I kept getting some errors on a Restore/Update procedure, linked to battery malfunction.
Easy repair to do. I had previously paid for a local repair to change my battery about 9 months back, but apparently it wasn't a high quality replacement because it gave me problems without even reaching a year of use. It was expensive and I had no control over the quality (or shelf life) of the spare. So this second time, I decided to do the repair myself. The procedure was simple (with the proper tools), and not only did I save myself about 60% of the cost of the first replacement, but I am confident that the battery quality is WAY better. So far, the phone is working fine. Since it is an older phone, my daughter is using it (she carries it around for emergencies and chatting, and moderate video downloads over WiFi), and I expect her to use it at least 'till Christmas, since she might be asking for her holiday present to be a modern iPhone.
Once the phone is out of warranty, do-it-yourself is the way to go. Saves more than a penny, and you are aware of what is done to the phone inside. It's good to practice with an older phone too. After this repair, I was ready to change the WiFi Antenna Flex Ribbon Cable on my own iPhone 6 (quite longer and harder procedure), but it all went well.