In a parallel circuit consisting of a 14V, 150A alternator maintaining a fully charged 12V, 800 CCA battery - How do I calculate how much current the alternator provides and how much current the battery provides if I put on a .12 ohm load?


.12 ohm load for 5 seconds for testing purposes.

  • 8
    \$\begingroup\$ This is going to depend on the and the impedance of the alternator and battery, as well as any other impedances present. With zero impedances the alternator and battery will only current share if they are EXACTLY the same voltage which will never happen. What will happen is the alternator will have a higher voltage than the battery and thus try to charge the battery while also supplying the load. You cannot charge and discharge a battery at the same time. If you don't know the impedances, then you should assume the the scenario I described where the alternator supplies everything. \$\endgroup\$
    – DKNguyen
    Commented Mar 23 at 1:53
  • \$\begingroup\$ Why exactly do you need this information? The reason I ask, is that the battery and the alternator are typically dynamic sources and if your 0.12 ohm load is actually a motor (possibly a starter, perhaps?), the problem is quite complex and time-variant. \$\endgroup\$
    – MOSFET
    Commented Mar 23 at 2:18
  • \$\begingroup\$ and @MOSFET I apologize, My question was for a 5 second test. I updated my question. \$\endgroup\$
    – stumblebee
    Commented Mar 23 at 2:25
  • \$\begingroup\$ What is your load? \$\endgroup\$
    – MOSFET
    Commented Mar 23 at 2:36
  • 2
    \$\begingroup\$ @stumblebee I'm not trying to discourage you. I'm trying to explain why your question is complicated to answer with certainty. It's not a "textbook question" when you include all the real factors. \$\endgroup\$
    – MOSFET
    Commented Mar 23 at 3:41

2 Answers 2


Assuming the alternator works properly in a modern vehicle, it provides all the current. If fully charged, the battery is floating, and the alternator is a stiffer source. The battery is there to start the vehicle, recharge quickly, and then float until needed. If the load demands more current than the alternator can provide when running, the voltage sags, and the battery supplies the missing current to keep the voltage up. It is self-regulating via Ohms Law.

Float voltage = 13.8 volts / .012 Ohms = 115 amps from the alternator while the battery floats.

Edit note: The vehicle alternator is a DC power source with precision voltage regulation. They use the same precision electronic voltage regulators as DC power supplies and battery chargers. The alternator voltage regulator is set to 13.8 volts, and when the battery is fully charged, its terminal voltage equals 13.8 volts. The battery and alternator voltage are equal, resulting in no current flow. The battery is FLOATING. When there is a demand for current, the voltage regulator turns up the alternator output to maintain 13.8 volts. The voltage at the battery terminals never changes; thus, no current flows from the battery.

Every DC power plant used in telecom, data centers, radio towers, CATV, and other critical mission systems follows the same principle. You set the output voltage of the rectifiers (or alternator) to the batteries' 100% SOC float voltage. The rectifiers supply the load equipment with power while the batteries float. If power is lost, the batteries supply the load seamlessly. When power is restored, the rectifiers go into current limit to charge the batteries and supply the load with power.

The voltage regulators in the alternator maintain 13.8 volts on the battery. The battery does not see the load, so it sits there and floats until needed. The alternator is at a higher energy state, making it the stiffer source.

  • \$\begingroup\$ I appreciate the answer, I clarified my question, also consider the comment I'm about to make below my question. \$\endgroup\$
    – stumblebee
    Commented Mar 23 at 2:38
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    \$\begingroup\$ OK, I edited my answer. Time has nothing to do with it. The vehicle voltage regulator maintains the voltage at the battery, so it just floats. \$\endgroup\$
    – Dereck
    Commented Mar 23 at 4:04

The alternator voltage is rectified obtaining a double half-wave which is then capacitively filtered so the resulting voltage is composed of a direct component plus a ripple. Assuming that the voltage produced by the generator is continuous, examination of the circuit gives the following results:

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