Re: New (possibly naive) Idea

[jonathan_dodge@juno.com (Jonathan L Dodge)]

Hi Chris!

I'll take a stab at this.

>I thought, "Wouldn't it be nice if I could flip a switch and have a 
>car with high performace turn into a more efficient car to go long 
>distances?"

Well, it's a great idea.  You're being creative, and we could all use a
lot more of that!  You can actually have the long range/high performance
feature built right into the car, with the "switch" being your foot,
i.e., how far down on the accelerator you tend to push it.

>The car would have a 144V pack:  either 24x6V, 18x8V, or 12x12V 
>depending on how many batteries fit.

Now we're in the land of trade-offs, big time.  Whatever type of battery
you choose to use, be it 12, 8, or 6 volt, flooded or sealed, your range
will basically be proportional to the weight in lead.  As you pointed
out, a high voltage pack tends to yield higher performance, depending on
how the controller limits current, etc. etc.

I think you are better off using a single string of batteries.  Use the
lowest voltage battery possible, which will be determined by your weight
(range), which you'll probably decide on first.  Cost, range,
performance.  After doing my EV conversion, this is the order of
priorities for me, with cost and range being much more important than
performance.  Low voltage batteries are cheaper and more durable, hence
less expensive.  If you want less than 1000 lbs. of batteries, your range
will probably be 50 miles or less.  You will probably also want to use 8
votl or 12 volt batteries.  Over 1000 lbs., 6 volt batteries at 120 volts
is definitely an option, as well as 18 8 volt batteries for 144 volts.

If you want to use a light (sealed) battery, you might consider a double
string only to get the amount of lead in the car to meet your range goal.

The reason for a single string is it's simple, so it's reliable.  Also,
your component selection will be easier since your controller and DC-DC
converter (if you'll have one) only need to deal with a specific range of
voltage.  High voltage EVs can be slightly more efficient than low
voltage, since current draw is less, so you lose less energy heating the
motor, controller, and cables.  However, increased efficiency by going to
high voltage is often negated by using a higher voltage battery, which
has more internal resistance and shorter life.  As I said, the land of
trade-offs.

In particular, let's consider a 144 volt battery pack.  It will give you
good performance if you put your foot down.  On the other hand, if you
drive conservatively, it will give you good range.  It's difficult
(spelled e x p e n s i v e) to have both at the same time, but there are
sealed batteries that are much better at this than flooded ones.

Hope this helps, and good luck!

Jonathan Dodge

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