Re: (ET) battery charger

["RJ Kanary" <rjkanary nauticom net>]

 I have in my possession, a Cornell Dubilier battery charger- eliminator,
that IS configured in such a  way that both the AC chassis ground, and the
rectified DC negative terminal, are tied together, and it WAS made that 
way.
 Chances are, not too many of these still exist, let alone on a functioning
state.To give you an idea just how old it is, it has two huge Fansteel®
selenium rectifiers.
        And, let's not forget, age, fair wear and tear, and just plain
inadvertent abuse, could compromise the isolation, which, in the expected
type  of use that the charger was designed for, would not cause a problem.
But used to charge a multiple battery array, simultaneously , as has been
noted, the discovery of this problem , would be quite dramatic. Checking
the chargers out, before trying it, is a capitol idea, indeed.
RJ Kanary @ Bandi Bros. Inc.
Member TRNi  Since 1998
ASE® Certified Master Auto Technician
Member Tech Line Associates Since 1987
rjkanary nauticom net

----- Original Message -----
From: "David Roden (Akron OH USA)" <roden ald net>
To: <elec-trak cosmos phy tufts edu>
Sent: Saturday, August 11, 2001 16:16
Subject: Re: (ET) battery charger


> On 10 Aug 2001, at 21:14, RJ Kanary wrote:
>
> > Just think of the excitement possibilities, if all the chargers have
three
> > wire, grounded cords, and at least two of them have the diode heat sink
> > attached to the case, which is commonly the situation.As soon as you
> > plugged the correct two in, ....
>
> Theoretically possible (and more than exciting - in fact rather beyond
> unpleasant - if it happened).  But I think it's pretty unlikely.
>
> When you see a diode or regulator bolted to the metal case of a battery
> charger or power supply, look closely.  It's most likely attached with
> hardware that isolates it electrically from the chassis -- a thin mica
> insulator, and either fiber screw inserts or nylon hardware.
>
> When I was young and foolish (as opposed to middle-aged and foolish, as I
> am now <g>) I built a power supply this way, bolting a regulator
> transistor right to the back of the minibox.  It wasn't long before I
> accidentally touched a stray clip lead to the box.  Fortunately, my
> supply wasn't capable of producing more than a couple of amps at perhaps
> 15 volts, so the outcome wasn't any worse than a minor blow to my ego
> (and pocketbook).
>
> But a battery can produce thousands of amps at 12 volts.  Imagine what
> would happen if a major battery charger maker built a charger this way.
> Inevitably, someone would use it to charge the battery in a positive-
> ground vehicle (I owned such a car).  Suppose he set the charger on the
> fender and clipped the leads to the battery.   BOOM!  Instant lawsuit.
>
> There may be a manufacturer somewhere in China or Thailand or some such
> place who would build such a charger on contract, but I doubt that even
> the shadiest sweatshop would be so careless.  Still, it won't hurt to
> check.
>
> 1.  Place the charger on an insulating pad, connect it to a battery,  and
> plug it in.  Now connect a 12 volt test light between each of the battery
> terminals and the case of the charger.  It should not light up on either
> terminal.
>
> 2.  Connect one lead of a 120 volt test light to electrical ground (a
> properly grounded outlet box) and touch the other to each battery
> terminal.  Once again, the light should not glow on either one.
>
> Just for the heck of it, I examined three different metal-cased chargers
> in my garage (a fourth type which I use has a plastic case and a 2-wire
> line cord).  All of them used internal, isolated heatsinks for their
> rectifiers and charge controllers.  They also all had transformers, which
> isolate the output from the AC mains.  If you'd like to see the guts of
> these chargers, go to
>
>     http://www.evdl.org/pages/iso12v.htm
>
> (I hope it doesn't take too long for the photos to download on this
> page.)
>
> In general, there are two ways a charger could be unisolated from the
> line.
>
> One would be if it used an autotransformer instead of a transformer with
> a primary and secondary.  An autotransformer has only one winding, and
> gives reduced or increased voltage by using taps on that winding.
> Autotransformers are sometimes used where the voltage change needed is
> relatively small, and there is no need for current multiplication (since
> there's only one winding, the entire winding has to be sized to the
> higher current).  It would be very surprising indeed to find an
> autotransformer in a 12-volt battery charger, where the voltage reduction
> is 10:1.
>
> The other would be if there were no transformer at all, but the voltage
> reduction were instead accomplished with a PWM circuit.  Again you're
> most likely to see this where the output voltage is relatively close to
> the input voltage.  This is the way those 1000 watt 230-to-115 volt
> European power converters work.  There are indeed some road EV chargers
> which also work this way -- for example, the Bohn chargers, and the K&W
> BC-20.  In such a charger, the battery isn't isolated from the AC mains,
> so using two of them can be big trouble.  (On the other hand, you can use
> ~one~ of them with another isolated charger.)  All of these unisolated
> road-EV chargers are high-voltage output, from perhaps 72 to 120 volts,
> and unsuited to use as 12-volt chargers.
>
> In sum, the only way you're apt to find a 12-volt charger that's
> unisolated is if it's defective.  I think you are very unlikely to run
> across a 12-volt charger which doesn't provide isolation by design.
>
>
> David Roden - Akron, Ohio, USA
> 1991 Solectria Force 144vac
> 1991 Ford Escort Green/EV 128vdc
> 1979 General Engines ElectroPed 24vdc
> 1970 GE Elec-trak E15 36vdc
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