There's been a lot of talk lately about our position on removable storage and removable batteries in smartphones. Most of the discussion has centered around what we've said in podcasts or alluded to in reviews, so we figured it's a good time to have the complete discussion in one central location.

Let's get through the basics first:

All else being equal, removable storage and user replaceable batteries aren't inherently bad things. In fact, they can offer major benefits to end users. 

The key phrase however is "all else being equal". This is where the tradeoff comes in. On the battery front, the tradeoff is very similar to what we saw happen in notebooks. The move away from removable batteries allows for better use of internal volume, which in turn increases the size of battery you can include at the same device size. There are potential build quality benefits here as well since the manufacturer doesn't need to deal with building a solid feeling removable door/back of some sort. That's not to say that unibody designs inherently feel better, it's just that they can be. The tradeoff for removable vs. integrated battery is one of battery capacity/battery life on a single charge. Would you rather have a longer lasting battery or a shorter one with the ability the swap out batteries? The bulk of the market seems to prefer the former, which is what we saw in notebooks as well (hence the transition away from removable batteries in notebooks). This isn't to say that some users don't prefer having a removable battery and are fine carrying multiple batteries, it's just that the trend has been away from that and a big part of the trend is set based on usage models observed by the manufacturers. Note that we also don't penalize manufacturers for choosing one way or another in our reviews.

The tradeoffs are simple with an internal battery, the OEM doesn't need to include a rigid support structure on the battery to prevent bending, and doesn't need to replicate complicated battery protection circuitry, and can play with alternative 3D structures (so called stacked batteries) for the battery and mainboard as well. Personally, I'd rather have something that lasts longer on a single charge and makes better use of internal volume as that offers the best form factor/battery life tradeoff (not to mention that I'm unlikely to carry a stack of charged batteries with me). It took a while for this to sink in, but Brian's recommendation to charge opportunistically finally clicked with me. I used to delay charging my smartphone battery until it dropped below a certain level and I absolutely needed to, but plugging in opportunistically is a change I've made lately that really makes a lot of sense to me now.

The argument against removable storage is a similar one. There's the question of where to put the microSD card slot, and if you stick it behind a removable door you do run into the same potential tradeoff vs. build quality and usable volume for things like an integrated battery. I suspect this is why it's so common to see microSD card slots used on devices that also have removable batteries - once you make the tradeoff, it makes sense to exploit it as much as possible.

There's more to discuss when it comes to microSD storage however. First there's the OS integration discussion. Google's official stance on this appears to be that multiple storage volumes that are user managed is confusing to the end user. It's important to note that this is an argument targeted at improving mainstream usage. Here Google (like Apple), is trying to avoid the whole C-drive vs. D-drive confusion that exists within the traditional PC market. In fact, if you pay attention, a lot of the decisions driving these new mobile platforms are motivated by a desire to correct "mistakes" or remove painpoints from the traditional PC user experience. There are of course software workarounds to combining multiple types of storage into a single volume, but you only have to look at the issues with SSD caching on the PC to see what doing so across performance boundaries can do to things. Apple and Google have all officially settled on a single storage device exposed as a single pool of storage, so anything above and beyond that requires 3rd party OEM intervention.

The physical impact as well as the lack of sanctioned OS support are what will keep microSD out of a lot of flagship devices. 

In the Android space, OEMs use microSD card slots as a way to differentiate - which is one of the things that makes Android so popular globally, the ability to target across usage models. The NAND inside your smarpthone/tablet and in your microSD card is built similarly, however internal NAND should be higher endurance/more reliable as any unexpected failures here will cause a device RMA, whereas microSD card failure is a much smaller exchange. The key word here is should, as I'm sure there are tradeoffs/cost optimizations made on this front as well. 

The performance discussion also can't be ignored. Remember that a single NAND die isn't particularly fast, it's the parallel access of multiple NAND die that gives us good performance. Here you're just going to be space limited in a microSD card. Internal NAND should also be better optimized for random IO performance (that should word again), although we've definitely seen a broad spectrum of implementation in Android smartphones (thankfully it is getting better). The best SoC vendors will actually integrate proper SSD/NAND controllers into their SoCs, which can provide a huge performance/endurance advantage over any external controller. Remember the early days of SSDs on the PC? The controllers that get stuffed into microSD cards, USB sticks, etc... are going to be even worse. If you're relying on microSD cards for storage, try to keep accesses to large block sequentials. Avoid filling the drive with small files and you should be ok.

I fully accept that large file, slow access storage can work on microSD cards. Things like movies or music that are streamed at a constant, and relatively low datarate are about the only things you'll want to stick on these devices (again presuming you have good backups elsewhere).

