The point is that even a small amount of current can cause a spark - and it doesn't matter which way the juice is flowing. And even a small (invisible to the naked eye) spark can be sufficient to touch off the hydrogen explosion that's the reason behind the method originally being asked about.
Precisely... Which is *EFFECTIVELY* what the original poster was asking about - "Why does it say to do the hookup by connecting an extension to the negative post, then connecting the charger to the far end of the extension?"
The answer, when boiled down, is "To get the inevitable spark away from the stuff that it could ignite, causing the battery to blow up in your face."
How well or poorly a diode conducts, in which direction, or at what voltage, has little or nothing to do with it. When you make or break a connection to the battery, even if what you're (dis)connecting is an unplugged charger, THERE WILL BE A SPARK. That spark MAY be enough to ignite the cloud of hydrogen that could be hovering around the battery. So you want to make/break the connection at a distance from the battery.
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. The fact that it is theoretically possible at million-to-one against odds is the only reason that the CYA language is there at all in the charger instructions. If some dumb-cluck blows up a battery in his own face by connecting with the charger powered, the language is there to protect the manufacturer. They can say, "see, we TOLD you to use an insulated pigtail and you didn't." Which is as it should be.
For all pracitcal purposes, you're right. There is a tiny reverse leakage current that can practically be measured in number of electrons per minute, which is for all intents and purposes zero.
And there IS the "junction capacitance" that Don is talking about, but its a tiny, tiny, tiny capacitance. Not enough to produce a spark- because the capacitance of the air gap as you move the charger lead up to the battery post (right before you connect it) is comparable to the diode junction capacitance and appears in series with it- in essence the diode "charges" before the physical connection is even made.
In this case yes, but the leakage on a diode can actually be a useful thing in high-Z circuits. If you look at an anti-knock sensor, for instance, it usually has a FET front end with a reverse-biased diode fabricated on the same substrate, to provide a very teeny path to ground for the FET gate. Piezo element flexes, voltage on the gate changes, and the ground path is enough to keep the FET biased right when it's idle but not enough to sink much of the tiny low-current signal from the piezo.
Junction capacitance can be pretty large on huge selenium stacks, like your local theatre used to use for projection arcs. Now it's a non-issue except in the RF world.
--scott
And there are all kinds of cool things to do with junction capacitance in the RF world. Since capacitance is related to reverse bias voltage, a reverse-biased diode makes a handy voltage-controlled capacitor (Varactor diodes are optimized for that purpose). Which is the technology that gave us radio and TV tuners without knobs or other moving parts.
Micramps for a few milliseconds? Have you ever measured the current flow when hooking a unplugged charger up to a charged battery?
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.
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.
OMFG!!! I work for a company that sells and maintains batteries all day long and TRUST me there's no science to it any more!!! The only time I have ever seen a battery "explode" was when a crew hooked up a bettery reversed polarity with a bad alternator putting out 22V. Don't make it hard on a person who is just trying to get general info... just give them the basics... I've been doing this for years. Just make sure the switch is off before hookup. And besides if you want to scare the hell out of someone who is trying to save money by doing it themselves,,, why don't you tell them if there battery has a short in it ,, it could sizzle, smoke and really cause a fire! The best bet is make sure you have a good battery charger!!! At PASCO, We are the Charging and Starting Specialists
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.
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.
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.
Measurement complete. Interesting results, actually.
The first test I did was with a 10-amp fully automated (fast then maintain) electronic battery charger that's about 10 years old. Its got dual-color LEDs on front to indicate "charging" or "charged." The LED turns amber ("charging") when the charger is unplugged but connected to the battery, so I knew it was going to have a measurable reverse current because that is what lights the LED. It did- 17 milliamps. Personally, I consider that completely safe to connect while unplugged- the probability of a spark that could ignite hydrogen from the battery would be vanishingly small. But there IS a current. I turned out all the lights and tried to see a spark- no dice.
The second charger I measured was a newer fully automatic fast/maintain charger. It was even better than I claimed- when connected to a fully charged battery but unplugged from the wall, it showed a whopping 0.2 MICROamps of reverse current. Yes, 200 NANOamps. The meter I was using only goes down to 10ths of a micro-amp, so really the measurement's at the limit of the meter's- it could be even less than that. Absolutely ZERO chance of that charger causing a spark if you connected it (while its unplugged) to a fully charged 12v battery.
Finally, I dug up the oldest-style charger we have. Its a semi-automatic Dayton, meaning that it charges until it senses 13.8 volts, then shuts off (a mechanical relay clicks open) and won't kick back on until the voltage falls WAY down to something like 11. It also has a 55-amp "start" setting. When unplugged, it draws about 30 mA reverse current from a fully charged 12v battery, and that might, conceivably, remotely, create a spark when hooked up unplugged. Yet its instructions don't say
*anything* but pretty much hook it up, positive first and then negative" and then plug it into the outlet. Nothing about a long insulated lead or anything. Since Dayton is still in business without having killed or maimed any users, and this was the "worst" reverse current of the chargers I had on hand, I think I will rest my case here.
At 20ma I can see a spark in the dark. At 17ma I would suggest you look again.
I have made connections to batteries also without blowing anything up. It just makes more sense if you completely remove the possibility by 'use of the extra cable.
Modern auto and marine battery chemistry is such that not a lot of "gassing" occurs unless seriously overcharging.. not the case here and second, the vents are ususally equipped with some sort of flame trap type arestor in the openings.
I used to work at a shop where oddball sized, heavy duty, lead acid batteries were "rebuilt", and before leading the terminal connections or heating the tar that was used in those days to seal the cells into the case, the tech used to wave the torch over the open cells to get any little hydrogen "pop" out of the way so that it wouldn't startle him later on!.
In my opinion (only), the danger of a hydrogen explosion is MANY times greater after the battery has been on charge, has reached full charge, is gassing freely and the charger is REMOVED without turning off the power... creating a spark when there would likely be gas in the vicinity of the battery terminals.
This "extra lead on the negative terminal" sounds like someones idea of a legal CYA more than an absolute requirement if common sense is used around a lead acid battery that has been gassing.
Bob
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