Real amps from a real car during jump starting


The question was (loosely), “Is a harmful amperage load placed on a donor alternator during a jump start?”

The first question would be, “How much IS the amperage load?” To answer that, I did the real thing-- I performed a jump start and did the amperage measurements. I disabled the ignition to prevent a start.


The voltage/amps at which the donee car engine ceased to crank because the battery got too weak: 11.6 volts, and 99 amps.

With donor car connected (but, not running), there were 107 amps (12.6volts) on the jumper cable, and 124 amps on the donee battery cable during cranking.

With donor car running (14 volts), there was 103 amps on the jumper cable, and 130 amps on the donee battery cable, during cranking of the donee engine.

Conclusion: In this instance, the donor battery and alternator supplied about 103 amps, at 14 volts. The Donee battery supplied about 27 amps. So, the amperage from the donor stayed the same, with, or without, the engine running. The difference was the voltage.

The next step would be to place a clamp-on amp meter on the alternator output cable during jump starting.


That is interesting Hellokit. Thanks for the info.


Just curious – are those values for alternator output the same or significantly different for a jump start as for starting one’s own car? Any idea?


When you are starting you own car, the current is coming from the battery. The alternator is not turning fast enough to provide any meaningful output while the engine is cranking. If your battery is dead, the alternator will try to charge the battery as quickly as it can, after the engine is running.


It’s not surprising that the current was similar and slightly lower from the donor car running vs. not running. The thing that causes the current to flow is the load and the supply voltage. In this case the load is mostly the starter motor. When running a motor such as the starter, the power required to turn the motor is what defines the load. With the donor car running, the supply voltage is slightly higher so with the same power being used by the starter you would expect to see a slightly smaller current (power is voltage X current). If it were a purely resistive load you would expect the oppesite - slightly more current with slightly higher voltage (the load would then dissapate slightly more power).

The interesting measurement to do is to measure how much of the 103 Amps is being supplied by the donor alternater vs, the donor battery. My guess would be about the same but there may be some non-intuitive things going on there.


Vehicle alternator maximum temporary output current is limited by magnetic saturation of the iron in the rotating field and the stator. The level of magnetic saturation can be controlled by the magnetic permeability and the physical sizes of the iron/steel parts.

Vehicle generators used a voltage regulating relay and also needed a current regulating relay to limit maximum output to keep them from self- destructing during a temporary heavy current demand situation but for vehicle alternators the maximum temporary current limit is designed in; no relay needed.

By the way, a common single cylinder gasoline engine-driven 115 or 120 volt a.c. power supply for use when your house has a power outage is not a generator, it’s an alternator.


Do you know what the max current at saturatiuon is? The alternater in my vehicle is around 120 Amps according to the “advertised specification” which I’m guessing is the theoretical max under ideal conditions.


I used the term “donee battery cable” to indicate the TOTAL amperage going to (and, through) the donee starter. With the donor engine running, the total amperage (donee battery plus the current from the donor car) was 130 amps. The engine cranked quickly, this way.


The question was (loosely), “Is a harmful amperage load placed on a donor alternator during a jump start?”

You can easily measure that directly, but how will you determine what a “harmfull Amperage” is?


All alternators are generators but not all generators are alternators. :slight_smile:


By the bubbles in the fusible link insulation. And the sudden quiet.


Alternators are rated at about 75 amps to about 150 amps. The user usually doesn’t know if this is 100% duty cycle, or what; so the number is used as general guide. A harmful amperage would be an amperage higher than the rated maximum amperage. It could, actually, be less.