RJT>George White wrote in a message to Roy J. Tellason:

RJT>

RJT> RJT> Even though it's been *ages* since I've actually seen one of
RJT> RJT> those, 

RJT>

RJT> RJT> The other thing that just comes to mind with those is that
RJT> RJT> underneath the mounting base were wire-wound
"resistors",  maybe
RJT> RJT> 2 or 3 of them

RJT> GW> If you had looked closely you would have spotted that they are 
RJT> GW> across the relay contacts. They're there to reduce (kill) arcing in
RJT> GW> the contact as they make and break to get a decent life out of the
RJT> GW> unit.

RJT>I did "look closely",  but at the time I was rather too
young to puzzle all of
RJT>this out...    :-)

RJT>

RJT> GW>> The duty on most car batteries and the rating of the alternators
RJT> GW>> is such that for most people by the time they've pulled out the
RJT> GW>> drive, and certainly by the time they've reached the end of the
RJT> GW>> street all the power taken from the battery to start the engine
RJT> GW>> has been replaced, and that is about the only time the full
RJT> GW>> alernator current can go to the battery.

RJT> RJT> That depends.  I've known of some people who did *lots* of fairly
RJT> RJT> short trips, and that ran their battery down because they weren't
RJT> RJT> running long enough to put the charge back into it.  This may
RJT> RJT> have been in colder weather,  may have been with the heavy use of
RJT> RJT> a lot of accessories,  too,  I don't recall

RJT> GW> Yes, that can happen. Usually in cold climates where you have 
RJT> GW> heated screens, fan blowers, lighting all running all the time. 
RJT> GW> This can limit the current available to charge the battery to 
RJT> GW> relatively few amps even when the engine is running fast enough to
RJT> GW> produce full output. If the vehicle was only used for school runs
RJT> GW> and shopping and in heavy traffic (choose any big city - I'm used
RJT> GW> to London (UK), New York (NY), Boston (Mass), Bristol (UK)) the car
RJT> GW> spends so much of it's time in heavy traffic with the engine idling
RJT> GW> the the alternator output falls below the accessory drain and the 
RJT> GW> battery is being discharged in normal use.

RJT>Or around here,  in winter,  when people are just "running
around town"...

RJT> GW> (On most systems I'm used to the alternator reaches full output at 
RJT> GW> about 25% max revs). The traditional solution is to up the engine 
RJT> GW> idle speed - but that only works for stick shift systems. For 
RJT> GW> automatics all you can do is cut down the power use (lower heater 
RJT> GW> fan speed, no extra lights) or put the battery on charge 
RJT> GW> overnight...

RJT>Not too likely for the senior citizens that are having this problem.

Present day alternators (last 15 yrs at least) will support most
everything normally used (A/C, headlights, radio, etc.) in a car AT
engine idle speeds.

Jay

RJT> GW>> In a modern car the alternator has a high current rating to drive
RJT> GW>> all the extra things in the car (windscreen wipers, lights,
RJT> GW>> heater/air conditoning fans, engine cooling fan, heated screens,
RJT> GW>> heated mirrors, heated seats, in car entertainment, etc...), most
RJT> GW>> of which only work when the enging is running anyway.

RJT> RJT> Just so.

RJT> GW> But you do still need to be moving along...

RJT>Yep.


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