From: galway@chtm.eece.unm.edu (Denis McKeon)
Subject: Re: Using a separate battery:  Perfect Interface or Bat Isolator
Date: Sat, 12 Sep 92 06:10:14 GMT
Organization: Connemara - Computing for People
Followup-To: rec.auto.tech
X-Mailer: Mail User's Shell (7.0.1 12/13/89)
Status: OR

In article <18qdl9INN2k4@usenet.INS.CWRU.Edu> ae454@cleveland.Freenet.Edu (Paul Simundza) writes:
>
>I want to add like a motorcycle battery and in case my regular battery fails,
>I can flip a switch to send current from the motorcycle battery to start
>the motor....

It should be possible - at least theoretically - but you might need a
heavy switch (or more likely a relay).  Some ambulance (rig) systems
used to have fairly massive switches to allow starting from a backup
battery - the ones I recall had 3 or 4 positions:

    use main batteries
    use backup batteries
    isolate both batteries - in case of electrical fire
    ? use both batteries - I can't recall if there was a 4th position ...

or if there was a 4th position, what it was, exactly.  In fact, I can't
tell you anything about the charging circuitry, but you might ask at
your local fire department if they have any spare mail-order catalogs
for this sort of thing.

You can imagine that it is somewhat embarrassing to load a patient into
a rig and then have to ask around for jumper cables.  

The problems in ambulances are a blend of the accessory-intensive, and
the motor-home deep cycle - if an ambulance is not part of a large city
service - and thus being used or under maintainance every many hours of
every day - the service cycle tends to look like:

    battery stands idle between runs, perhaps for days
    heavy starting load
    heavy accessory load (lights, radios, etc.) 
	during run to scene
	at scene, possibly without engine running
	during run to hospital 
    rig returns to station, optionally is plugged into trickle charger

You can tell that a given ambulance comes from a small town (or is
rarely used) when there is an external port for a battery charger at the
driver's side door - so the crew (often volunteer) will be sure to
unplug the charger before racing to an accident scene :-)

Sometimes it is easier to put the port at the rear end of the rig, 
with a rope to auto-magically unplug as one drives away.

On a more general topic - I have been told that modern vehicles do not
usually have much extra alternator capacity.  I heard this in the
context of a vehicle which was failing to "charge up" a drained battery
- and a motorcycle mechanic said that the classic method of "jump start
it and drive around for a while" was less appropriate nowadays, because
alternators had less capacity, and that "charging up" a drained battery
was better accomplished in the driveway or garage, with a plug-in charger.
(Let's assume that the alternator drive belt was tight.)

Clearly there are tradeoffs - gasoline, time, tires, vs. risk of
overcharging with a dumb charger - but can anyone respond to the
question:  are '80s or '90s cars sold with less (surplus) alternator
capacity than '60s or '70s cars (with alternators)?  By "surplus" I mean
"available to recharge battery" (while driving with several accessories
on - say, headlights & radio, but not de-icer and seat heaters.) 

Or is this an alternator vs. generator (pre-mid-'60s) issue?  
Or an urban legend, like the zillion MPG carb :-)?

Please note - Followup-To: rec.auto.tech

--
Denis  McKeon                            Good.   Fast.   Cheap.
galway@chtm.eece.unm.edu                       Pick two.
Newsgroups: rec.audio.car