Electricity basics

[David Herron <davidh@7gen.com>]

I want to verify some understanding of the way the measurements work.  It
seems I need to understand this in order to know how much batteries to get
etc.  I looked in "Convert It!" for this kind of info, but it wasn't 
there (granted that book isn't one for theory, but instead more for
the how-to aspects of making an electric vehicle).  I remember a
discussion of this sort in the Curtis battery book but I couldn't
find my copy of that.

1 kwh is 1 kilo-watt for an hour.  So the KWH used in an application is a
unit of energy that is the summation under the curve of watts over
a period of time.

At an instant in time the power source (batteries and/or APU) deliver
an amount of watts.  Not watt-hours, but watts.

Here is one place where I'm having some trouble is this .. In discussing
energy use y'all say that a vehicle uses a number of watt-hours/mile (in
the ballpark of 100 Wh/mile, depending on vehicle and speed).  So if a
power source is able to provide N watts, how to size this to provide the
proper number of Wh's for the need? 

It seems like it is 6 KW's but that's partly a guess.  reasoning; 100
WH/mile means the amount of energy that would be 100 Watts in an hour, but
is delivered in a minute (1 mile/minute at 60 MPH).  If the usage were
kept up for an hour it would be 60 times that, or 6 KWH.  If one were to
measure the instantaneous KW's going through the system (that is, just
sample volts & amps at an instant rather than over a period of time), is
it 6 KW? 

The question is whether I've taken the right step in reasoning here.  Do
you take the KWH's used in an hour and size your power source to that?
That is, as I have done here the example is to extrapolate the number of
KWH used in an hour of driving (100 WH/mile, for 60 miles, equals
the number of KWH used in that hour).  So to size the power source
you make it capable of providing the number of KW's you need, and size
it to provide those KW's for the amount of time you need. (KWxHOURS=KWH)

If I want a 100 mile range, that means 100 minutes of driving time.
That gives me 10 KWh of needed energy capacity in the system.

I'm talking theory and general figures here.  I realize the energy use
varies with speed and the amount of weight being moved.  There are
inneficiencies (how does the resistance of the battery pack factor into
the energy capacity required?), and also adjusting for the need to not
drop the state of charge below approx 20-30% (how do you factor that into
the size of the pack?).

Is there a book that discusses these issues?

If I studied the code in the EV calculator page would I get some answers? 

I saw at Costco yesterday a portable gasoline generator claiming
to produce 6 KW and it produces 220 V directly.  Is that generator
enough to power an EV directly?

It's comforting to see that the numbers here roughly correlate with other
numbers I've seen in discussons on this list and in vehicle spec's. 


	David


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