[Healeys] (no subject)alternator

Brits'n'Pieces (Eric Frenken) lists at brits-n-pieces.com
Wed Aug 20 08:12:37 MDT 2008


Bob, your equation is completely right, but additionally the current flow is
limited by the capability of the source. Dynamos built into AHs (Lucas C45
and C42) were capable of an output of max. about 20-22 amps (even if the
resistance would be 0 ohm) and the related cables were adjusted to that.
Modern replacement alternators can do at least double the output. Guess what
can happen when a modern alternator starts charging a flat battery and the
cables have not been exchanged.

Right, an alternator isn't meant to be a current-limiting device, but its
capabilities are limited. Otherwise I wouldn't see a reason for alternators
with different amps.

Best regards

Eric
Heinsberg/Germany

Bob Spidell wrote:
Not unless the alternator is overcharging.  The current flow to the battery
is determined by the voltage of the source and the internal resistance of
the battery (i.e. E=I*R, or I=E/R).  In other words, a battery in a given
state of charge will sink the same current from a generator producing a
nominal charging voltage (e.g. 13.8V) as from an 
alternator producing the same voltage.   If the internal resistance of 
your battery is low enough--close to a dead short--a generator can 
produce enough current to fry the charging wire.   Remember also that 
the battery has to supply enough power to excite the field coils
(generator) or armature (alternator) to provide current in the first place,
so a battery near a dead short will a) not start the car in the first place
and b) not supply enough juice to keep it running.  If you jump a car with a
dead short battery from another car or battery, the 
500+ amps available will fry a pretty large cable (check how hot the
cable to the starter gets after only a few seconds of fruitless cranking).

An alternator or generator isn't meant to be a current-limiting device 
(unless it's Lucas ;).  That is accomplished by the total load of all 
your electrical devices or, in a pinch, a fuse, fusible link or circuit 
breaker.


bs


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