I feel like a lot of the demand for microSD support stems from the fact that internal storage capacity was viewed as a way to cost optimize the platform as well as drive margins up on upgrades. Until recently, IO performance measurement wasn't much of a thing in mobile. You'd see complaints about display, but OEMs are always looking for areas to save cost - if users aren't going to complain about the quality/size/speed of internal storage, why not sacrifice a bit there and placate by including a microSD card slot? Unfortunately the problem with that solution is the OEM is off the hook for providing the best internal storage option, and you end up with a device that just has mediocre storage across the board.

What we really need to see here are 32/64/128GB configurations, with a rational increase in price between steps. Remember high-end MLC NAND pricing is down below $0.80/GB, even if you assume a healthy margin for the OEM we're talking about ~$50 per 32GB upgrade for high-speed, high-endurance internal NAND. Sacrifice on margin a bit and the pricing can easily be $25 - $35 per 32GB upgrade.

Ultimately this is where the position comes from. MicroSD cards themselves represent a performance/endurance tradeoff, there is potentially a physical tradeoff (nerfing a unibody design, and once you go down that path you can also lose internal volume for battery use) and without Google's support we'll never see them used in flagship Nexus devices. There's nothing inherently wrong with the use of microSD as an external storage option, but by and large that ship has sailed. Manufacturers tend to make design decisions around what they believe will sell, and for many the requirement for removable storage just isn't high up on the list. Similar to our position on removable batteries, devices aren't penalized in our reviews for having/not-having a removable microSD card slot.

Once you start looking at it through the lens of a manufacturer trying to balance build quality, internal volume optimization and the need for external storage, it becomes a simpler decision to ditch the slot. Particularly on mobile devices where some sort of a cloud connection is implied, leveraging the network for mass storage makes sense. This brings up a separate discussion about mobile network operators and usage based billing, but the solution there is operator revolution.

I'm personally more interested in seeing the price of internal storage decrease, and the performance increase. We stand to gain a lot more from advocating that manufacturers move to higher capacities at lower price points and to start taking random IO performance more seriously.



View All Comments

  • repoman27 - Wednesday, November 27, 2013 - link

    Well, Anand and Brian are also right because their arguments are based on actual physics and valid logic. Reply
  • Spunjji - Wednesday, November 27, 2013 - link

    Please cite the valid logic you speak of and I'll cite a ton of counter evidence... Reply
  • vortexmak - Wednesday, November 27, 2013 - link

    Evidence: GS4 - slim, larger, longer battery life includes an sd card slot as well. I don't see any other non-replaceable battery phones having a significantly longer battery life Reply
  • repoman27 - Wednesday, November 27, 2013 - link

    Reality: Given two batteries of equal energy density and differing volumes, the larger battery will provide more energy. Sealed batteries are more volumetrically efficient than those that are user replaceable. Therefore, given the same volume and chemistry, the sealed battery will have a higher energy density and provide more energy.

    More reality: All microSD card slots require interior volume in a device. That volume is equivalent to 4-6% of the volume of the batteries found in the current flagship smartphones.

    Hypothetical: If you asked all smartphone users whether they would rather have 4-6% longer battery life or a microSD card slot in their next device, what would you fancy the outcome of that poll would be? What if you then asked them if they would like to have both but in a slightly larger device?
  • Hairs_ - Thursday, November 28, 2013 - link


    Improving screen technology so that they suck less power, and changing the way mobile networks are deployed would instantly gain far more battery life than either of the sacrifices you're talking about.

    Would you rather lose 4% battery life and gain cheap storage and a phone that isn't bricked because a single replaceable component failed, or gain 100% battery life with a better designed screen?
  • repoman27 - Monday, December 2, 2013 - link

    Neither screen nor mobile network technology is relevant to the argument pertaining to microSD card slots and removable batteries; they are simply red herrings. Furthermore, no smartphone has ever shipped with a screen that was 50% less power efficient than any screen of similar size, pixel density, maximum brightness and color gamut available in production quantity at that time. Also, no handset OEM directly controls the deployment of any mobile network.

    The total increase in battery life achievable by using sealed batteries and omitting microSD card slots is realistically in the 10% range, and potentially higher than that. If the on-board NAND flash memory fails, the device will be non-functional regardless of whether there is a microSD card installed or not. Thankfully, failures of this nature are incredibly rare. If a sealed battery fails, or the maximum charge level falls below a reasonable percentage of the original capacity, you can have it replaced. A device with a failed user replaceable battery is just as bricked if you don't own a second battery. Most owners of smartphones with removable batteries never purchase a second battery for their phone.
  • Hairs_ - Monday, December 9, 2013 - link


    You claim that a 5% increase in potential battery volume is significant enough to warrant ditching expandability, but claim that any other measure to increase battery life is "irrelevant"? Are you really going to set out a stall on "this needs to be done to increase battery life" and then ignore factors which influence battery life to a much greater extent based on simple physics on the basis they're "red herrings"?

