On Thu, Jan 10, 2002 at 11:54:58AM +1300, Russell McMahon wrote:
> > The supply will have a 12V 7Ah gel cell to act as a mini UPS. So the
> > input voltage will be from 12 to 14 volts.
> >
> > I want to get a regulated supply of 12V +- 5% @ 4A.
>
>
> Be aware that a 12v gel cell "floated" off the main battery by a diode will
> be undercharged when the main battery is charged. The extra 0.6v or so can
> make quite a difference to cell life. This comment is based on practical
> experience with 100m + car based systems using this arrangement and a
> standard silicon diode.  If this is one off or small volume
> then use of a low voltage drop diode can be useful eg Schottky diode. If you
> don't mind having to replace the gel cell every year or 2 this may not be
> too much of a problem.

Neither is an issue because it is a limited run (less then 10 units total).
In my prototype I did use the Schottky diode.

I was always concered with overcharging. My altenator (sp?) runs about 14.2V.

[FET deleted for brevity]

Russell,

I appreciate the pointer, but charging the gel cell is the least of my
concerns in this setup. The fundamental question is how to get a reliable
12V +- 5% when the gel cell is in its main discharge curve and the output
voltage is near 12V.

BTW you probably are not aware that your now famous buck converter is in the
middle of this mix. I plan to use it to generate my 5V supply.

So you've helped me immensely already!

Thanks,

BAJ

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email listserv@mitvma.mit.edu with SET PICList DIGEST in the body