Current reversal in 2 connected car batteries?

No, no, it was a good one. I'm just kinda thick sometimes (or very thick all the time, depending on whom you talk to....)

Eric Lucas

No, reverse bias is the normal mode of operation for many semiconductors, including the transistor collector-base junction, and rectifier diodes which are reverse biased for most of the AC cycle. What's potentially damaging to some semiconductors is reverse biasing at a voltage high enough to cause breakdown. If done repeatedly to the emitter-base junction (which is normally forward biased), it can result in reduced transistor hfe. Of course, since substantial current flows when breakdown occurs, there's also the possibility of complete destruction, e.g., melting of the silicon, due to heating. Zener diodes are intentionally operated in this mode, and the designer has to make sure they don't get sufficient current to be damaged.

In contrast, forward overvoltage is difficult to achieve. Because of the exponential I-V relationship of a forward biased diode junction, it takes a great deal of current to raise the forward voltage much. That's why a reverse connected shunt diode makes an effective limiter for negative transients. (Negative transients forward bias it.) There is the possibility that some junctions in the device to be protected will turn on at or near the same voltage as the snubber diode, but they're likely to be current limited in some way. If not, a hot carrier diode, with its lower forward voltage, might be used.

The best approach is to use devices like ICs which are specifically designed to survive in the automotive environment -- where another very significant hazard to electronic components is potentially extreme temperatures.

Roy Lewallen

What's knackered mean?

: But wouldn't this result in the donor battery forcing a current through : the weak battery such that it flows through the weak battery in REVERSE?

Yes. This is known as "charging".

Ian

Hi Eric,

This assumes negatively grounded 12-volt vehicles and that the donor vehicle's charging system and battery are good. Unless you suspect that the dead battery has a shorted cell, a reversed cell or is frozen, I believe that it is better to start the donor engine first and then connect the donor battery to the dead battery (with the last negative connection going to the frame rather than the battery post to less the possibility of an explosion). I suggest allowing the donor charging system to run at "high idle" for some time to partially recharge the dead battery before attempting to restart the dead vehicle. The reason is that if the donor battery is not fully charged, you could end up with two stranded vehicles. I also recommend that the battery that was jumped be fully charged with an external battery charger, surface charged removed, and capacity tested to check for latent damage, especially wet "Maintenance Free" (Ca/Ca) car batteries.

Please see Section 6 in the Car and Deep Cycle Battery FAQ on

Kindest regards,

BiLL.......

English and Merkin?

English = Language of the country England.

It's our language and we'll make the decisions over who can and can't call themselves 'English speaking'.

Americans don't fall into that category.

I thought you lived in Wales? :-)

Bollucks. Most "English" these days can't speak their own language, merely a debased variant.

Correct, inasmuch that American isn't the same as English. FFS the Queens' English originates from Scotland.

Is being French a bad thing?

cheers, clive

:-p

England squandered its world dominance and became a has-been second-world economy between 150 and 60 years ago. Does this *still* make you so bitter that you need to go around sniping at how other people speak, to make up for it? Well over 420 million people call English their first language, and fewer than 1/7 of those people "deserve to", in your eyes. Perhaps it's really time to finally consider getting over yourself.

Eric Lucas

To most Brits I know...and to anybody that believes that language is a dynamic, living entity that is constantly evolving, despite the belief of some that they can dictate how others communicate.

Eric Lucas

I can only assume you're American.

This will be the good old USA, where your industry is about to go the same way as ours has over the last 20 years.

Erm, no, he's not referring to industry...

Well, industry is the main reason behind a strong economy. In fact, even without industry, we have a stronger economy than the USA.

If he's referring to us handing the countries we illegally invaded many centuries ago back to the native population, I have to ask what's wrong with that?

It's certainly better than randomly invading anyone you don't like the look of, nearly bringing the entire middle East into a civil war the likes of which we've never seen before.

Where have *you* been for the last 60 years?

Eric Lucas

Appreciating that the world doesn't end at our national borders.

Oh, come on--do you even listen to yourself? Your attitude toward other speakers of English is the most elitist and isolationist that I've heard in a long time. The world doesn't end at your national borders...just peoples' right to call themselves speakers of English. Sheesh.

Eric Lucas