Rod McInnis wrote:
"w_tom" wrote in message
...

If switching from a weaker bank of batteries to a fully
charged bank, then the armature coil inside alternator cannot
discharge fast enough. Load dump results.

Both SAE J1455 and ISO 7637-1 defined load dump for 12 V
automotive alternators as up to 270 volts and energy up to 50
joules.


You can certainly get this when switching from a discharged battery to
"OFF".

Even a fully charged battery will absorb a tremendous amount of current if
the voltage rises very much. If the battery is any good, I doubt that you
could get the voltage up above 16 volts. Just the capacitive load of the
battery could absorb a fair amount of the energy.

The problem occurs when the alternator was pumping amps into the battery and
it is switched to OFF. It is a basic inductor problem, the current can't
change instantaneously. It WILL go someplace, and it is likely that it will
damage things in the process.

Rod


After reading the comments regarding load tripping, I was going to
write just about what you said.

Good on ya.

The surge supressor previously mentioned would protect the diodes in
the alt from inductive field collapse induced overvoltage on dropped
load as long as it lasted. Sometimes surge supressors die quietly,
leaving the system vulnerable, sometimes they work perfectly for
ages. Eventually, all electronics go poof.

This brings us full circle back to the topic overcharging. In the
above posting's scenario, the alternator would overcharge the
battery switched to for about a millisecond. No big deal.

A warning: make before break switches fail too; a little dirt in arc
pitting can transform such a switch into an intermittent break
before make device, putting us back into a blown alternator scenario.

Alternators immune to the phenomenon would include an internal
crowbar type ciruit to protect internal diodes from excessive peak
reverse voltage.

Terry K