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

  • solipsism - Wednesday, November 27, 2013 - link

    @ THX,

    And every time you switch your battery your phone is turned OFF. It's perfectly fine if you're OK with powering down, taking apart, switching components, and turning your phone back on in order to get a decent battery life, as well as having to buy an additional component you need to travel with just to charge the battery or else do your "musical batteries" maneuver twice to charge the other battery in the middle of the night, but this is not the simplest or most common action make or want. You need to accept that your method, while perfectly fine for you, is as archaic as one of the asshats on AT saying that people that don't build their own PCs shouldn't own one.
  • THX - Wednesday, November 27, 2013 - link

    Haha and "YOU" need to "ACCEPT" that just because tons of other people do something that "YOU" don't do it's not weird or archaic. Or are you just playing up to your username? Reply
  • Spunjji - Wednesday, November 27, 2013 - link

    Dude, blimps are huge. ;) Reply
  • greywolf0 - Monday, November 25, 2013 - link

    Can't you just buy one of those cases with large integrated batteries? I mean, if you have to carry extra batteries around, I don't see why not just integrate it all together. Nowadays, 90% of the phone is nothing but the battery anyway. You won't need to deal with removing the back cover and wearing it down or the phone, you won't have to worry about losing your spare batteries, and you won't have to deal with the whole physical operation or having to turn off your phone and swapping batteries and waiting for your phone to turn on again, saving precious minutes of busy city life. No downtime. Your phone will always be on. Reply
  • THX - Tuesday, November 26, 2013 - link

    Less than a minute to get your phone back to 100% battery life vs having it attached to the wall (or carry around a wired charger)? Not sure where you're seeing the benefit in the latter.

    Been doing that to two Android devices for years previously and nothing ever broke. And never lost a battery either. Would prefer not to add bulk to my phone with a thick battery case.

    To each his own...
  • solipsism - Tuesday, November 26, 2013 - link

    He mentions an extended battery and you talk about caring around a wall charger? WTF?! Reply
  • Tams80 - Tuesday, November 26, 2013 - link

    Battery cases add bulk and unless you have an iPhone, or one of the very few Android devices with one, then there just aren't any. Reply
  • yelped - Monday, November 25, 2013 - link

    Great article! Reply
  • gunsman - Monday, November 25, 2013 - link

    As someone who does cyanogenmod nightlies, i have a need for the micro sd slot, constant nandroid and titanium backups destroy my storage, and i have had to reformat my internal 5 times so far just because i keep losing it, i can just put important things on the external, format, and be on my way, although removable batteries i could care less about as i have a portable 7000mAH charger if needed Reply
  • heartinpiece - Monday, November 25, 2013 - link

    If you can get like a 20% increase in battery life, for internal, and unremoval batteries, I'd rather have a exchangeable battery. That'll be 200% battery charge compared to the 120%.
    Now Anand's claim that laptop batteries have moved on to be non-exchangeable is quite sensible, I think Anand missed the size of the batteries of phones and laptops. Phone batteries are quite small. And I think due to this smallness, people like me might want to keep extra batteries in our backpacks just in case. Unlike laptops, phones are more essential in daily life. You need these things to make emergency calls(like tell someone you'll be running 4 minutes late, etc). Laptops aren't used for such emergency (well not as much)

    So, yeah, I think in conclusion I prefer exchangable batteries.

    But again, non-exchangeable batteries do seem to make the phone more well made... More sturdy, firm, and perhaps results in a better finish.
    I myself got a nexus 5, and because the nexus 5 does have some great form I'm happy with it.
    But being a former galaxy nexus user, and since I had kept spare batteries in my backpack, the fear of drying out of battery in the middle of nowhere does give me the spooks....

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