Charging System Under Load on a Fleet Van
A fleet operator brings in a light commercial van complaining that the battery warning lamp flickers at night when lights, wipers, and the rear demister are all on. The van has a 110 A alternator and a 90 Ah battery. At idle with all accessories on, measured charging voltage at the battery is 12.9 V and the alternator output is 55 A. At 2000 rpm the voltage rises to 13.6 V and output reaches 95 A. The belt is tensioned correctly and shows no glazing. The battery passes a capacity test. The fleet wants the alternator replaced because the lamp flickers.
1. Assess whether the alternator is faulty or whether the operating conditions explain the readings, and state what further tests you would run.
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The readings are consistent with a healthy alternator operating near its limit at idle, not a failed unit. At idle, alternator output is proportional to speed, so 55 A from a 110 A machine at low rpm is plausible. The 12.9 V at idle with full load is below the typical 13.8–14.4 V regulated target, which explains the flickering lamp: the lamp is sensing low system voltage, not necessarily an alternator fault.
At 2000 rpm the output of 95 A and 13.6 V show the machine can deliver current and regulate. The remaining questions are whether the sense wire is reading true battery voltage and whether the load exceeds capacity at idle.
Further tests: measure voltage at the alternator B+ terminal versus the battery post to find cable drop; check the voltage sense circuit for high resistance; confirm the demister and lighting current draw with a clamp meter; and check the regulator's temperature compensation. If total load at idle approaches or exceeds 55 A, the flicker is a system design limit, not a failed alternator.
2. Recommend a practical fix and explain the trade-offs, given the fleet wants a cheap solution and the van is used for night deliveries.
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The cheapest correct fix is to reduce idle load or raise idle speed during high-load operation. Options include fitting LED interior and marker lamps to cut draw, checking that the demister is not staying on unnecessarily, and confirming the idle speed is within specification. If the vehicle has an idle-up function, verify it operates when the demister or lights are switched on.
A higher-output alternator is the robust fix but costs more and may require a larger belt and wiring upgrade. A second battery or a battery isolator does not solve the flicker, because the lamp reflects system voltage, not stored energy.
The trade-off is cost against reliability. For night delivery work, the flicker indicates the battery is not being fully recharged at idle, which shortens battery life. I would present the low-cost load-reduction option first, measure the result, and recommend the alternator upgrade only if the flicker persists at correct idle speed with normal loads.