The technical aspects of Battery Charging
Posted: Thu Feb 15, 2001 7:06 am
Charles Shiplee wrote,
"I have taken no offence and am safe in the knowledge that ... I am
allowed
at least one stupid question."
Charles, what a magnanimous comment to try to end this thread and how
pleasant it is in contrast to the inappropriate reply that started it
all. Thank
you! However, I think you have improperly characterized your original
question.
All the banter got me thinking about why, when charging a car battery,
it
doesn't make any difference how the battery is grounded. I realized
that
the issue is more subtle than it appeared and I think it is reasonable
for
a prudent person to wonder about it.
It certainly COULD matter how the battery was grounded. The fundamental
reason it doesn't matter is that the DC side of battery chargers always
float relative to ground (i.e., are not electrically connected to
ground).
This is not true for all types of DC power supplies. Some supplies are
even
switchable so the output may either be floated or be tied to the power
supply ground. If the power supply is properly grounded through the AC
plug, grounding the output would tie the DC to earth ground. If such a
power supply was used to charge a battery whose ground did not match the
charger ground, an inadvertent connection between the car and earth
ground
(a jack would do fine) could cause trouble. Such earth-ground circuits
typically are not very low resistance, so the low voltage from a charger
would not be likely to produce a huge ground current. Still, the fix is
trivial - float the output - and that is the way battery chargers are
made.
My point is that the original inquiry was not a stupid question. To the
experienced gearhead, it may have had an obvious answer, but how many of
us
had thought about the underlying reason?
Russ Wilson