I have a vehicle alternator with following parameters: 14V, 140A. In parallel to this alternator is connected 14.4V vehicle lead acid battery. The alternator is used as a power source for a load which consumes 115A. Please see the attached circuit diagram.

enter image description here

This circuit works in an operating point where the alternator delivers 105A and the rest 10A for the load are drawn from the vehicle battery. Due to this fact the vehicle battery is being discharged which is of course undesirable. I am not sure whether I understand why the vehicle battery is being discharged in a situation when the current drawn by the load is less than the alternator nominal current. Does anybody know the reason? Thanks in advance for any ideas.

The alternator is Bosch 0124 525 064. The combustion engine is simulated by a three phase induction motor supplied from the voltage source inverter. The induction motor has 1500 rpm synchronous speed and there is a belt transmission with ratio 2:1 so the alternator works at approximately 3000 rpm.

  • \$\begingroup\$ If the battery is delivering 10A then of course it will discharge. Faulty alternator? \$\endgroup\$
    – Chu
    Oct 14, 2019 at 6:56
  • 1
    \$\begingroup\$ Can you provide a datasheet for the alternator? I would guess that it can deliver its nominal power under nominal torque input - perhaps the engine driving the alternator is not powerful enough? \$\endgroup\$ Oct 14, 2019 at 6:59
  • \$\begingroup\$ Perhaps the alternator can deliver 140A in certain circumstances, rpm, engine ECU tells the battery type, voltage and state of charge,.... \$\endgroup\$ Oct 14, 2019 at 7:12
  • \$\begingroup\$ Alternator max output is set by RPM and the regulator. IF the RPM are high enough to exceed the delivered load AND the regulator is providing max field current then the alternator is faulty. Otherwise the regulator is faulty. \$\endgroup\$
    – Russell McMahon
    Oct 14, 2019 at 9:58
  • \$\begingroup\$ Measure the voltages across the alternator output and the battery. I suspect drops in the cables are preventing the alternator from delivering the full load. \$\endgroup\$
    – Finbarr
    Oct 16, 2019 at 15:04

2 Answers 2


Why limit the alternator to producing 105A when the load requires more than that?

So, allow the alternator to function with the "normal" regulation and it will cover the load as well as charging the battery.

If the engine is under-sized and cannot produce sufficient power then the alternator won't be able to produce full output.

Also, if the alternator is not driven at sufficient rpm then it won't produce its max output either


The question is erroneous.

given Source= 14V; Load = 115A

battery load = 10 A is impossible at 14V since the current direction will always be charging above 13.5 +/- 0.x and discharging below a voltage 13V toward 12.5 to 12.8 where SoC=100%

therefore the assumptions and conditions supplied are invalid.

Conclusion: Invalid question.

More valid is low RPM idle with Alternator dropping below 12.5V and battery dropping from 14 to same to supply 10A while all accessory’s drawing much less that 115A but max current at this low RPM, dimming the Lights, reducing fan speed and draining the battery but never 115A at this low RPM

  • \$\begingroup\$ The voltage at the vehicle battery in the above mentioned operating point is not 14V. It is 12.1V. I think that this voltage along with the internal resistance of the vehicle battery set the discharging current 10A. \$\endgroup\$
    – Steve
    Oct 14, 2019 at 12:34
  • \$\begingroup\$ The Alt V = Bat V. , where did it say 12.1 in question. battery ESR = 5V/ CCA e.g. < 10 mOhm for 500A CCA thus 12.1 =12.5 - ESR^I. CCA is rated @ 7.5V \$\endgroup\$ Oct 14, 2019 at 12:38

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.