Samsung's 2016 Galaxy A3 recently got its update to Android 7.0 (Nougat), and according to one user, that update has caused their phone to no longer charge beyond 87%. Additionally, charging now takes far longer than it used to, and some users are even reporting their batteries draining more quickly than they should. These problems have been confirmed by a number of users, and seems to be affecting all or at least most users who upgrade their devices to Nougat. The usual troubleshooting steps, and even re-calibrating the battery, don't seem to be doing any good, and almost all users who manage to successfully roll back to Android 6.0.1 (Marshmallow) are reporting their batteries working perfectly again.
For the time being, users who want to keep their batteries performing as they should have no recourse except to abstain from the update, if they haven't taken it already, or roll back to a previous Android version. The issue has reportedly been brought to Samsung's attention on the company's official support forums. While Samsung has yet to issue any official statement on the matter at this time, Samsung is said to be aware of the issue, has acknowledged that it is indeed on the software side, and will get a patch out with a fix in the near future, according to a Samsung representative.
For those who do have the update and are having this issue, the steps to roll back include using the ODIN flashing suite or the open-source Heimdall alternative, and it is highly recommended to make a backup of your phone's internal firmware and any personal files or data stored on it before going through with the flash. There is no official way to roll back your Android version in order to avoid this problem, so you'll need to grab the correct firmware file for your phone, then extract the file you get with an archive tool, and use ODIN to flash the proper parts of the firmware to the proper partitions on the phone. If you don't have ODIN set up, all you have to do is download the Samsung USB drivers, install them, connect your phone, then boot it into download mode and fire up ODIN. At that point, the application should be able to see your phone.