    And when did your volume estimates for ditching these factors jump from ~5% to 10%? Did you calculate incorrectly the first time? Or are you just fishing for random numbers to back up an argument that didn't stack up the first time?

    Note that I never claimed "Phone A shipped with a screen 50% more efficient than Phone B". I said, very clearly, that if an engineer is given a choice between ditching SD cards and gaining 5% battery capacity, or working on improved screen technology to lower battery drain by 50%, one is very much a larger benefit than the other.

    If you choose to ignore that, then you're not much of an engineer.
  • repoman27 - Monday, December 16, 2013 - link

    Until phones are powered by unicorns, the size of the battery determines how long a given device can function. Every OEM makes their devices as efficient as is technically possible while still ticking all the feature boxes and trying to get the best scores possible in the benchmarks. Then they fit it with the smallest battery they can get away with before people start massively complaining about how bad the battery life is, so that they can tout how thin and light their device is compared to the competition. In fact, the less power the screen or radios draw, the bigger impact a given reduction in battery volume will have on device run time.

    Clearly when I said, "The total increase in battery life achievable by using sealed batteries and omitting microSD card slots is realistically in the 10% range," I added the effect of doing both (see how I used the words "total" and "and"). Does something appear incorrect about my estimate? Why don't you go ahead and do the maths yourself. If you do, you'd realize that if making the battery on the iPhone 5 non-user removable allowed Apple to increase its overall size by as little as 150 µm in each dimension, the result would be a greater than 5% improvement in battery life.

    And I'll note that you made the rather specious claim that it was possible for a smartphone to, "gain 100% battery life with a better designed screen." That would only be possible if such screens existed in the supply chain in sufficient quantity and at a reasonable cost during the same time period that the device was slated for assembly. Do you really think the screen suppliers are sandbagging and just holding out on some new super low-power tech? Considering that Samsung and LG both make screens themselves, and a supply contract for an iPhone screen is a 3 comma deal, I find it highly unlikely that they're just not bothering with display power optimizations.
  • Klug4Pres - Thursday, December 19, 2013 - link

    "Then they fit it with the smallest battery they can get away with before people start massively complaining about how bad the battery life is, so that they can tout how thin and light their device is compared to the competition."

    According to you, this is presumably a good thing, and we should all be grateful that OEMs pay so much attention to the bottom line. You keep going on about BOM and whether x or y missing feature is going to lead to a loss of profit.

    Some people are actually more interested in the usefulness of a device, and I think the arguments in these comments show that there are many useful things one can do with SD cards and removeable batteries.

    In fact, it is precisely Anand's and Brian's excessive concentration on the producer interest that is annoying everyone here.

    Of course, there is the absurd figleaf that we can collectively rectify the poor value offered to consumers by "demanding" that phones come with more internal storage and a reduced markup on additional storage tiers. While we're at it, let's all demand that OEMs make thicker phones with bigger batteries - oh wait: "Then they fit it with the smallest battery they can get away with."
  • repoman27 - Friday, December 20, 2013 - link

    I think you've missed my point when it comes to the BOM. The battery is the least expensive item on any smartphone bill of materials. OEMs could double the size of the battery they include and it wouldn't even move the needle on their margins, yet they would trounce their competitors when it comes to run time. There is clearly a belief that keeping these devices as thin and light as possible will result in better sales than longer battery life will.

    I'm not saying we should be grateful to the OEMs for being good bean counters, or looking out for their profits, I'm merely saying that if you want to understand their design considerations, the bottom line is a good place to start. I don't believe Brian and Anand want their readers to empathize with the manufacturers either, they're simply pointing out that if you look at it from the OEMs perspective, it's easier to see why we are where we are, and where we're likely headed.

    So do you propose that things will get better if we encourage every OEM to stick with 16GB of NAND and a battery that lasts 3 hours as the baseline, as long as there's a microSD slot and the battery is swappable? So that those who are so inclined can buy and carry additional microSD cards and batteries to correct the shortcomings inherent in the device? I'm much more of the opinion that this site, and we as consumers, should reward the companies willing to deliver a better package—one that comes with a reasonable amount of storage and a decent battery in the first place.

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