Re: Easy Question: Charging Car Battery Removed From Vehicle?

tnom@xxxxxxxxx wrote:

In both cases, current flow equals possibility of spark sufficient to ignite hydrogen.


I just don't buy it. The capacitance of a reverse-biased diode is so tiny that you're talking microamps for microseconds. That would be such a vanishingly small spark that the concentration of hydrogen and oxygen at the point of spark would have to be absolutely PERFECT for ignition. Just aint gonna happen in the real world.


Micramps for a few milliseconds? Have you ever measured the current
flow when hooking a unplugged charger up to a charged battery?

Since I'm 50 feet from a lab where I can actually make that measurement, and its my lunch hour, I will go do that within an hour. I'll let you know- but I don't think my lab has a meter that can measure that small of a current.

Diodes are not the perfect switch you seem to imply. In order for a diode to pass large amounts of current it is required to have a
minimal resistance in the forward direction. Since a typical power
diodes used in battery chargers have about a 10:1 resistance ratio
that means it still passes current in the reverse direction. Just not
at the same level.

I don't know where you get this "10:1 resistance ratio" business. Its certainly not valid. I have pulled from my shelf and opened on my desk right now the textbook "Solid State Electronic Devices, 2nd Edition" (published 1980- darn I'm getting old) by Ben G. Streetman, presently chairman of the Electrical Engineering department at the University of Texas at Austin. Section 5.3 "Forward- and Reverse-biased junctions" is applicable. Page 150 defines the components of current that can cross a P-N junction (aka a diode), primarily the "drift current," the "diffusion current" and the "generation current" components. All 3 contribute to current flow under forward bias. However, page 151 states clearly:

"Under reverse bias, both diffusion currents are negligible and THE ONLY CURRENT (my emphasis) is the relatively small and VOLTAGE INDEPENDENT (my emphasis again) generation current from n to p."

Note that the reverse bias current flow across a diode doesn't even depend strongly on the applied voltage, therefore it doesn't even begin to have a curve that looks like a "resistance," and therefore the "10:1 resistance ratio" doesn't even exist at all.

"Generation current" is due to the quantum effect of electron-hole pairs spontaneously appearing in the junction region of a diode, and the electron being swept one direction and the "hole" the other due to any applied voltage at all. Photodiodes use the fact that light can cause hole-pair generation, but in regular rectifier diodes its just a probabalistic event, with a super-low probability. The reverse current flow is TINY, up until you exceed the reverse breakdown voltage of the diode... and a 12v battery aint gonna do THAT to a charger rectifier diode.



You also seem to forget that many chargers have accessory circuits
like voltmeters that draw 20 milliamps or so when the leads are hooked
up. 20 milliamps will cause a spark.

Again, it would be irresponsible to suggest that all one has to do is
have the charger unplugged in order to hook a battery up.

If I'm a corporate lawyer knowing that some goober will hook it up live, backward, or whatever, then I force my tech writers to put in the requirement for a long insulated lead to cover my company's ass. Someone WILL blow up a battery with one (or more) of the chargers my company sells, so its imperative that I make it clear that my company explained all precautions, both reasonable and unrealistic.

On the other hand if I'm out in my garage and all I care about is my own life (which I value greatly) then I have NO problem hooking up with the charger unplugged, because the odds of me getting hit by lighting inside my garage are FAR greater than the odds of connecting this way causing the battery to blow up. IOW, a rational and competent user only cares about the REALISTIC precautions.

.

Relevant Pages

  • Re: Diode reverse protection and current limiting.
    ... After several unpleasant experiences with battery chargers (which did ... It is a constant voltage power supply with no current limiting ... My calculation is that at 100A, the diode with voltage drop of 0.7 ... The smart charger I bought uses a bi-color LED to ...
    (sci.electronics.design)
  • Re: Diode reverse protection and current limiting.
    ... After several unpleasant experiences with battery chargers (which did ... It is a constant voltage power supply with no current limiting ... My calculation is that at 100A, the diode with voltage drop of 0.7 ... The smart charger I bought uses a bi-color LED to ...
    (sci.electronics.design)
  • Re: Reversed Biased MOSFETS & BJTs?
    ... possibility that someone will connect a battery reverse polarity. ... want this to destroy the charger. ... Emitter or Drain Source reverse biased? ... Most power MOSFETs have a "substrate diode" between source and drain. ...
    (sci.electronics.components)
  • Re: Reversed Biased MOSFETS & BJTs?
    ... possibility that someone will connect a battery reverse polarity. ... want this to destroy the charger. ... Emitter or Drain Source reverse biased? ...
    (sci.electronics.components)
  • Re: Diode reverse protection and current limiting.
    ... After several unpleasant experiences with battery chargers (which did ... battery charger is to use it to replenish some battery that does a lot ... It is a constant voltage power supply with no current limiting ... My calculation is that at 100A, the diode with voltage drop of 0.7 ...
    (sci.electronics.design)
Loading