I's basically 3 phase ripple an I think a few 100 mV p-p. It's one of th alternator tests I usually do. heavy charging gives you a little more ripple.

This https://www.hotrod.com/articles/0206sr-understanding-your-alternator/

says
QUOTE
An alternator has a normal range of operation. Most alternators need to spin at about 2,400 rpm at idle, have their maximum output above 6,000 rpm, and should never exceed 18,000 rpm
UNQUOTE

So 7.2 to 54 kHz

 

OK, now we have an independent verification. Thanks for that! Now depending on how much current is required, the simple circuit can be built with an LM324 quad op-amp and just a few cheap parts. The LM324 can be powered directly from the 12 volt supply, and the input capacitor can be 0.01 Mfd, rated at least 50 volts. The first stage +input will need to be biased at half the supply using 2 resistors, 10K each. This point also gets a 0.1mfd cap to the supply negative, since this voltage is also used as a comparator reference.. Since a fair amount of gain is needed, a 10K feedback resistor from the first stage opamp output to the inverting input of the first stage. another 0.01 cap should feed the second stage. The first stage output goes to the non-inverting input of the second stage, connected as a unity gain buffer, output tied to the inverting input. The output of this second stage is to another 0.01 Mfd capacitor in series with a diode (cathode end) feeding the non inverting input of the third opamp. This non-inverting input also has a 10K resistor connected to the supply common negative. The inverting input is connected to the comparator reference voltage from the first stage + input.
If all is working well, the output of this third stage should be low, near zero, when there is no noise on the DC supply line, and it should jump up to near the 12 volt supply voltage when noise is present. That jump will be enough to turn on a transistor to either power the load or operate a relay to power the load.