I will post pictures, but as I said, still in the early stage, but at least I know it works, no secrets either, I'm using a small Brushless motor that use to drive a small propeller. As you know brush-less motors used as an alternator is exactly that , it alternates the voltage , which is AC and not DC. Since its 3 phase, then a bridge rectifier uses 6 diodes, simple enough and you now have a + and - DC voltage output. So far so good, but it needs to spin quite fast similar to your car alternator, Installed it in a dremel tool which spins very fast, but I don't know the RPM, and was getting 21.5 volts DC. I only need 12 volts and 13 is what I want. so I plugged the dremel (with only one speed) into a variac transformer or if you wish a variable output transformer. With the voltmeter connected to the alternator, I increased the rpm of the dremel, it started turning at about 50 volts from the variac. So I increased the voltage to have the dremel turn fast enough until I could read 14 volts. , this of course is with no load, next step I connected a 3 amp lipo charger with a 2s lipo plugged in, switched the dremel on to the preset voltage on the variac, and voila was charging the lipo battery at 3 amps as expected , so now this part is ok and works.
I did find a 4 to 1 toothed belt pulleys I had in the shop with the belt, now to adapt this to mount the large pulley on my small drill press to check if 4000 rpm on the big pulley will be enough , which should give me 16,000 rpm on the alternator, stay tuned