How to Check a Used Phone’s Battery and Condition Before Buying?

Why the battery is the „heart” of the phone and how it ages

When buying a used smartphone, the condition of the battery is one of the most critical aspects that determines its future usability. Studies show that for the majority of users, battery life is a priority. Unfortunately, all modern phones use lithium-ion (Li-Ion) batteries, which are chemically aging components. This means that their capacity decreases not only over time but also due to the number of charging cycles.
[Image of lithium-ion battery chemical aging process diagram]
It is important to distinguish between instantaneous battery life (how long the phone lasts on a single charge) and battery health (how much maximum capacity remains compared to new). Manufacturers like “Apple” indicate that a battery is considered worn out when it reaches 80% of its original capacity (usually after 500–1000 full charge cycles). A worn-out battery not only causes inconvenience due to frequent charging but can also lead to processor slowdowns (to prevent sudden shutdowns) and device overheating.

Visible warning signs that should not be ignored

While software displays numbers, a physical inspection often reveals the most dangerous defects. When buying a phone “from hand,” pay attention to these symptoms:

Swollen battery and body deformities

This is the most serious signal. Due to chemical failure, gases accumulating in the battery can cause the body to swell. Place the phone on a flat table: if it spins like a top or you see a gap between the screen and the frame, the battery is dangerous (it may catch fire) and needs to be replaced immediately.

Temperature and charging behavior

Try charging the phone for at least 10–15 minutes. If the device becomes unusually hot or the percentage jumps suddenly (e.g., from 20% to 40%), this indicates a faulty controller or chemically degraded cell. Also, check the charging port: look for corrosion (greenish deposits) and whether the cable fits securely.

iOS tools, cycle counting, and computer diagnostics

In the Apple ecosystem, the check is quite standardized, but sellers may try to hide the true condition.

Integrated Settings

Go to Settings -> Battery -> Battery Health & Charging. The number “Maximum Capacity” is the key indicator.

• 90–100 %: Excellent condition.
• 80–89 %: Average condition, the phone will work well, but capacity is decreasing.
• Below 80 %: The battery is considered worn out, likely needing replacement.

Be sure to check the “Peak Performance Capability” section. If you see a message about “unexpected shutdowns,” it means the battery can no longer supply the necessary power to the processor.
[Image of iPhone battery health settings screen]

Deeper Analysis (Cycle Count)

To find out the exact number of charging cycles (which Apple hides in older models), it is recommended to use computer programs:

• 3uTools (Windows) or CoconutBattery (Mac): When you connect the phone via cable, these programs show not only the “health” percentage but also the exact cycle count and whether the battery is original. This is the best way to expose sellers who have programmatically “reset” the battery percentage but did not change the actual battery.

Secret codes, manufacturer tools, and third-party apps

The situation in Android phones is more complicated due to the variety of models, but there are reliable methods.

Samsung and Xiaomi specifics

• Samsung: Use the “Samsung Members” app (section “Diagnostics” -> “Battery Status”). It will show the condition (“Good”, “Normal”, “Weak”). The old code *#0228# often no longer works on new models.
• Xiaomi / POCO: Dial the code *#*#6485#*#*. In the opened window, look for parameters:
  • MB_06: Shows the condition (e.g., “Good”).
  • MF_05: Current real capacity.
  • MF_06: Factory capacity. Dividing these numbers will give you the exact health percentage.

Universal Apps

If the codes do not work, the best tool is AccuBattery. However, it is not suitable for instant checks – it requires at least a few charging cycles for the app to calculate the real capacity. For immediate checks, Ampere or CPU-Z are suitable, which will show whether the battery is charging at a stable current and if its status is “Good.”

IMEI check, accounts, and other components

The battery is important, but it is not the only risk. To avoid purchasing a stolen or blocked phone, it is essential to follow these steps:

IMEI and status check

Dial *#06# to get the IMEI code. Check it on websites (e.g., imei.info or manodraudimas.lt databases) to see if the phone is reported stolen. Also, ensure that security systems are disabled: “Find My iPhone” (Apple) or “Factory Reset Protection” (Android). When buying the phone, ask the seller to reset it to factory settings in your presence and successfully complete the initial setup.
[Image of checking smartphone IMEI number on screen]

Functional Test

• Screen: Check color uniformity (look for burn-in on AMOLED screens) and touch sensitivity across the entire area.
• Cameras and sensors: Take a photo against a white background (look for spots), check “FaceID”/fingerprint.
• Connections: Insert a SIM card, make a test call, check Wi-Fi and Bluetooth functionality.

Replacement costs, options, and economic evaluation

If you find a phone in excellent condition but its battery is worn out, this can be a point for negotiation. However, evaluate the replacement costs:

Official vs. Unofficial Service

Official service (in the case of Apple – authorized partners) ensures original parts, warranty, and water resistance restoration, but the price can reach 70–100 Eur. Unofficial services offer cheaper alternatives (30–50 Eur), but you risk receiving a lower quality battery, and newer iPhone models may display a warning about an “unknown component.”

Self-Replacement

This is the cheapest option but requires specific tools and knowledge. Modern phones are glued together, so attempting to open the case at home risks breaking the screen or damaging other components.