Remaining capacity detection of electric vehicle batteries: methods, tools, and precautions

1、 Four core detection methods

1. Discharge testing method (most accurate)

• Principle: Discharge the battery to the termination voltage with a constant current, using the formula; Capacity (Ah)=Discharge current (A) × Discharge time (h)  Calculate the actual capacity.
• Tool: Professional discharge tester (such as Songke pilot battery module testing cabinet).
• Operation:
  • Connect the load resistor and set the discharge current (recommended 0.5C, i.e. 2-hour discharge rate).
  • Record the time when the voltage drops to the termination value (such as 10.5V for lead-acid batteries).
• Advantages: Accurate results that can intuitively reflect the degree of battery degradation.
• Limitations: It takes a long time (2-10 hours) and requires professional equipment.

2. Conductivity testing method (rapid screening)

• Principle: By measuring the internal resistance or conductivity value of the battery, the capacity can be indirectly evaluated. The conductivity value is positively correlated with the capacity.
• Tool: Conductivity tester (such as BSIDE Elmer Q11).
• Operation: Contact the probe with the battery pole, and the instrument will display the conductivity value and capacity percentage.
• Advantage: Easy to operate, results can be obtained within 10 seconds.
• Limitations: Accuracy is affected by temperature and battery aging mode, making it suitable for rapid preliminary judgment.

3. Voltage testing method (simple daily use)

• Principle: Capacity is determined by open circuit voltage (OCV) or load voltage.
• Tool: multimeter.
• Operation:
  • Open circuit voltage: Measure after leaving the battery idle for 2 hours. A 12V battery with a voltage ≥ 12.6V is considered fully charged.
  • Load voltage: It is normal for the voltage to be ≥ 11.6V after adding a 10A load.
• Advantage: No need for professional equipment, suitable for daily self inspection.
• Limitations: Unable to accurately quantify capacity, only able to determine approximate status.

4. Specific gravity meter testing method (for lead-acid batteries)

• Principle: The capacity is reflected by the specific gravity of the electrolyte. At 20 ℃, a specific gravity of 1.25-1.29kg/L is considered fully charged, while<1.06kg/L requires charging.
• Tool: Hydrometer.
• Operation: After absorbing the electrolyte, read the specific gravity value.
• Advantage: Intuitively reflect the status of lead-acid batteries.
• Limitations: Only applicable to open lead-acid batteries, not suitable for lithium batteries.

2、 Essential Tools and Equipment List

3、 Testing precautions: 5 key points to avoid stepping on potholes

1. The battery needs to be fully charged:
   • Fully charge before testing, otherwise the result will be lower. Lead acid batteries need to be fully charged with the original charger, while lithium batteries need to be balanced with BMS cells.
2. Control the ambient temperature:
   • The optimal temperature is 25 ℃, with a capacity decay of 1/3 at low temperatures (<15 and="" accelerated="" aging="" at="" high="" temperatures="">35 ℃).
3. Standardize the connection circuit:
   • Ensure good contact between the clamp and the pole to avoid errors caused by virtual connections. Suggest using sandpaper to polish the oxide layer.
4. Reasonable selection of discharge current:
   • Excessive discharge current (>1C) may damage the battery, while insufficient discharge current (<0.1C) takes too long. Recommend 0.5C (such as 10A discharge for 20Ah batteries).
5. Charge promptly after detection:
   • Charge immediately after the discharge test to avoid damaging the battery due to over discharge (voltage<10.5V).

4、 Interpretation of Test Results and Action Guidelines

5、 Advanced technique: Utilizing BMS data and OBD diagnosis

1. BMS data reading:
   • Read battery SOC (remaining capacity), SOH (health), cell voltage and other parameters through an OBD diagnostic instrument (such as Yuanzheng X431).
   • Focus on  Individual pressure difference (>50mV to be cautious) and; Cycle times (may decay if>800 times).
2. Charging curve analysis:
   • Use a power analyzer (such as Ulide UT230A) to record the charging voltage and current curves.
   • Abnormal performance: Charging time shortened by more than 30% (capacity decay), large current fluctuations during constant voltage stage (increased internal resistance).

6、 Summary: Scientific testing extends battery life

• Daily self inspection: Measure open circuit voltage with a multimeter every month and conduct load testing every quarter.
• Professional testing: Conduct in-depth analysis using a battery tester every year and discharge testing every 2 years.
• Exception handling: If capacity decay>20% or internal resistance>30m Ω is found, contact after-sales service in a timely manner.

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