Re: (ET) battery charger

["David Roden (Akron OH USA)" <roden ald net>]

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
= = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =
Thou shalt not send me any thing which says unto thee, "send this to all 
thou knowest."  Neither shalt thou send me any spam, lest I smite thee.
= = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =