Describe the change before choosing a part
A phone that loses charge during navigation presents a different question from a phone that drops sharply while untouched overnight. Write down the model, software version, approximate battery age and when the behaviour began. A recent update, new app or change in daily use belongs in the history; it is a clue rather than proof of a fault.
For a useful comparison, record the starting percentage, finishing percentage, elapsed time, screen use and connection type. Compare similar periods instead of treating two very different days as a controlled test. Avoid repeatedly running a battery flat just to produce a dramatic result.
Use the phone's own usage information
Google's Pixel guidance starts with software and app checks and includes tools for investigating battery use. Menu names differ across manufacturers, so use the instructions for the exact device. Look for whether the reported activity fits what the owner actually did, rather than declaring the largest number on a chart to be a defective app.
An app may lead the usage list because it was used most. Ask whether it was active on screen, working in the background or associated with the period of unexpected drain. Keep screenshots limited to the relevant settings, with private notifications and account details excluded.
Change one condition at a time
Choose a reasonable next comparison: a period on a reliable Wi-Fi connection, a normal restart, or observing whether the problem follows one recently installed app. Record exactly what changed. Changing brightness, networks, apps and charging equipment together makes the outcome difficult to interpret.
A technician's note might say: 'Owner reports drain during the commute; idle behaviour at the workshop has not reproduced it.' That is more useful than saying the battery is healthy after a short visit. A brief check cannot reproduce every usage pattern or establish an accurate remaining service life.
Know when the next step is service
Unexpected shutdowns, visible swelling, physical damage or unusual heat change the priority. Stop charging or using a damaged or swollen device and arrange qualified service. Do not press the battery back into place or attempt to improve its shape.
If ordinary checks do not explain the change, bring the usage log and charging accessories to a technician. Ask what evidence supports any recommended replacement and what will be tested afterwards. Battery percentage alone is not a complete diagnosis, and a factory reset should never be a casual first experiment with someone else's data.
Sources and further reading
Manufacturer and technical references for the checks discussed in this guide. Follow the instructions for your exact device and equipment.
Repair procedures involve heat, electricity, fragile parts and customer data. Practise under qualified supervision and work only on devices you are authorized to service.