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.



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  • thesavvymage - Tuesday, November 26, 2013 - link

    I find it awesome how in south korea nearly every phone has a removable back and comes with a spare battery and an extra charger for it Reply
  • gnx - Monday, November 25, 2013 - link

    "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."

    The authors' logic is overall sound and well taken, but at the end, what incentive or gain (do the authors reason or forsee) exists for manufacturers to lower the price of additional internal storage?

    Are there any? It'd be nice to know. Or is this more of just a whimsical wish? (And are we customers doomed to be fleeced for higher corporate profit margins?)
  • Spunjji - Wednesday, November 27, 2013 - link

    If the author's wish of the SD card going the way of the dodo comes true then there will be no incentive whatsoever for that to happen because there will be 0 competition. It is a whimsical wish. If there was any desire for these companies to compete with each other they'd have started already. As it is, the indication is that since the arrival of the iPhone, every manufacturer has struggled to find a way to raise their prices rather than lower them. Reply
  • WelshBloke - Monday, November 25, 2013 - link

    I'm curious as to how SDcard performance really matters. How fast does a card need to be to load a 25mb app into memory?
    If I can stream my HD video or off line sat nav maps does it really need to be any faster?

    I'm also curious as to the "trends" you mentioned. It would be interesting to see a break down of phone numbers sold with/without sdcards rather than just saying that there's more models being made without SDcard slots (that may well have appalling sales figures).
  • dawheat - Monday, November 25, 2013 - link

    The thing is all things are not equal. Apple charges $200 for a 48GB increase in internal storage. A new class 10 microSD card costs $42 for a 64GB increase in storage. That's a huge value proposition for consumers and if there was more of an end-consumer focus on this site, microSD support wouldn't be just a 'nice' feature, but a praised 'must-have' feature UNTIL manufactures stop abusing customers. There's too much no man's land in your comments - microSD isn't needed but manufactures don't offer reasonable alternatives today.

    And those buying large capacity iPhones I'd bet are generally filling up the extra storage with the exact type of media that is ideal for microSD - music, movies, and photos.

    Reviewers seem to sometimes live in their own world - if the world operated within their reality distortion field, the HTC One would be the best selling Android phone in the world. Instead, HTC is losing money and no longer a top 5 manufacturer.
  • A5 - Monday, November 25, 2013 - link

    Samsung isn't winning because they make the best phone. Reply
  • Drumsticks - Monday, November 25, 2013 - link

    @Dawheat: I never get this argument. Samsung doesn't make the best phones. Note the better selling iPhone without microSD or removable batteries.

    Both companies who make good, but not *clearly* the best phones, who have taken completely opposite routes and ended up pretty similar.
  • Spunjji - Wednesday, November 27, 2013 - link

    Word. Reply
  • stotticus - Monday, November 25, 2013 - link

    First, this article and its arguments have somewhat put to rest my DIY foolishness and the resulting resentment that I can't open my N4 up in any way. Not mentioned but supporting the point about carrying replacement batteries around is that there are now many good universal rechargeable batteries on the market. People can exercise that option without being able to open their phones.
    Also, I particularly liked the witty toss away, "but the solution there is operator revolution." Well done!
  • iwod - Monday, November 25, 2013 - link

    On the Note for Battery. How much more Battery life could we squeeze out? Since Battery Capacity just simply isn't improving. Assuming we could have CPU running 4x more powerful at the current lowest state of battery level. Which means most of the multi tasking in background should no longer sucks up much battery life. Assuming we get even better Display Screen Tech in the pipeline and those extreme efficient LED, assuming Stacked Memory comes in, assuming better LTE controller and state of art Antenna system.

    Even with all of these aren't going to get us double the battery life. More like 30 - 40% max. And some of these are theoretical.

    So I think the answer in the short term isn't about getting bigger battery. Is that Battery needs to recharge superfast that makes most battery problem non issue. If you could fully charge a battery in less then a minute?

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