Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.milw.wisc.edu!leah!rpi!batcomputer!garnett
From: garnett@batcomputer.tn.cornell.edu (Roger Garnett)
Newsgroups: sci.electronics
Subject: Re: Whatizzit?  ("Battery Booster")
Message-ID: <7724@batcomputer.tn.cornell.edu>
Date: 12 Apr 89 16:15:04 GMT
References: <27792@cci632.UUCP>
Reply-To: garnett@tcgould.tn.cornell.edu (Roger Garnett)
Distribution: na
Organization: Cornell Theory Center, Cornell University, Ithaca NY
Lines: 35

In article <27792@cci632.UUCP> rdi@ccird3.UUCP (Rick Inzero) writes:
>I was just given a car "battery charger" (at least that's what I 
>was told).  Anyway, I can't figure out if/how it works, since it's pretty old, 
>and the technology pre-dates me.  When I plug it in (with both output leads
>unconnected- just floating with no load), I get only AC out, *no* DC, 

	You probably get rectified AC (ie. pos only, 1/2 wave), but as the
charger doesn't have (or need) a filter Cap, it may be hard to get a DC reading. 
>opening up the 
>metal case, I see only 3 components:  a transformer, a current-sensing 
>shutoff device rated at 10 Amps (to protect against the output leads 
>shorting), and one really mystifying component... it's a flat 3" square of 
>metal, insulated from the box by insulating paper, in the center is a 2" disc 
>of brass, with a brass hole thru the middle, 1" brown insulating disks on top 
>and bottom, and a bolt passing thru the hole to hold the metal device in place 
>plus connect the center brass hole to the metal case. [etc...] 

	Probably a selenium or similar rectifier. It could be replaced with
a large, modern diode, IF it isn't working. Hook it up and find out.  

>Unfortunatly, no output VOLTAGE is specified, so I don't even know if
>it's meant to be used with 12V or 6V car batteries.
 
	Most car battery chargers are just crude, unregulated power supplies.
 Current is determined by a combination of the transformer windings, and the
battery voltage/state/resistance. Therefore, a 6 Volt battery will charge at
about twice the rate of a 12 V. Too small of a transformer won't be able to
produce enough current to charge a 12 V battery, and too large will overcharge
whatever. (6 V first) Most chargers with multiple charging rates just have a
multiple-tap transformer to select a charging rate.
___________________________________________________________
Roger Garnett           (garnett@tcgould.TN.CORNELL.EDU)
Cornell Phonetics Lab   (plab!roger@cornell.uucp)
Ithaca  N.Y.            (sggy@cornellC)
(607) 255-0704		(sggy@vax5.ccs.cornell.edu)