Battery advice has a habit of outliving the batteries it was written for. Nearly every rule people still repeat comes from nickel-cadmium cells, which have not shipped in a phone this century. Lithium-ion behaves differently, and a few of the habits built around the old chemistry actively shorten the life of the new one. “Let it die completely before charging”: This one is exactly backwards. Deep discharges are the single most stressful thing you can do to a lithium-ion cell. It is happiest between roughly 20% and 80%, and a phone topped up in short bursts through the day will outlast one run flat and filled overnight. “Charging overnight destroys the battery”: Modern phones stop drawing current at 100% and most now learn your alarm and hold at 80% until an hour before it. The cost of overnight charging is not the charging — it is the hours spent sitting at full, which the phone is already managing. “Only the original charger is safe”: Any charger that carries a real USB-PD or Quick Charge certification negotiates voltage with the phone before delivering anything. What matters is the certification, not the logo. Uncertified cables are a genuine risk; third-party is not. “Battery health percentage is the whole story”: It is a useful number and a poor summary. A cell at 85% health that has been kept cool will often outperform one reading 92% that has spent two summers on a car dashboard. Heat ages a battery faster than cycles do. “A replaced battery means the phone is worn out”: A replacement is the cheapest repair a phone will ever need and it resets the part that wears fastest. On the resale market a device with a new cell is worth more, not less — which is why we price it as its own option rather than burying it in the condition grade. What this means when you buy refurbished: Ask for the battery health figure and ask whether the cell has been replaced. Both are on every listing here. A device at 85% or above with an honest grade will serve you better than a higher number attached to no information at all.