School Bus Battery Drain: Finding 1.8 A Before Upsizing the Battery
A 24-volt school bus with two 12-volt, 100 Ah batteries in series is reported to have a flat battery every Monday morning after a 60-hour weekend idle. The alternator is rated at 80 A. At 1,500 rpm with headlamps and blower on, it produces 27.6 V and 30 A; the AC ripple measured at the battery terminals is 0.3 V. With all switches off and the keys removed, a clamp meter still shows 1.8 A flowing out of the battery. The transport manager proposes fitting two 150 Ah batteries and a bigger alternator instead of finding the drain. The bus has a tachograph, an emergency exit buzzer, a radio memory, and a recently fitted GPS tracker. A battery load test shows both batteries at about 80% capacity.
1. Calculate how long the 1.8 A drain takes to flatten the 100 Ah battery. Compare this with the 60-hour idle period and judge whether the manager's proposal is technically sound.
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For batteries in series, the voltage adds but the Ah capacity does not: the pack is 24 V, 100 Ah. Theoretical time to zero = 100 Ah/1.8 A = 55.6 hours. If the battery is not to be discharged below about 50% state of charge, the usable time is only about 28 hours. A 60-hour idle period will therefore leave the battery deeply discharged or flat, which matches the complaint. A healthy vehicle parasitic drain is usually below about 0.05 A, so 1.8 A is roughly 36 times the normal limit. Fitting 150 Ah batteries would increase the time to zero to about 83 hours, but the drain would still be present and would still discharge the pack, while the larger batteries would take longer to recharge. The proposal masks the fault rather than repairing it.
2. Outline a test sequence to isolate the drain and to check whether the alternator is contributing to it.
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With everything off, measure the drain at the battery negative cable using a clamp meter or a series ammeter. Remove one fuse at a time and note when the current drops; this identifies the circuit. Check the recently fitted GPS tracker first, then the radio memory, tachograph, emergency exit buzzer and any interior lamps or relays. Inspect for a stuck relay or a glovebox or luggage lamp that remains on. To test the alternator, disconnect its B+ output with the engine off and re-measure the drain; if the current disappears, a faulty rectifier diode is leaking. The charging voltage of 27.6 V and 30 A at 1,500 rpm with loads is broadly reasonable for a 24 V system, and 0.3 V AC ripple is not by itself proof of a failed alternator. The isolation test, not the voltage reading alone, identifies the source.
3. What corrective action should the inspector or workshop recommend, and what should be verified before the bus returns to service?
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The drain must be repaired at its source. If the GPS tracker is the cause, it should be connected to an ignition-switched supply or a low-voltage disconnect, not left on the permanent battery feed. If a relay, lamp or alternator diode is leaking, it should be replaced. After repair, re-measure the parasitic drain with all systems off and confirm it is within the manufacturer’s limit, typically below about 50 mA. Both batteries should be recharged and load-tested; 80% capacity suggests they may have been damaged by repeated deep discharge and may need replacement. The charging system should be checked under load, and the bus should be left idle overnight as a verification test. Upsizing the battery is not a substitute for fixing the drain, and the repair should be recorded for the vehicle’s maintenance history.