Charge Lamp Glowing and Repeat Battery Failures Under Load
2018 transit bus, 24 V, 200 A alternator with remote voltage sense taken at the battery disconnect box and an internal regulator. Complaint: the charge warning lamp glows at idle and gets brighter with headlights, blower on high, and wipers. Batteries have been replaced twice in 14 months and are due for a third set. At 1200 rpm with a 120 A load: 28.6 V at the alternator B+ stud, 27.1 V at the disconnect box positive stud, 27.1 V at the alternator sense terminal, field duty 100% (8 A on the field lead). Belt tension per spec, no slip marks. Alternator regulation spec: 28.8 V ± 0.3 V at 25 °C. Battery temperature 31 °C. Charging circuit spec: combined positive-leg and negative-leg drop ≤ 0.5 V at rated current.
1. Compute the drop and the power loss. Is the alternator at fault? Explain why relocating the sense lead would not fix this.
Read the worked answer
Drop = 28.6 − 27.1 = 1.5 V around the charge loop at 120 A, which is 1.5 × 120 = 180 W dissipated as heat (1.5 ÷ 120 = 0.0125 Ω). The spec is 0.5 V total, so the circuit is three times out of spec.
The alternator is not yet condemned. The regulator senses at the disconnect box and is commanding 100% field trying to pull that point up to its setpoint; it cannot, because 28.6 V at the stud is already at the regulation ceiling of 28.8 ± 0.3 V. The 1.5 V drop therefore subtracts directly from the voltage the batteries actually see, so they live permanently undercharged - which explains two sets in 14 months far better than a defective alternator does.
Moving the sense lead to the alternator stud would make it worse: the regulator would read 28.6 V, decide the system is charged, and cut field current, masking the fault while the batteries stay undercharged. The sense lead belongs at the battery end; the cable and connections must be repaired.
2. Where do you place the meter to find the loss, and how do you avoid misdiagnosing it?
Read the worked answer
Measure under load - at least 120 A, key on with headlights, blower and wipers - and split the loop into two legs, because the 1.5 V you measured is the sum of both.
- Positive leg: alternator B+ stud → cable lug → megafuse or breaker → disconnect switch in and out → battery positive post.
- Negative leg: alternator case → engine block → frame or chassis ground strap → battery negative post.
Chasing only the positive cable can miss a corroded engine-to-frame strap that produces the same total. Likely culprits are crimp corrosion under heat shrink, pitted disconnect switch contacts, a loose megafuse holder, paint or sealer under a ground stud, and an undersized replacement cable.
Also verify the sense lead is intact and fused. Here it reads 27.1 V, matching the battery, so it is healthy; if it ever reads low with a normal battery, an open sense lead is driving the regulator to full output and cooking the batteries. Finish with a true-RMS AC ripple check at the battery - a failed diode heats batteries and mimics a bad set.
3. Before releasing the bus, what charge-system values do you record to protect the new battery set, and why?
Read the worked answer
Record, with the new batteries at operating temperature:
- Voltage at the battery posts at idle and at 1200 rpm, accessories off and then all on. It must sit inside the regulator's temperature-compensated window (roughly 27.6–28.8 V at 25–30 °C for a 24 V lead-acid system).
- Voltage on each 12 V battery separately, to catch an imbalanced series pair.
- True-RMS AC ripple at the battery (typically well under about 1 V AC; higher points at diodes).
- Field duty at idle with lights and blower on, which should be well below 100% once the cable is repaired - a still-saturated field means residual resistance or an undersized circuit.
- Total charge-loop voltage drop, repeated hot, to confirm the repair holds under heat.
Writing these numbers on the work order gives the next technician a baseline, proves the undercharge cause was corrected rather than masked, and gives the operator a documented reason to expect normal battery life.