Capacitor before a DC-DC

**panelrodder_81** · 04-25-2004, 04:08 AM

I've been crusin' the different forums just reading and not posting. This is my first post and haven't found anything like this here yet. Couldn't you simply hook up a car audio capacitor before your DC_DC power supply to avoid the cranking issues? If you had a relay triggered by the ignition switch to turn on the computer, or just had a switch to turn it on, and the main power coming off of the cap you should be able to crank all day long and not have problems, unless your cap is small and doesn't have the power on hand. Right? If your cap was big enough you could even run the computer for a little bit after the car was off, really only for shut down or entering hybernation though. It seems to me that it should react the same way as having a UPS or something hooked up right? Again, my first post and I'm new to the car PC thing to boot!

PanelRodder
www.brigham.net/~jacobh/barndoors/

**adamis** · 04-25-2004, 04:40 AM

Well for a first post it's not bad but there has been plenty of study on this one. Heck, a few of the guys around here have even done the math to figure out how big of a cap it would take and the end result is it would be one big fat cap. Way bigger then the caps used for car audio stuff. Why spend 200 dollars on a cap when you can spend 15 bucks on a battery?

I'm sure others will chime in with the details on this subject though. Keep up the reading and welcome to the boards.

**Rob Withey** · 04-25-2004, 04:50 AM

Originally Posted by panelrodder_81

you should be able to crank all day long and not have problems, unless your cap is small and doesn't have the power on hand. Right?

Yep, but you'll need a huge capacitor:

http://www.mp3car.com/vbulletin/show...63&postcount=5

Capacitors aren't a magic store of infinite power. There are very few success stories, if any:

For those who tried the 1F tank!!!

A second battery is a better choice. A decent DC to DC power supply that will cope with the dip in voltage is even better.

**none** · 04-25-2004, 01:21 PM

The flaw with Ricky's equation is that he is assuming total voltage loss when starting. That doesn't happen, only a momentary dip - the largest amplitude of that dip lasting only milliseconds. The equation also does not even address current, which is just as important.

That said, for 99.9% of these applications a battery is better. But a capacitor can work with the right PSU (Keypower especially but NEVER a mini-box PW-XX, those things are crap.)

**panelrodder_81** · 04-26-2004, 04:57 AM

Ok. I guess I just opened a the MOTHER of all can of worms! For some reason I wasn't able to find all those posts on the capacitor before I posted. It seems that we should just leave this alone. The logic of both sides seems to make sense. It seems that no one has yet to actually try a 1F cap on the system though. I've got a friend with one in his audio system. When I actually build the system I'll borrow it and try it since it seems no one else is willing to.