Your typical smartphone Lithium Ion battery will last for 300 to 500 cycles before its remaining capacity is 70% of its original charge. A cycle is one full charge and discharge. So if you recharge your battery at 50%, it counts as just one cycle. Your phone will still display a full 100% charge even as your battery life diminishes, but after 300 to 500 cycles, 100% really will just mean 70%. Given that modern smartphones are charged daily, even those with higher quality batteries will be down to 70% of their original battery life in 15-18 months. This is a bit problematic, since we are usually stuck with our phones now on 24 month to 30 month contracts.
Back in the old days, when we would charge a phone once every four days, getting the battery life cut down by half was not all that big a deal. With todays smartphones barely lasting a day with moderate use, lasting 70% of a day is a bit problematic. Making things a bit more difficult is today, you cannot swap batteries on many phones anymore, and usually this is a problem with the more expensive phones.
Pick up one of several low cost budget models, and you usually have a removal rear cover and removable battery. Go into the mid-level and higher end models, that is no longer the case. You cannot replace the battery on Apple's iPhones. User replaceable batteries were standard on HTC phones. Now since the One X, all high end have batteries sealed into the phones case. LG's Optimus G and Nexus 4 also do not have user replaceable batteries. The same is true for Sony smartphones. The last higher end Sony smartphone with a user replaceable battery was the Sony Xperia V. Even Nokia has gotten into this trends, with the battery on their Nokia Lumia 920 not being user replaceable.
One reason given for the loss of the user replaceable battery is the need to make phone thinner. But this is not really true. Samsung Galaxy S 4 has a user replaceable battery and is all of 7.9 mm thick. The BlackBerry Z10 also has a user replaceable battery, and it is as thin as a HTC One. It is not really the thinner part that makes user replaceable batteries difficult to incorporate into current phones, but the use of materials other than plastic. Notably, both the Samsung Galaxy S 4 and BlackBerry Z10 have plastic cases.
The sad part is without user replaceable batteries, instead of these older models ending up in second hand shops in Greenhills after two hard years or use, they will wind up in dumpsites, landfills or hopefully some recycling facility.
The demise of the user replaceable battery has gone unnoticed. Apple users are used to this and many slap on an external battery pack on their phones on its second year. This is not going to be a good solution for a 5-inch superphone. Most Android and Windows Phone users, have phones with sealed batteries which are just a year old. So its another six months to one year, before diminishing battery life turns todays prized possession into a paperweight.