[ausev] Battery choice

[Brian Lasseter]

On Mon, Dec 8, 2008 at 10:33 AM, The Mullins <ckmullins at earthlink.net> wrote:
> Why choose to use 20-6V batteries @67 LB producing 1340 LB of lead and
> 120Volts, when
>
> I have space for.. ..  20-8V batteries @69 LB producing 1380 LB of lead and
> 160Volts?

The short answer that you are missing is energy storage capacity, in
the form of Amp-Hours (Ah).

US-125 6V batteries have 242 Ah of power, while US-8VGCHC batteries
have 183Ah of power.  (From your weight and voltage measurements... I
presume those are the batteries you are using... feel free to correct
me.)

While having a higher voltage is good... you are actually keeping
fairly even on the amount of overall energy you are storing.  Multiply
your Ah by the volts to find total energy storage.  In this case... it
appears that you are only getting 29kwh of power for your ~1350lbs of
lead.

20 * 8V * 183Ah = 29.2kwh
20 * 6V * 242Ah = 29.0kwh

So... by having a higher voltage with the 8V... you will be able to
travel faster... but not quite as far.  With the 6V batteries, you
will be able to travel further, but not as fast.   (Though that
difference is mitigated to a large extent by the motor controller...
making any real difference very small.)

--

Another consideration is amperage coming from the batteries... and how
big your wires need to be.  To give you a real life example.  It takes
me about 57.6kw of instantaneous power to accelerate (fast) away from
a stop light.

That's 144 volts for me times 400 amps for 57.6kw at any given
instant.  (not kwh)

http://en.wikipedia.org/wiki/American_wire_gauge#Table_of_AWG_wire_sizes
Looking at a wire gauge table, the 00 gauge wire in my car is rated
for 175 Amps continuous current.  So it can take the 400 Amps briefly
to accelerate away from the stop light, and 100 Amps continuously to
cruise down the interstate.

If I were to double the voltage of my batteries, while keeping the
amount of lead in my car constant (as in my previous discussion
above)... then the current I would need would be cut in half:

That would be 288 volts times 200 amps gives me the same 57.6kw of
instantaneous power to accelerate away from the stop light... and only
50 amps of power to continuously to cruise down the interstate.  This
means I could get away with 1 gauge wire instead of the 00 gauge wire.

With the higher voltages and smaller currents... I could get away
smaller wires.  Smaller wires are cheaper, and lighter.  So win.   :-)
  This is also why in most production electric vehicles and hybrids,
you see obscenely large voltages close to 400V.  400V lets you have
small currents, but isn't so large as to be in danger of causing
insulation failure.   (Most wires and components are only rated for
600V... if you get near 600V, then your other fixed costs go up
because you will need "special" high voltage wire and parts.)

--

If the differences between "power" and "energy" confuses you... I
might take a look at the following wikipedia articles...
http://en.wikipedia.org/wiki/Watt
http://en.wikipedia.org/wiki/Kwh

-- 
TTFN,
Brian "Lasso" Lasseter

"No Sane man will dance."   -Cicero (106-43 B.C.)