Battery Diagnosis and Service
What to Know
Understand battery construction (plates, electrolyte, cells), nominal 12.6 V state of charge for a fully charged lead-acid battery at rest, and the effects of temperature on capacity and cranking. Know that a battery must be tested after being fully charged, and that surface charge must be removed (lights on briefly or a load applied) before an open-circuit voltage reading is meaningful.
Testing Methods
A conductance tester gives state of health without discharging the battery. A load test (typically half the CCA rating for 15 seconds) reveals capacity. A hydrometer check of each cell identifies a weak or shorted cell in serviceable batteries. Always check state of charge first; a discharged battery is not a failed battery.
Service and Safety
Know charging procedures, gassing and ventilation, and that hydrogen gas is explosive-no sparks near a charging battery. Understand series/parallel connections in multi-battery school bus installations and the importance of matched batteries. Clean and torque terminals; corrosion causes voltage drop and cranking complaints.
Traps
Questions often present a battery that "won't hold a charge" and expect you to distinguish a parasitic drain from an internally failed battery. Another trap: testing a battery that is not fully charged and condemning it. Also watch for confusion between CCA (cold cranking) and reserve capacity.
Rule of Thumb
"Charge, rest, then test." A battery below 12.4 V at rest is not ready for a valid load or conductance test.
Worked Example
A bus cranks slowly. Open-circuit voltage is 12.5 V after sitting. Load test drops to 9.0 V in 10 seconds-below the pass threshold-so the battery is failing under load even though resting voltage looked acceptable.