Re: 42 Volt Electrical Systems

[Jim Cash <j.cash@xxxxxxxxxxxx>]

Scott Miller writes about a 42 volt auto system.

>  didn't ask Larry how they came to choose 42 volts, as opposed to some
> multiple of 1.5 volts as we are used to with batteries, but then I'm not an
> electrical engineer.  I expect we'll be seeing more on this in the future.
> 
> (That may be the closest to breaking news that I've ever posted to the
> Digest!  Or are all of you already aware of this, and I'm the last to know?)
> 
> Scott Miller


Scott - there has been discussion about this in the auto industry for
some time.  Ithink there was an article in either the Roundel or the BMW
magazine some time ago.

This is the first time I have heard of 42 volts.  THere has been
speculation that it will be either 24 or 48 since they will both address
the issue, and these voltages are already common for some other
applications (automated controls often use 24V, and telecommunications
has a lot of 48V - so there could be synergy with other items that have
already been developed. For instance the telephone industry has designs
for 48 volt fuses with built in detection of a blown fuse condition -
handy!.

Regarding the comment that Ford has maxed out alternators at 135 amps -
BMW has uses a 140 amp alternator for several years now - in the E38
seven series and in the E39 5 series.  Although I noticed that the E39
reverted to a 120 amp alternator as the maximum in 1998.


Regarding the batteries - flashlight batteries run at about 1.5v per
cell, but lead acids usually have a cell voltage of about 2.17, and
float under power at about 2.2.  That way you get a little of 13 volts
our of 6 cells.

Which does raise an ugly issue  - a 42 volt system will be great for
operations - but that does mean checking the acid level, and SG, in 19
cells. (have you checked your 6 cells lately?)


Cheers

Jim Cash
London, Ontario, Canada
BMW CCa 102929
E39  97 540iA

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