12V Battery (Flooded, EFB, AGM) & Alternator Testing: Cold Cranking Amps and Voltage Reference
Open-circuit resting voltage vs State of Charge (SoC), minimum cranking voltage drop, smart alternator charging ranges, and AGM/EFB replacement rules.
| 100% Resting Voltage (12h rest) | 12.65–12.70 V (Flooded) · 12.80–12.90 V (AGM) |
|---|---|
| Discharged Threshold (< 50%) | ≤ 12.20 V resting (sulfation risk begins) |
| Minimum Cranking Voltage | ≥ 9.6 V at battery posts during starter engagement |
| Alternator Charging Range | 13.8–14.7 V (Smart BMS: 12.8–14.9 V) |
1. Never Downgrade an Start-Stop AGM or EFB Battery
Vehicles equipped with automatic Start-Stop and regenerative braking deceleration use Absorbent Glass Mat (AGM) or Enhanced Flooded Battery (EFB) chemistry. Replacing an AGM battery with a conventional flooded battery leads to rapid plate shedding within 6–12 months. Always register the new battery in the BMS if equipped.
2. Parasitic Draw Limit After Module Sleep
After locking the vehicle and waiting 20–40 minutes for CAN-bus modules to enter sleep mode, steady-state parasitic current draw should not exceed 20–50 mA (0.02–0.05 A). Draws above 80 mA will drain a 60 Ah battery over a long weekend.