Q. Is it safe to use a universal phone charger?

In addition to harming the phone's battery the universal charger will drastically reduce its life span. Most importantly they are not guaranteed safety. It's not healthy for the phone's battery to be at the full battery or low battery thus the phone intentionally keeps it at around 99–90% battery maybe a touch less. Even with an extra charger mine still does it. Don’t let the phone's battery go below 40 percent for too long and don’t let it fully charge if planning on using it for more than a few days. However letting the phone's battery go completely dead every so often and then fully charge it again will help maintain its battery life.