Well I am being a cheap skate so not purchasing a DC/DC converter. I am sure I saw some where a way to achieve it by tapping across the alternator and battery some how.
It can be done by connecting the negative terminal of the battery to the positive terminal on the alternator. Your 24 V would be across the alternator ground and positive battery terminal.
Please, please do not take this information as a recommendation. It should be used for academic purposes only.
I don't think that approach will meet nemmi69's original requirement of keeping the rest of the car on 12 volts. Assuming conventional automotive alternator and battery, and assuming he doesn't want to disconnect the alternator/vehicle from the battery (most likely welding himself to the car in the process!), it cannot be done without buying some hardware. In modern alternators, the rectification and regulation are done in an internally mounted module, and it's not practical to get at any of the connections. In order to keep the vehicle at 12V (really 14V on a fully charged battery, but what's a couple of volts, right!) the regulator drops the current to the field windings, to maintain voltage. Under normal use, there isn't really any higher voltage to tap.
A DC-DC converter, or a second battery with a charging isolator (ala fishing boat trolling motor style,)
would be the cheapest solutions. The DC-DC converter would definitely be the lightest solution for limited power consumption.
Couldn't you wire in a simple switch between the battery and the alternator, and once the car is running flip the switch to allow the alternator and battery to run in series and parallel? I do not know if this is viable or not, as I am only a lowly mechanical engineer. My only experiences in electrical engineering usually end up getting me in trouble.
In theory, yes, but that would be a very expensive switch, as it would need to be able to carry very large amperages (50-100A) across it's contacts, for normal battery charging. You would also likely need to be able to do it remotely, which would get even more expensive. I don't see how you can practically do it.
I think I would purchase a 70-150 dollar DC-DC converter, before I would screw up my wiring in a manner that just screams WRONG!!!!!! from an Electrical Engineer's perspective.
Thanks for the clarification. As I said, I'm OK with electrical theory, but it is nice to have someone with a technical electrical background to keep me in line as far as practical applications.
well it was suggested that as I am controlling is an electro pneumatic solenoid that a baby DC/DC would work. My aim is cheap!!! and small.....any ideas.
Oh, if any one is wondering what I am up to I am building a boost controller.
I don't get it... why are you forced to use a 24V solenoid if all you are doing is building a boost controller? There are dozens of boost controller kits on the market and they're all 12V, so what's so special about your application?
As far as switching between series and parallel once the vehicle is running, that's not as easy as you'd think. On top of the large currents mentioned by TStaples, you'll also have to deal with surge currents... ever heard of load dump? The moment you remove the parallel battery connection, the alternator field is going to wind up and give your system a fat voltage spike. That wil be bad not only for every electronics system in your vehicle, but potentially explode the battery if you do it enough.
You're either not giving us all of the proper information or you're not approaching the problem from the correct viewpoint.
There are many commercially available solenoids in ratings of 6V, 12V and 24Vdc. One thing to consider is that the coil may be rated for 24Vdc, but that is ususally a maximum continuous rating, or else is specified with a max duty cycle. However, it's quite possible that your 24Vdc rated valve will work just fine on the 12-14Vdc that is present in the vehicle. Typically, the solenoid opening pressure, and/or opening time is de-rated at lower voltages, but is often adequate for the application. If not, then Mike is correct; get the proper valve for the job. There are many valve companies on the planet. Parker Skinner, and ASCO, are two of the biggest in the US industrial market. There are many others. Check out:
