.
To get the energy capacity of a pack, multiply the voltage by the mAh rating. Note that this isn't perfect, especially when comparing packs by different manufacturers, because they don't use perfectly consistent methods of measurement.
If the total volts for 4 batteries is 14.4 volts then mAh of each battery is 3200. Thus, the total 4 batteries mAh is calculated or the mAh remains constantly if we add several batteries by maintaining the volt constantly.
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1Where did 3200 come from? There's no information here that leads to such a number. Jan 13, 2016 at 1:52
Tl;DR: see last sentence.
The laptop pack is direct output of the batteries although there may be an electronic switch in the way. It's usually 11.1v or 14.8v, a multiple of 3.7v which is the nominal voltage of a LiCoO2 cell (the most common chemistry used in laptop battery packs).
For your 4-cell pack the 2620mAh is the rating of each cell, but if they made an 11.1v 6-cell or 14.8 8-cell version with the same cells those would be rated 5240mAh (2x as the additional batteries would be in parallel).
The actual power in watts the pack holds can be calculated by multiplying the Ah (1000mAh == 1Ah) with battery voltage (That is very theoretical though as the voltage will decrease as the battery discharges, and there are other factors such as power loss in various places). So, with equal mAh rating the 14.8v pack would have more power.
Now for the power pack I suspect they measure the sum mAh of all cells rather than the output voltage, i.e. as if all cells were in parallel. This would particularly make sense for those that have multiple output voltages (else which one would be used anyway??)
Furthermore it's likely easier to have all cells in a pack in parallel (internal 3.7v output), or even a single large Lipo cell, which could reasonably explain the way of calculating the mAh. Unlike laptop batteries where the laptop converts 11.1v/14.8v input to the various voltages used by the computer (3.3v, 5v, 12v), power packs needs to output a very specific voltage, usually 5v and 12v for newer packs supporting USB Hi-powered ports. This means with a 3.7v pack all that is needed on top of the charge controller is a DC step up circuit for 5v, 12v, etc.
Any serial configuration would also require a step-down for the 5v ports which require additional circuitry. More importantly, Li-Ion batteries using cells in series needs a balancer circuit to ensure each group of cell remains balanced while charging and doesn't go under-voltage while discharging. LiCoO2 can break down and burn when overcharged or over-discharged, and reading 11.1v on a 3-cell pack doesn't mean the cells are actually at 3.4v + 4.1v + 3.6v. The balancer will steal/dissipate excess power to individual cells so they don't overcharge and cut the load if any cell goes too low.
Of course if all cells in the pack are in parallel, you get 3.7v, mAh is the sum of all cells and you never have to balance the cells. It also helps marketing a higher mAh rating!
Therefore, I suspect if you do the math, your 5500 mAh pack is actually the equivalent mAh of a 1375 mAh 14.8v pack (5500 * 3.7 / 14.8) minus power loss in the step up circuit. Since LiCoO2 is pretty much the highest energy density you can get, the volume/weight of the the power pack vs. laptop battery should also be a good indication this is the right math. The power pack should probably be about twice as big/heavy (or a bit less for Lipo) if it had twice the amount of power.
