Battery Test

This procedure tests the battery condition and charging system on a 2007-2013 Mercedes-Benz S65 AMG to diagnose battery or alternator issues.

Warnings

Tools required

Parts

• Battery terminal cleaning compound (anti-corrosion) × 1 — Use OEM specification

Preparation

1. Open trunk and locate battery compartment on right side of trunk floor
2. Remove battery cover by loosening cover screws (4 screws total)
3. Ensure all vehicle electrical accessories are OFF (radio, lights, climate control)
4. Let vehicle sit with ignition off for at least 2 hours before testing for accurate resting voltage
5. Verify battery terminals are accessible and not heavily corroded

Procedure

1. 1

   Remove battery cover
   Using 10mm socket, remove the four cover screws securing the battery cover. Lift cover away to expose battery terminals and top surface.
   Torque spec

   Cover Screws15 Nm (11 lb-ft)
2. 2

   Perform visual battery inspection
   Inspect battery case for cracks, bulging, or leakage. Check terminals for excessive corrosion (white/blue powder or heavy oxidation). Verify battery hold-down bracket is secure. Note battery manufacture date code on label; batteries over 5 years old typically require replacement regardless of test results.
3. 3

   Test resting voltage
   Set digital multimeter to DC voltage. Place red probe on positive terminal, black probe on negative terminal. Resting voltage should be 12.6-12.8V for a fully charged AGM battery. 12.4V indicates 75% charge, 12.2V indicates 50% charge, below 12.0V indicates discharged or failing battery.
4. 4

   Clean battery terminals if needed
   If corrosion is present, loosen battery terminal bolts using 10mm socket (do not remove cables unless necessary). Clean terminals and cable ends with wire brush until shiny bare metal is visible. This ensures accurate load test results. Re-tighten terminals.
   Torque spec

   Battery Terminal Bolts9 Nm (7 lb-ft)
5. 5

   Perform battery load test
   Using battery load tester, connect clamps to battery terminals (red to positive, black to negative). For S65 AGM battery, apply load equal to half the CCA rating (typically 450-500 amps for 10 seconds). Battery voltage should not drop below 9.6V during load test at room temperature. If voltage drops below 9.6V, battery has insufficient capacity and requires replacement.
6. 6

   Test alternator charging system
   Start engine and let idle. Measure voltage at battery terminals with multimeter. Charging voltage should be 13.8-14.6V with engine running. Rev engine to 2000 RPM; voltage should remain stable in this range. If voltage is below 13.5V, alternator may be failing. If above 15.0V, voltage regulator may be faulty.
7. 7

   Perform electrical system load test
   With engine running at idle, turn on high electrical loads (headlights, heated seats, rear defrost, HVAC blower on high). Voltage at battery should remain above 13.2V. If voltage drops below 13.0V with loads applied, alternator output may be insufficient for vehicle demands.
8. 8

   Check for parasitic draw (if battery drains when parked)
   If battery repeatedly discharges when vehicle is parked, measure parasitic draw. Turn off ignition and wait 15 minutes for modules to sleep. Using multimeter in DC amp mode (10A range initially), disconnect negative cable and measure current between negative terminal and negative cable. Draw should be less than 50mA after all modules sleep. Higher draw indicates electrical fault requiring diagnosis.

Reassembly

1. Apply thin coat of anti-corrosion compound to battery terminals after testing
2. Reinstall battery cover and torque cover screws to specification
3. Ensure battery compartment carpet and trunk trim are properly repositioned

Verification

• Verify battery voltage reads 12.6-12.8V at rest or 13.8-14.6V with engine running
• Confirm no warning lights on instrument cluster related to battery or charging system
• Test vehicle start: engine should crank vigorously without hesitation
• If battery failed load test (below 9.6V under load), recommend replacement with AGM battery meeting Mercedes-Benz specifications (minimum 92Ah capacity, 850+ CCA)

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