From: Dr. Bob [dr.bobf@worldnet.att.net]
Sent: Friday, April 26, 2002 2:05 PM
To: 928
Subject: [928] Brain Failures (was jump starting and charging)

Just some casual research showed that the threshold of pain on the
controller is about 16 volts.  My Explorer runs at almost 15 volts all the
time, and probably exceeds that amount when there's a rapid change in load.
Jump starting the 928 from the truck would qualify as one of those rapid
changes I think.

The wiring in the car gets old, connections get dirty and corroded over
time, and eventually there's a real chance that the voltage in the front and
the voltage in the back are different.  A test of this is to attach a
voltmeter between the positive battery terminal and the positive terminal on
the starter itself.  Do the same test between the bagttery positive and the
'jump start' terminal.  Run the starter and watch the voltage drop, then
note again the drop (in the other direction) as the flow comes from the
alternator now.  When jump-starting the car, you risk seeing the normal
system voltage plus that drop at points close to where you are attaching the
cables.

Most battery chargers are no more than transformers (to reduce the voltage)
and some form of rectifier to convert the AC from the transformer to DC for
the car.  Lead-acid batteries really do like the current/voltage ripple from
this system, but your electronics don't.  You might think that this is a bit
of a tough situation, since the altarnator makes the same ripple, but it is
smoother since there are three phases rectified.  So the charger is tough on
the electronics, to the point that peak charging voltage can exceed the
average voltage by as much as 40%.  Do you really want to charge the battery
from that front point near the brain, at the end of an aging and corroded
cable that is the only connection to the battery (current/voltage sink)?
Might not be the best idea.


The dr's recommendation:

Jumper cables attach to a battery that isn't attached to the car.  Lift the
ground strap at the wing-nut or bolt at the rear apron in the tool panel
area.  Attach the cables to the battery.  Let the donor car charge your
battery. The preferred method for this is with the battery out of the car,
but having the covers off and the area well ventilated will reduce the
accumulation of hydrogen and oxygen in the well.  Once the battery in your
car is charged, disconnect the jumper cables, reconnect the ground starp
with the wingnut or bolt, and start the car.

If you use a charger, attach it directly to the battery, plain and simple.
Better if the battery is disconnected, best if the battery is out of the
car.

This also points to the need to regularly inspect your cables and
connections.  The battery positive cable to the front of the car is a known
weak spot, along with the connecting cable from the starter post to the
accessory post ('jump start terminal') under the hood.  Add in the ground
strap between the engine and the right frame rail, and you have something to
do.  Failure of the cable , erratic connections, flex damage, corrosion on
the connections or inside the jacketing-- any of these will cause your very
own  alternator to send voltage spikes to the brain.  The problem is
exacerbated when the battery gets old or weak/run down, and the voltage
required to keep it charged gets excessive.  This points to a need for
battery replacement at the first signs of weakness.


So do your electrical maintennce, don't jump-start the car, charge
carefully.


My two sense on the subject.


dr bob