Battery and Starting Systems
Battery and Starting Systems
A transit bus battery bank must supply high cranking current, absorb accessory loads, and stabilize voltage for electronic modules. Know battery ratings-cold-cranking amps, reserve capacity, and amp-hour capacity-and why batteries in a series string must be the same size, age, type, and state of charge. On a 24-volt bus, two 12-volt batteries are series-connected; tapping one battery for 12-volt loads unbalances the string and causes early failure. State of charge can be estimated by open-circuit voltage after surface charge is removed (about 12.6 V = 100%, 12.2 V ≈ 50% for a 12-volt battery), or by specific gravity in flooded batteries. Conductance testers are common, but a load test remains the decisive check.
The starting circuit has a control side (key/start request, neutral-start or transmission inhibitor, start relay, magnetic switch) and a power side (battery cables, solenoid contacts, starter motor). Diagnose a no-crank by proving control voltage at the solenoid, then checking voltage drop on both the positive and negative cables while cranking. A slow crank with normal battery voltage often means cable or connection resistance or a failing starter; do not condemn the starter until control and ground paths are proved.
Trap: measuring voltage drop with no current flowing. Resistance only shows up under load. Another trap: assuming a relay click proves contact closure-test voltage at the load.
Worked example: battery reads 12.6 V at rest and 10.8 V at the starter terminal during crank; the 1.8 V drop exceeds the typical 0.5 V total cranking-circuit limit. Measure each cable and connection to locate the excessive drop.
Rule of thumb: keep total cranking voltage drop under 0.5 V, with no single connection over 0.2 V.