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I know this is kind of a basic question, there might be a few ways to answer it...

Recently I found out there is a slim extended battery which has the same size as OEM battery (I mean really same size height and width, not using a different battery cover,) how is that possible?

In simply physics, if the OEM Battery already max out the capacity of storing mAh in such a size, how is the Extended Battery able to have more capacity with the same size?

For example, I found original battery capacity is 2800mAh But here the extended battery can be up to 3800mAh, 4400mAh, that's a huge difference...

Are those extended batteries just fake products?

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    \$\begingroup\$ A good back-of-the-envelope calculation is to take the known energy density of the chemistry and apply it to the dimensions of the product being examined. \$\endgroup\$ – Ignacio Vazquez-Abrams Nov 26 '15 at 2:21
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This can be false marketing. This can also be more or less true and here's how.

First, battery technology slowly advances. One major power tools producer launched its 10.8 volts line of professional power tools in perhaps 2004 and they shipped with batteries assembled of 1Ah Li-Ion cells. In perhaps 2007 they switched to 1.3 Ah cells of the same size. Later they somehow found 1.5 Ah cells and 2.0 Ah cells still of the same size. Nowadays tools from that line come with 1.5 Ah and 2.0 Ah batteries. Newer batteries can be used on older tools and older batteries can be used on newer tools. I have no independent data to back the claim that batteries capacity indeed increases as claimed but batteries are key elements to power tools so I assume a major tools producer would not fool us like that.

Second, it depends on how you measure. You can charge the battery to different states. One major power tools producer claims certain Li-Ion batteries are charged fully in 35 minutes. Answers to this question explain that it's simply impossible and the most likely scenario is that the battery is actually charged to about 80% which isn't that bad. So if you know that the target device only charges a battery to 80% you can charge your battery to 100% when measuring capacity. Customers will be unable to use that full capacity but you can simply disclose your measuring procedure in the finest print available.

Third, you could find a battery with slightly thinner case which simply holds more chemicals and thus has larger capacity.

Fourth, you could find a battery that uses more compact safety features or lacks some of them. Major battery producers make some effort to ensure that batteries don't explode or burn under heavy loads. That also takes some volume of the battery. Make those less reliable or remove them - and you can fit more chemicals into the battery and get larger capacity.

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  • \$\begingroup\$ So based on what you said "Battery technology slowly advances" . Since the OEM battery released only 1 year ago, I don't think they can have such a big improvement within one year. Even if the battery do provide the capacity as advertised, it might because removing necessary security materials..... That's lame :( And thanks for the answer. \$\endgroup\$ – King Chan Nov 27 '15 at 6:09
  • \$\begingroup\$ Additionally, the way a battery is discharged can affect the total capacity. Most batteries have an optimal discharge rate. Discharging rapidly causes the battery to lose energy. Ratings can be exaggerated by claiming the capacity for an ideal discharge curve, even if the customer would never use the battery like that. \$\endgroup\$ – Robert Stiffler Nov 27 '15 at 8:56
  • \$\begingroup\$ @KingChan When a new phone is designed only well known batteries are often considered. So it's natural to have a phone release with a rather old battery model - maybe even several years old because a lot of effort was put into pre-selection and thorough testing of the battery. \$\endgroup\$ – sharptooth Nov 30 '15 at 11:00
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Usually just marketing

Most aftermarket batteries are labelled in way that is extremely optimistic (if not an outright lie).

It is possible to change chemistries to go to one with a higher energy density, but this is usually very unsafe as the phone's internal battery charge regulation circuitry is not tuned for the new chemistry and can cause a fire or explosion as a result.

It is also possible to gain capacity by utilizing the volume better. Most cell-phone battery packs have integrated safety electronics (current-limit, thermal fuse, short-circuit protection, and/or similar safety features). Using more advanced/smaller packaging for these electronics can yield more volume for active battery material.

Similarly, you can use more advanced/thinner substrates to get more active material into the same volume -- but these improvements are typically small (maybe no more than 10% in most cases).

So...

Yeah, it's probably just a fake product built on the (probably correct) assumption that nearly all buyers will not test the battery's capacity to verify the claim (e.g. they won't get caught).

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    \$\begingroup\$ Thanks! I love your answer, simple and easy to understand. But @sharptooth also mentioned how battery technology slowly advances, which I didn't know. So I mark him as answer. (I wish I can mark you as answer as well!) \$\endgroup\$ – King Chan Nov 27 '15 at 6:11
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Hypothetically, battery technology could have improved between the release of your phone and the release of the third-party battery. It's not very likely though, as the other commenters have said, given that battery technology hasn't moved much in the last few years.

My first mobile phone though, came with an NiCd battery. A later released battery, from the original manufacturer, was an NiMH battery, and was indeed both slimmer and had higher capacity than the original one (NiMH has up to three times the energy density of NiCd).

In the end I bought an NiMH battery even thicker than the original, and never had to charge my phone outside weekends. Those were the days. :-)

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A larger capacity battery is always heavier ,thicker than the lower capacity battery you are replacing usually . If they are li-Ion as I am sure they are there is only one type approved for cellphones and that is the Lithium Cobalt Oxide(LiCoO2) because of their relatively long life and safety .Normal type are 150–200Wh/kg. but these higher capacity ones you are looking at maybe specialty cells which can provide up to 240Wh/kg ( 37% more ) which matches your 2800mAh to 3800mAh increase quoted , a 36% increase .

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    \$\begingroup\$ Is always, or usually? this is contradictory. \$\endgroup\$ – Icy Nov 26 '15 at 8:26
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    \$\begingroup\$ "approved for cellphones" where? And what keeps an off-brand aftermarket manufacturer from using a different chemistry or building some of the electronics inside the pack in a way that they will discharge the battery deeper, especially if not safety but just life expectancy is worse - the nice thing with batteries is that you can always blame reliability problems on usage patterns to end users, and given most people do not even know that managing the battery is the device's job with lithium.. \$\endgroup\$ – rackandboneman Nov 26 '15 at 11:29
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In my experience, getting higher capacity cheap batteries is not that hard. However, they are rarely worth the trouble since they lose 10% of the advertised capacity already on the shelf, are down to the nominal capacity of the original battery within a month, to the actual remaining capacity of the original battery within another month, and dead within another. I guess that they don't have enough or proper material inside to withstand and contain the onslaught of the rather aggressive chemicals storing the energy.

It would be nice if batteries had to advertise the charge they are expected to provide over their lifetime rather than merely a one-time charge capacity.

So I try steering clear of cheap high-density offerings. Particularly if they don't even fall short all that much of the original capacity claims, they are likely to degenerate far too fast for my liking.

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A modern new stock Li-Ion battery of the same size cannot have more mAh . If you see a Li-Ion battery for your phone that states more mAh it is a lie. The only way to increase mAh is to make the battery larger. They usually supply a different back panel for the phone that allows for the thicker battery. With today's smart phones I'm not seeing that as much as I did 10 to 15 years ago. I bought a couple of the so called 4600mAh batts for my Note 8 that had 3300mAh batts originally. I saw no difference in battery time. These larger mAh claims are fake specs!

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  • \$\begingroup\$ Hi and welcome to EE.SE! Thank you for becoming a contributor! I just wanted to make sure you noticed this question is from 2015. It's OK to answer old questions, but keep in mind it will bump them to the top of the stack and also that the OP is not likely to appear. When you do answer a very old question, try as much as possible to be sure your answer is complete and correct, check spelling and grammar. \$\endgroup\$ – K H Jan 13 at 1:06

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