Current reversal in 2 connected car batteries?

computar2006 ( snipped-for-privacy@ntlworld.com) gurgled happily, sounding much like they were saying :

Complete crap.

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember "computar2006" saying something like:

Your battery was on its last legs anyway. No way could jumpstarting knacker your battery.

I don't think that's true. There are at least a couple of easy ways to ruin a battery during jumpstarting...hooking it up the wrong way round, and connecting cars that use different voltage standards are two that come to mind. The first is possible if you're careless about hooking up the cables. Both the first and second are more a problem when jumping older cars, which sometimes used either a + ground, or a completely different voltage (I think I've heard of 6V systems). As far as I know, all modern cars have 12V, negative ground systems, so the main problem is hooking up the cables wrong.

Eric Lucas

( snipped-for-privacy@sbcglobal.net) gurgled happily, sounding much like they were saying :

Some use 42v now, notably 7-series BMWs, and - of course - most trucks are

24v

Thanks for the correction--I thought the auto industry had finally standardized. I guess I can see an advantage to a higher voltage in starting the (usually) bigger engines in trucks, but I wonder why BMW chooses to use such a non-standard voltage for one of their line of cars.

Eric Lucas

Because it's the NEW standard. All (or most) of the auto manufactures are on board this new standard.

Bill K7NOM

New one on me (not surprising), but that still doesn't answer the question, why was a new standard necessary? What does 42 V do (besides answer the Ultimate Question of Life, the Universe and Everything) that 12 V doesn't? Was it simply to reduce starter current? Why is that an advantage--I'd think you would still have the same power losses in wiring, etc. Certainly

42 V batteries are more expensive than 12 V, so there must have been a good reason for the added expense.

Eric Lucas

( snipped-for-privacy@sbcglobal.net) gurgled happily, sounding much like they were saying :

There's rather a lot more electrickery in your average new 7-series than just the starter motor...

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember saying something like:

Are you serious? I was referring to a standard jumpstart with the operator not being a clueless fool.

Thanks for the info--interesting.

Yes, but most of it is electronics, which runs perfectly well on +12V/-5V, as long as the current capability is there. I can understand the higher voltage being required in hybrids, because there, you actually need the higher voltage for both the electric motors and the quick-start of the gasoline engine. It just wasn't clear to me what "electrickery" in non-hybrids needed the higher voltage.

Eric Lucas

Define "standard" when there are several voltage standards on the road.

I'm sorry if you took it that way, but on Usenet, one never knows the ability of one's audience. Do you know for a fact that the OP (who toasted his battery during a jumpstart) didn't accidentally hook it up wrong? If you don't then a misconnect (or a battery mismatch) is a reasonable possibility.

Eric Lucas

It doesn't need the higher voltage, but the higher voltage lets you get away with thinner cable & smaller relays, which is cheaper & lighter. & makes things like tailgate motors much more practical.

Given he diidn't need a new battery straight away that would seem to be a reasonable assumption.

Interesting, that makes sense. You do, of course, have to balance the weight saved in thinner wires and smaller relays, against the extra weight of 42V batteries vs. 12 V batteries. But then again, that extra battery weight would only be about 3.5x the 12V battery weight, or something like 30 lbs. I suppose the weight saved in wiring could easily equal that. It just seems like more chance for mistakes--a la the confusion that existed in the

60s over + ground vs. - ground. The electronics industry has come to understand that standardization is good for business, and that switching standards really does require a compelling benefit. It was unclear what that was with a 42 V standard.

I'm not sure how many BMW 7-series drivers have tailgate motors....

Thanks again for the reasonable and informative response.

Eric Lucas

There's not actually any extra battery weight, as you need less current for the same power you end up with only a fractionally larger battery for the same power to the starter motor.

I see some real benefits in establishing a new voltage standard for cars.

Until it becomes common, aftermarket radios, headlights, and a large host of other products won't work in the new cars, so dealers can charge what they want for parts.

As it becomes more common and the parts market increases, all auto parts dealers will have to buy a whole new inventory of higher voltage parts, from radios to headlights, other lights, and probably even fuses, while retaining their current inventory of 12 volt parts. So it's a great deal for the auto manufacturers and a great deal for the parts manufacturers.

I don't see any real benefit to the consumer, but hey, since when was that a concern?

Roy Lewallen

Right. I can also see an opportunity for manufacturers of 24 (or whatever they go to) vdc to 12 vdc converters. But they will probably be overly complicated to allow a larger price tag..

As I understand the plan there will be two voltages used at least initially. !2 volts will be available for those after market accessories and 42 volts for the built-in loads. Suppliers are designing (cheap) converters to get 12 volts from the 42 volts

There is consideration of using electrical motors for such things a breaks and steering. And of course as some one mentioned there is the hybrid car and it's power requirement.

Bill K7NOM