How to Wire Dual Car Alternators Together for High Current?

My guess is that your generators might be low in voltage output most of the time.

This explains why the highest is the one that works. You are working with a low margin to use diodes, that depending on which one, create another voltage loss of in the worst scenario of let's say 1.1 volts ( you should try with Schottky diodes in case you haven't done it yet)

15 Volts (in the best wind) provides for 13.9 Volts and your batteries don't get charged optimal.

Has it any use to say that charging 6 Volts batteries with this setup will work better? They will at least put all your gen's to work. It all depends how much wind you get and what output they provide.

Well first of we have to know if your using generators with perm mag or alternators with rectifiers, are they all the same? You only have one connected at this time, is that correct? Do you have any voltage regulation in place....? If you are using car alternators they should have the rectifiers on them, and there is no need for the additional blocking diodes... I don't think it's a good idea to combine the outputs without matching voltage, this would cause backfeeding and damage to the components...

If your generators are all rated to charge 12 volt batteries then their own speed will dictate when they will start charging. The one that has the highest speed is going to provide charging power first and as the others get up to speed they will add on as well.

If they are actually permanent magnet type alternators they will have diodes built in so you can safely connect the outputs of all of them together without feed back issues but if they are brush type permanet magnet DC generators each one will need a diode to prevent power from backfeeding into it from the other units if they are running as well.

The only practical way to get each one to contribute at lower speeds and output voltages would be to use a dedicated MMPT (Maximum Power Point Tracker) on each unit.

Why not split the system so that each generator charges a smaller bank of batteries, so that each generator charges its own bank, and then parallel the batteries to the delivery network using diodes downstream of them?

To achieve the result you need you will need a boost converter or maximum power point tracker on each supply. Silicon Chip magazine designed a useful MPPT check their website. For commercially built items try Redarc of Perth Western Australia website, a similar problem is encountered with camper trailers so they make a boost converter to suit. Both sites will give an insight into the issues involved.

In parallel, after rectification.

VWs have for more than 10 years had a 120 amp alternator on some of their car diesel engines.....just one will do the job completely.

Several US companies sell car alternators with a high output as well....

...except the OP apparently has three different small windmills, each with it's own generator, so probably not enough torque to produce anywhere near that much current.

Its easier to combine electric currents than it is to combine torques.

Absolutely correct, but in the case where only one is delivering current as he seems to suggest, I thought it might help to know a bit more.....

Combining currents/ voltages is also not simple if done properly/safely.....

Use two batteries in parallel with each generator. Thus you will be able to charge 2 batteries with one generator. If you are not there while charging you will want to but in a fuse to each battery. This way one battery shorting will not take out the others and possibly your house. You will also need charge control to prevent over charging the batteries. This can be done with a 13,5 volt Zener Diode and PNP Power Transistor with the cathode tied to the battery, anode to the generator and gate to the Zener. Also add a one k ohm resistor from anode to gate to bias the zener to the correct voltage. As the battery voltage increases the Base to cathode voltage will decrease shutting down the charge. The zener voltage will need to be selected to allow for the voltage drop across the transistor.

Maximum power point tracking is usually effective for solar panels.

If you have automotive alternators on all of your windmills you need to take a close look at them to see if they have more than a single output terminal. Some 3 terminal alternators have a voltage sense input terminal that you can run back to the battery post so the alternator internal controls can sense the actual battery voltage and compensate for any voltage drops in your system, like the voltage drop across your blocking diode. (The alternator will automatically put out a higher voltage.) You may be able to fine tune the output of each alternator by installing an adjustable resistor in each voltage sense line to control the voltage output of each alternator.

Here's a description of the 3-wire alternator--its aimed at the older car enthusiast and there's lots of breathless hype but it tells you how to use the voltage sense wire.

If your battery is reading 14.8V, it is pretty much fully charged as a 12V system. Maybe your system is working adequately now? Have you checked the actual charging currents from each alternator to see if you truly are getting any charge current when the battery voltage is down around 12V?

If all 3 alternators used remote voltage sensing and all put out current when you have a load on the system, like lights or the like, you probably are getting what you will get without fine tuning the output voltage of each alternator.

Jon.