From: rslau@usc.edu (Robert Lau)
Newsgroups: rec.audio.car
Subject: Re: batteries for stereo comps
Date: 5 Nov 1992 17:38:08 -0800
Organization: University of Southern California, USA
	<23OCT199214002592@lims02.lerc.nasa.gov>
	<1992Oct23.190926.14493@leland.Stanford.EDU>
	<RSLAU.92Oct29233328@tarazed.usc.edu> <gilchr.720616824@ee.ualberta.ca>
NNTP-Posting-Host: tarazed.usc.edu
In-reply-to: gilchr@ee.ualberta.ca's message of Sun, 1 Nov 1992 11:20:24 GMT

Time to put my EE cap back on, jeez it's been decades :)

In article <gilchr.720616824@ee.ualberta.ca> gilchr@ee.ualberta.ca (Andrew Gilchrist) writes:

   This is only really swell if you want to run your stereo a lot when the
   car is off.  I'd recommend a standard battery isolator rather than
   an electromechanical system.

   >This relay is called a contactor and you can get ones that handle well over
   >100 amps.  It has more benefits than drawbacks but for some reason, I see
   >more battery isolators than contactors...  Basically, you can switch the
   >second battery in/out of the system at any time.

   I would like you to list these benefits and drawbacks, please.
   The only benefits I can see are low power disipation, and manual switchability
   (if that's what you need).

Okie doke.  BTW, there have been several good articles in various car
audio rags about battery systems...  A standard battery isolator is
basically two high-current diodes.  This results in anywhere from a .7
to 1.7 volt drop on the isolated side.  Since the typical regulator
output voltage is 13.8 volts, your amps see only 12 volts.  Amps are
usually spec'd at 13.8 or even 14.4 volts.  If you're competing, you
need your amps to perform to spec and you can't afford any voltage
drop.  Clark's, John's, and a few other comp cars I've seen don't
really use isolators because of this, they've wired up contactors
which effectively bypass the isolator for competition...  On the down
side, contactors are basically relays on steroids.  High-current ones
take a heck of a lot of control current to throw.  Also, yes, you have
to manually reconnect the battery for charging.  You can get around
this by wiring the control to your ignition.

   However, the alternator WILL dump a whole bunch of current into the
   battery.  This may be bad, but I doubt your alternator will pump
   enough juice into it to do any real damage.  That's kinda beside the
   point though, since there are other reasons you shouldn't deep-cycle
   car batteries.  It is a bad thing.

Yup, a normal car battery can be fully discharged 5-10 times before it
cannot be charged again.

   >2. use a smart charger onboard that regulates the charge current and keeps
   >your second battery fully isolated

   I'd like to hear how they explain the purpose and function of such a
   "smart charger."  Please quote.

Other way around, you don't want to put excessive strain on your car's
electrical system by throwing what amounts to a current sink into the
circuit.  All you need is a regulated power supply, DC-DC.  Max draw
is between 10-12amps with a dead battery.  Add control logic for
voltage sweep and current limiting.  Voila, no more wierd load for
your regulator to deal with.

   If an inorganic chemist, or other suitably qualified/informed reader
   would like to flame my comments on lead acid batteries, please feel
   free to do so.

You know, there were lots of things that they didn't teach us in
undergrad EE (heck, there was lots that undergrad CS missed as well)
We studied caps to death but not batteries, and a battery ain't no
cap.  In the simplified world of undergrad EE circuits classes, there
were only voltage and current sources.  You never placed voltage
sources in parallel and current sources in series.  Then again, I
might have been asleep most of the time :)

-- 
Robert Lau - Systems Programmer, Unix Systems     213-740-2866
--  University Computing Services                 Internet: rslau@usc.edu
--  University of Southern California             Bitnet:   rslau@uscvm
--  1020 W Jefferson, LA, CA  USA, 90089-0251     UUCP:     ...!uunet!usc!rslau